How to compute property tax: the complete step-by-step guide

Last updated 2026-07-09

What is the basic formula for computing property tax?

Property tax comes down to three parts:

Taxable Value x Tax Rate = Property Tax Bill

That's the whole thing. Everything else is just figuring out what feeds into those two numbers. Get the terms right, because assessors and tax bills use them loosely, and that sloppiness costs homeowners money.

Assessed value is the dollar figure your local assessor assigns to your property. It may or may not equal market value. In many states it's a fixed percentage of market value, called the assessment ratio.

Taxable value is assessed value minus any exemptions you qualify for (homestead, senior, veteran, and so on). This is the number you actually multiply by the rate.

Tax rate usually shows up as a mill rate. One mill equals $1 of tax per $1,000 of taxable value. A rate of 20 mills is 2%, or $0.020 per dollar. Some places just write the rate as a percentage.

So the full formula, spelled out, is:

((Assessed Value x Assessment Ratio) minus Exemptions) x Mill Rate / 1,000 = Annual Tax Bill

If your county already publishes assessed value as the post-ratio number (many do), skip the ratio step. Subtract exemptions, apply the rate, done ^[1].

What is assessed value and how does your assessor calculate it?

Assessed value starts as an estimate of what your property would sell for. Assessors reach that estimate three ways, and which one they use depends on your property type ^[2].

Sales comparison approach. The assessor pulls recent sales of similar homes (comps) in your neighborhood and adjusts for differences in size, age, condition, and features. This is the workhorse method for residential property.

Cost approach. The assessor estimates what it would cost to rebuild your structure today, subtracts depreciation, and adds land value. It shows up more often for unusual properties or new construction where comps are thin.

Income approach. Mostly for rentals and commercial buildings. It converts the net income a property could produce into a value estimate.

Most homeowners get the sales comparison approach. And here's the part that matters: the assessor's office runs a mass appraisal model, not an individual inspection of your house. Your assessed value is a statistical estimate. It can be wrong. A 2021 study from University of Chicago researchers found that across Chicago, the effective assessment rate for the lowest-value homes was nearly double that for the highest-value homes, meaning cheaper houses get over-assessed on a percentage basis ^[10].

Once the assessor lands on a market value, they apply the assessment ratio if your state requires one. Illinois assesses most residential property at 33.33% of market value in counties outside Cook, and Cook County assesses residential at 10% ^[4]. New York City runs four separate assessment classes with different ratios ^[5]. California plays by its own rules: under Proposition 13, assessed value is locked at the purchase price and grows no more than 2% a year, no matter what the market does ^[6].

So if your home's market value is $500,000 and your state uses a 50% ratio, your assessed value on paper is $250,000. That's your starting point before exemptions.

What are mill rates and how do you convert them to a percentage?

A mill rate (also called millage) is the tax rate written in mills. One mill equals $0.001, one-tenth of a cent. A rate of 25 mills means you pay $25 for every $1,000 of taxable value.

Converting is easy. To turn a mill rate into a percentage, divide by 10: 25 mills is 2.5%. To go the other way, multiply by 10: a 2% rate is 20 mills.

Your total mill rate is almost always several levies stacked together. A typical bill breaks down like this:

Taxing Jurisdiction	Mill Rate
County general fund	8.50
Municipal government	5.20
School district	14.30
Community college	1.10
Library district	0.75
Fire district	0.80
Total	30.65

That 30.65-mill rate is a 3.065% effective rate on taxable value. Your bill either lists each line or lumps them, depending on your county. The school district piece is almost always the biggest.

The Lincoln Institute of Land Policy's 50-State Property Tax Comparison Study (2023 data) put the average effective property tax rate on owner-occupied housing near 1.1% nationally. That average hides enormous spread: New Jersey sat around 2.2% and Hawaii around 0.3% ^[3]. Knowing your own mill rate lets you check that your bill matches what was actually levied.

How do exemptions reduce your taxable value?

Exemptions subtract a set dollar amount (sometimes a percentage) from your assessed value before the mill rate touches it. They can cut a bill hard, and plenty of homeowners never claim the ones they qualify for.

The big one is the homestead exemption, offered in most states for your primary residence. The amount swings wildly. Florida's basic homestead exemption removes the first $25,000 of assessed value from all taxes plus another $25,000 from non-school taxes, for a combined $50,000 reduction on homes worth more than $75,000 ^[7]. Texas raised its homestead exemption on school district taxes to $100,000 starting in 2023 under HB 5 ^[8]. Illinois cuts equalized assessed value by $6,000 in most counties through its General Homestead Exemption ^[4].

Other common exemptions:

• Senior exemptions. Extra reductions for homeowners over a set age, often 65. Some states freeze the taxable value at the level it hit when you turned 65.
• Veteran and disabled veteran exemptions. These range from a few thousand dollars off assessed value up to a full exemption in some states.
• Disability exemptions. For homeowners with qualifying disabilities.
• Agricultural exemptions. Lower valuations for land in active farm use.

To find your taxable value: Assessed Value minus every applicable exemption = Taxable Value.

Say your assessed value is $300,000 and you have a $25,000 homestead exemption. Taxable value is $275,000. At 20 mills, that's $275,000 / 1,000 x 20 = $5,500 a year. Without the exemption you'd owe $6,000. The $500 difference costs nothing to claim, and in most counties you apply once and never think about it again.

Can you walk through a concrete end-to-end property tax calculation?

Here's a full example with realistic numbers from a mid-size county that uses a state assessment ratio.

Scenario: You own a home in a state that assesses at 40% of market value. The estimated market value is $425,000. You qualify for a $15,000 homestead exemption. The total mill rate in your district is 22.5 mills.

Step 1: Calculate assessed value. $425,000 x 40% = $170,000 assessed value

Step 2: Subtract exemptions. $170,000 minus $15,000 = $155,000 taxable value

Step 3: Apply the mill rate. $155,000 / 1,000 x 22.5 = $3,487.50 annual property tax

That's your bill. Plain arithmetic once you have every input.

Now the part that helps you fight back. The number most likely to be wrong is the market value in Step 1. If your assessor pegs your market value at $500,000 instead of $425,000, watch what happens:

$500,000 x 40% = $200,000 assessed value $200,000 minus $15,000 = $185,000 taxable value $185,000 / 1,000 x 22.5 = $4,162.50 annual tax

That's $675 more a year, and the only thing that changed was the assessor's opinion of your home's value. Challenging that opinion with better comparable sales is exactly what a property tax appeal does. Build the case yourself and you skip the contingency firm that would take 30% to 50% of your savings. TaxFightBack's appeal kit walks you through pulling comps and formatting an evidence package your assessment board will actually read.

A handful of cities compute things differently. NYC property tax uses four property classes with separate ratios and rate structures that make the standard formula hard to trace. LA County property tax uses Prop 13 base-year values instead of current market value.

How do you find your assessment ratio and mill rate?

All of these numbers are public. Here's where they live.

Assessment ratio. Your state's department of revenue or taxation publishes the statutory ratios. Search "[your state] assessment ratio statute" or check your county assessor's FAQ. Some states bake the ratio into law: California uses 100% of base-year value under Prop 13, and Texas uses 100% of market value ^[6][8]. Others set it locally.

Mill rate. Your county or township puts out an annual rate sheet, sometimes called a levy rate table or millage table. Find it on the county auditor, treasurer, or assessor website. It breaks out every taxing district.

Your assessed value and exemptions. These print on your assessment notice (mailed yearly or every other year in most states) and appear on your county assessor's online property search. Most counties have a searchable portal. Type your address, see your current assessed value, applied exemptions, and prior-year figures.

Can't find your mill rate? Your bill hands it to you. Take your annual tax, divide by taxable value, multiply by 1,000:

Mill Rate = (Annual Tax / Taxable Value) x 1,000

A $4,500 bill on $200,000 taxable value: ($4,500 / $200,000) x 1,000 = 22.5 mills.

For county-specific lookups: Santa Clara property tax, Miami-Dade property taxes, Hennepin County property tax, and Contra Costa County property tax all have dedicated guides on this site with the exact assessor portals linked.

How do effective tax rates differ from mill rates?

The effective tax rate is the tax you pay as a percentage of your home's actual market value. The mill rate is the rate applied to assessed value, which is often a fraction of market value. That gap is why the mill rate alone tells you almost nothing about how heavy your tax burden really is.

Compare two places:

• Jurisdiction A: 40 mills on 50% of market value. Effective rate = 40 x 0.001 x 0.50 = 2.0% of market value.
• Jurisdiction B: 25 mills on 100% of market value. Effective rate = 25 x 0.001 x 1.00 = 2.5% of market value.

Jurisdiction A looks like the high-tax place at a glance (40 mills against 25). It's actually the cheaper one once you factor in the assessment ratio.

The Lincoln Institute's 50-State Property Tax Comparison Study uses effective rates for exactly this reason, so you can line up New Jersey's 2.2% against New Mexico's 0.6% on the same scale ^[3].

When you're testing whether your bill is fair, effective rate is the metric to use. Get your home's market value (a recent sale price, a Zillow estimate as a rough check, or a formal appraisal), then divide your annual tax by that market value. If your effective rate sits well above the county median for similar homes, you may be over-assessed and worth appealing.

For how the system fits together, including how levy limits and rate caps constrain what local governments can charge, see our property tax taxation overview guide.

What causes property tax bills to change from year to year?

Your bill can jump for three separate reasons. Knowing which one is behind the increase tells you whether you can fight it.

1. Your assessed value went up. The assessor re-estimated your market value higher, usually because comparable homes nearby sold for more. This is the most common cause of a bigger bill and the most commonly disputed. If the new assessed value sits above what your home would really sell for, you have grounds to appeal.

2. The mill rate went up. Voters passed a school bond, the county raised its levy, or a special district added a line item. You can't appeal a rate increase through the assessment appeals process. That's a political question, not a valuation question. Your options are voting against the levy or petitioning local government.

3. An exemption expired or dropped off. Forget to renew your homestead exemption in a county that requires annual renewal, or rent out your home and lose the primary-residence requirement, and your taxable value jumps. Read your notice carefully and confirm your exemptions still show up.

California homeowners have a strong shield. Prop 13 caps assessed value increases at 2% a year unless the property changes ownership or gets new construction ^[6]. So a California bill can only jump meaningfully after a sale or a renovation. That's why Santa Clara property tax bills for two identical houses on the same street can differ by tens of thousands of dollars depending on when each owner bought.

Some places phase in big valuation changes over several years. Maryland spreads assessment increases across a three-year cycle ^[1]. That smooths the pain, but it also means you keep paying on a high assessment long after a market downturn.

How do you check if your property tax calculation contains an error?

Errors show up more than most homeowners expect. They come in two flavors: math errors and valuation errors.

Math errors are rarer but quick to check. Grab your assessment notice and tax bill and confirm three things:

• Taxable value on the bill equals assessed value minus exemptions.
• The mill rate matches your county's published rate table.
• The arithmetic holds: taxable value / 1,000 x mill rate = total tax.

If any of those don't reconcile, call your county treasurer or assessor. Math errors get fixed fast, no formal appeal needed.

Valuation errors are more common and hit harder. To check them:

1. Look up your property on your county's online portal. 2. Note the assessor's market value estimate and the assessment ratio applied. 3. Pull 3 to 5 recent sales of comparable homes (similar size, age, condition, location) from the past 6 to 12 months. Your county recorder or Zillow's "sold" tab works. 4. Calculate what your assessed value should be if your actual market value matched the average of those comps. 5. Compare that figure to your current assessed value.

If your assessed value runs more than 5% to 10% above the comp-based number, you likely have a case. The University of Chicago study on Chicago assessments found the effective assessment rate for the lowest-value homes was nearly double that for the highest-value homes, which points to systemic over-assessment at the bottom of the market ^[10].

For step-by-step help finding and formatting comps, see our evidence-and-comps guides. Also check Collin County property tax and Williamson County property tax for Texas examples where fast appreciation has produced widespread over-assessment.

How do property tax calculations work differently in major cities?

A few big jurisdictions deserve their own callout because their formulas veer off the standard model.

New York City runs four property classes. Class 1 (1-3 family homes) is assessed at 6% of market value. Class 2 (apartments) and Class 4 (commercial) use different ratios and rate structures. The Department of Finance publishes the rates every year ^[5]. NYC's system is genuinely tangled; our NYC property tax guide walks it step by step.

California (every county, including LA County and San Mateo County) uses Prop 13 base-year values. Assessed value locks at the purchase price (100% ratio) and grows no more than 2% a year. The base rate is 1% of assessed value, plus voter-approved bonds and special assessments that usually tack on another 0.1% to 0.3% ^[6].

Texas (counties like Collin and Williamson) assesses at 100% of market value and has no state income tax, which pushes property tax rates among the highest in the country. The 2023 jump in the homestead exemption to $100,000 off school district value helps owners of modest homes a lot ^[8].

Michigan (including Detroit) uses a taxable value cap. Assessed value is 50% of market value, but taxable value can rise only by the lesser of 5% or inflation each year until the property sells. Long-term owners pay on much lower taxable values than new buyers.

Florida (including Miami-Dade) caps annual assessment increases at 3% for homesteaded properties under the Save Our Homes amendment, a close cousin of California's setup ^[7].

If you pay online, your payment portal usually shows a breakdown of each levy, which helps you double-check the pieces of your bill.

When should you appeal instead of just paying your bill?

Appeal when the assessor's market value estimate is higher than what your home would realistically sell for, and you can prove it with recent comparable sales. That's the whole test.

Signals that an appeal is worth the effort:

• Your assessed value (converted back to full market value using the ratio) sits more than 5% to 10% above what similar homes recently sold for.
• Your neighborhood saw declining sale prices over the past 12 months, but your assessment went up anyway.
• Your property has physical problems (foundation issues, deferred maintenance, functional obsolescence) the assessment ignores.
• You bought recently at a price below the assessed market value.

The deadline is firm and varies by state. Most states make you file within 30 to 90 days of getting your assessment notice. Miss it and you usually wait until next year. Some states let you appeal every year; others reassess only every few years and cap the appeal window to match ^[1][2].

Contingency firms charge 25% to 40% of first-year savings for work you can do yourself with the right templates and a few hours of research. TaxFightBack's DIY appeal kit gives you the same comp-gathering method and hearing scripts those firms rely on, and you keep every dollar you save.

One honest caveat. If your property is commercial, mixed-use, or you're fighting an income-approach valuation, the process gets technical fast, and a contingency attorney may earn the fee. For a standard single-family home, doing it yourself is very doable.

Frequently asked questions

How do I calculate my property tax if I don't know my assessment ratio?

Check your state's department of revenue website or search for your state's assessment ratio statute. Most publish the ratio prominently. Or back into it from a recent local sale: divide the assessed value (from the county portal) by the sale price, and you get the effective ratio being applied. Some states like Texas and California use 100%, so there's no separate ratio to hunt for.

What is a mill rate and how do I find mine?

A mill rate is the tax rate written as dollars per $1,000 of taxable value. One mill equals $0.001. Your county auditor or treasurer publishes an annual mill rate or levy rate table, usually online. You can also back-calculate it from your bill: divide annual tax by taxable value, then multiply by 1,000. A $3,000 tax on $150,000 taxable value equals 20 mills.

Why is my neighbor's property tax lower than mine on a similar house?

A few reasons. Your neighbor may hold a homestead, senior, or veteran exemption you don't. In states like California and Michigan, long-term owners pay on a base-year value locked at purchase, so someone who bought 20 years ago pays far less than a recent buyer. It's also possible your property is over-assessed relative to theirs, which is a valid basis for an appeal.

How much can my assessment go up in one year?

It depends entirely on your state and locality. California caps increases at 2% a year under Prop 13. Florida's Save Our Homes cap is 3% for homesteaded properties. Michigan's taxable value cap is the lesser of 5% or inflation. Most other states set no cap, so a hot market can produce a large single-year jump. Check your state's statutes for any limits.

Does a higher sale price always mean a higher assessment?

Not immediately, and not always by the same amount. Many counties reassess on a fixed cycle (annually, biennially, or every few years), so a sale price bump today may not reach your assessment for a year or two. In California, the sale itself triggers a reassessment to the purchase price, but later market increases don't matter until the next sale. Some states use statistical models that smooth changes across neighborhoods rather than reacting to individual sales.

Can I calculate my own property tax before buying a home?

Yes, and you should. Find the current assessed value in the county's online portal. Confirm the assessment ratio so you can estimate the new assessed value after your purchase price becomes the base (required in some states). Subtract any exemptions you'll qualify for. Multiply by the local mill rate. Add special assessments (stormwater, lighting) if they apply. The county treasurer's website usually lists all of these.

What exemptions automatically apply, and which ones do I have to apply for?

Almost none apply automatically. The homestead, senior, veteran, and disability exemptions all require you to apply. The application is usually a short form filed with the county assessor, often a one-time filing with annual certification for seniors. Miss the filing deadline and you lose the exemption for that tax year. Check your county assessor's site for the form and deadline, which commonly falls between January 1 and March 1.

How does property tax change after a renovation or addition?

New construction or a significant addition triggers a reassessment of the improved portion in most states. In California, only the new construction value gets reassessed; the existing structure keeps its Prop 13 base value. In most other states, the assessor revalues the whole property after a permit is pulled and inspected. A kitchen upgrade may or may not trigger reassessment, depending on whether a permit was required and whether the assessor was notified.

Is property tax deductible on federal income taxes?

Yes, up to a limit. The Tax Cuts and Jobs Act of 2017 capped the state and local tax (SALT) deduction, which includes property tax, at $10,000 per year ($5,000 for married filing separately) through at least 2025. If your combined state income tax and property taxes clear $10,000, the deduction stops helping past that point. You must itemize to claim it, and the standard deduction is higher for most taxpayers ^[9].

How do I know if my property tax bill has a math error?

Pull your county's published mill rate table and your assessment notice. Confirm taxable value equals assessed value minus your exemptions. Then multiply taxable value by the mill rate divided by 1,000. If that result doesn't match your bill, there's either a rate entry error or a missing exemption. Call the county treasurer with the specific discrepancy; math errors get corrected without a formal appeal.

What is the difference between market value and assessed value on my tax bill?

Market value is what the assessor estimates your property would sell for in an arm's-length deal. Assessed value is often a fraction of that, set by the state's assessment ratio. In Texas and California the ratio is 100%, so assessed value equals estimated market value (or base-year value in California). In Illinois it's typically 33.33%. Your tax runs off assessed value, not market value, which is why a high market value doesn't always mean a proportionally high bill.

How often do counties reassess property values?

It varies by state and sometimes by county within a state. Some counties reassess annually. Others run a two-year or four-year cycle. Maryland uses a three-year cycle with phased increases. Indiana reassesses every year using market data. California triggers a full reassessment only on sale or new construction. Check your county assessor's website for your schedule, or read the date on your most recent assessment notice.

Can I appeal my property tax even if I just bought the house at the assessed price?

Yes, if the assessed value now tops what similar properties are selling for, or if your purchase price came in below the assessed value. In many states a recent arm's-length sale is strong evidence of market value, so if you paid $320,000 and the assessment is $350,000, that sale price is a powerful data point. File within your county's appeal deadline, usually 30 to 90 days from the notice date.

Sources

1. Maryland Department of Assessments and Taxation, Property Assessment Overview: Maryland phases in assessment increases over a three-year cycle; appeal rights and deadlines tied to assessment notice.
2. International Association of Assessing Officers (IAAO), Standard on Mass Appraisal of Real Property: The three standard approaches to value (sales comparison, cost, income) used by assessors for mass appraisal.
3. Lincoln Institute of Land Policy, 50-State Property Tax Comparison Study (2023): Average effective property tax rate on owner-occupied housing is approximately 1.1% nationally; New Jersey near 2.2%, Hawaii near 0.3%; New Mexico near 0.6%.
4. Illinois Department of Revenue, Property Tax Overview: Illinois assesses residential property at 33.33% of market value in most counties; Cook County at 10%; General Homestead Exemption reduces EAV by $6,000.
5. California State Board of Equalization, Proposition 13 Overview: Under Proposition 13, assessed value is set at purchase price and grows at no more than 2% per year; base property tax rate is 1% of assessed value.
6. Florida Department of Revenue, Property Tax Exemptions: Florida's homestead exemption removes first $25,000 of assessed value from all taxes and an additional $25,000 from non-school taxes; Save Our Homes caps annual assessment increases at 3% for homesteaded properties.
7. Texas Comptroller of Public Accounts, Property Tax Exemptions: Texas HB 5 (2023) increased the residential homestead exemption for school district taxes to $100,000; Texas assesses at 100% of market value by statute.
8. IRS, Topic No. 503: Deductible Taxes: The SALT deduction cap of $10,000 per year ($5,000 married filing separately) applies to combined state and local taxes including property tax through at least 2025 under TCJA.
9. University of Chicago Harris School of Public Policy, The Assessment Gap (Avenancio-León and Howard, 2021): Across Chicago, the effective assessment rate for the lowest-value homes was nearly double that for the highest-value homes, indicating systemic over-assessment of lower-value properties.