Property taxes are calculated with a simple formula: take your property’s assessed value, subtract any exemptions you qualify for, and multiply the result by the combined local tax rate (the mill rate). In plain terms, (Assessed Value − Exemptions) × Mill Rate = Your Tax. The arithmetic is easy. What makes bills vary so much from house to house, and year to year, is how each of those three numbers gets set.
How the Assessed Value Is Set
A government assessor assigns a value to every taxable parcel in the jurisdiction. For a typical home, that value comes from one of three approaches:
- Sales comparison. The assessor looks at recent sales of similar nearby homes and adjusts for differences in size, condition, and features. This is the standard method for residential property.
- Cost approach. The assessor estimates what it would cost to rebuild the structure today, then subtracts depreciation. Newer construction and unusual buildings are often valued this way.
- Income approach. For rentals and commercial buildings, value is based on the revenue the property can generate.
Assessors also review building permits and sometimes inspect properties in person, so improvements like a finished basement or an added deck can show up in the next valuation. These practices follow professional standards, including the Uniform Standards of Professional Appraisal Practice.
Assessment Ratios
In many places, the taxable assessed value is not the full market value. The assessor applies an assessment ratio set by state law. If your home is worth $400,000 and the ratio is 80%, the assessed value used for tax purposes is $320,000. Some jurisdictions assess at 100% of market value; others use a much lower ratio. This is where people get confused when they compare the assessor’s number to what they think the home would sell for. Before deciding the assessment is too high, check whether a ratio applies and work the math from there.
Reassessment schedules also vary. Some jurisdictions revalue every property each year; others use two-, three-, four-, or eight-year cycles. Certain events trigger an immediate update outside the regular cycle, including a sale, a major building permit, or damage from a fire or natural disaster. If you just bought a home, expect the assessor to move your value toward the purchase price.
What a Mill Rate Is and Who Sets It
A mill is one-thousandth of a dollar. One mill means $1 of tax for every $1,000 of taxable value. A rate of 20 mills works out to $20 per $1,000, or 2% of taxable value.
The rate on your bill is not one number set by one authority. It is the sum of separate rates chosen by each local body that taxes your property: the school district, the county, the municipality, and any special districts for fire protection, libraries, water, and the like. Each of these bodies decides how much money it needs for the coming fiscal year and votes on a rate, usually at a public budget hearing. Your bill typically lists each district’s rate on its own line and then combines them into a single figure that gets applied to your taxable value. Four or five overlapping rates on one bill is normal.
Mill rates can rise or fall each year regardless of what your property is worth. A district needing more revenue raises its rate; one seeing strong growth in the local tax base might hold steady. Over time, rates tend to drift upward.
Special Assessments Are Not Part of the Formula
A line on your bill labeled as a special assessment is not calculated with the mill rate. Special assessments fund a specific project that benefits your parcel directly, such as a new sewer line, street repaving, or sidewalks. The charge is based on project cost and the benefit to your property, not on the home’s value, so it sits alongside your property tax rather than being computed from it.
Exemptions That Come Off Before the Rate Applies
Exemptions reduce the assessed value before the mill rate is multiplied in, which is why they cut the bill dollar for dollar at the applicable rate. The most common one is the homestead exemption, which shelters a fixed dollar amount of your primary residence’s value from taxation. The size varies widely by jurisdiction.
Additional reductions are commonly available for:
- Senior citizens who meet age (often 65 or older) and income limits, sometimes as an extra reduction and sometimes as a freeze on assessed value.
- Veterans with service-connected disabilities, sometimes eliminating the tax entirely for those with severe disabilities.
- Homeowners with documented permanent disabilities, on terms similar to the senior break.
Almost every exemption requires that you own and occupy the home as your primary residence on a specific date, often January 1 of the tax year, and almost none apply automatically. You have to file. If you buy mid-year, find out right away whether you need to submit paperwork for the following tax year. Miss the window and you pay the full amount until the next cycle.
Abatements are related but work differently. Instead of lowering assessed value across the board, an abatement waives the tax increase that would otherwise result from a qualifying improvement. Some jurisdictions abate the added value from solar panels, wind systems, or other clean energy equipment for a set number of years. Historic renovations can qualify for similar treatment.
Putting the Formula to Work
The three steps in order:
- Start with the assessed value. If your jurisdiction uses an assessment ratio, that is market value times the ratio.
- Subtract every exemption you qualify for. The result is your taxable value.
- Multiply the taxable value by the combined mill rate. Move the decimal three places to the left on the mill rate, or divide it by 1,000, before multiplying.
A worked example. Say your home has a market value of $350,000 and the jurisdiction assesses at 100%, so the assessed value is also $350,000. You qualify for a $50,000 homestead exemption, bringing taxable value to $300,000. The combined mill rate across all your local districts is 25 mills, which is 0.025 as a decimal. Multiply: $300,000 × 0.025 = $7,500 for the year.
Now watch what happens when the inputs move. A reassessment that adds $30,000 to your value costs another $750 at the same rate. A two-mill bump across the districts costs $600 on the original $300,000 taxable value. When market values and rates both rise in the same year, those effects stack, which is how homeowners end up with jarring increases after a hot local market.
If the Number Looks Wrong
You have the right to challenge an assessed value you believe is too high. Assessors’ offices process large volumes of these appeals every year, and many succeed on straightforward facts.
Start with your property record card, available from the assessor’s office or website. Check for factual errors first: wrong square footage, a bathroom that doesn’t exist, a basement listed as finished when it isn’t. Errors like these are the easiest wins because the assessor can verify them quickly.
If the facts are correct but the value still looks inflated, gather recent sales of comparable homes that sold for less than your assessed value. Note the differences that matter, such as a renovated kitchen in the comparable versus original cabinets in yours. Document anything that reduces your home’s worth: structural problems, flood-zone location, a noisy highway next door, an unfavorable zoning change. A recent independent appraisal is strong evidence if you have one.
Then file. Your assessor’s office publishes the appeal form and the deadline, usually online. You will need the parcel identification number from your tax bill or assessment notice. Deadlines are strict: often 30 to 90 days after assessment notices go out, and missing the window generally forfeits your right to appeal for that entire tax year. File early, keep proof of submission, and check whether there is a filing fee.
The assessor’s office reviews your evidence. Sometimes the value is adjusted without a hearing. If not, you’ll present your case to a review board (commonly called a board of equalization or assessment appeals board), which weighs the evidence from both sides and issues a written decision. It can lower, confirm, or in some cases raise the assessed value. If the board rules against you, most jurisdictions allow further appeal to a state tax commission or court, though that step brings higher costs and more formal procedure.
One practical warning: filing an appeal does not pause the obligation to pay. If a deadline hits before your appeal is decided, pay the bill on time, keep pursuing the appeal, and collect a refund if you win. Penalties and interest on a late bill start almost immediately and can be steep.