What a mill is, and why the bill has three moving parts
A mill is one dollar of tax per $1,000 of taxable value. Thirty mills is three percent; 22.5 mills is 2.25 percent. The word survives from the Latin millesimum, a thousandth, and it exists because a rate per thousand keeps the arithmetic in whole numbers when values run into six figures.
Three numbers between the house and the bill. The assessor sets a value, usually on a cycle of one to five years. The state sets an assessment ratio — the fraction of that value which is taxable, which is 100 percent in many states and as little as 10 percent in others. The taxing bodies — county, city, school district, fire and library districts — each set a millage, and the bill you receive is their sum.
Because two of the three vary by state, mill rates cannot be compared across state lines. A 90-mill rate on a 25 percent assessment ratio is 2.25 percent of market value, exactly the same burden as 22.5 mills at full value. Only the effective rate on market value is comparable, which is why this calculator reports it.
Exemptions come off the assessed value, not the tax. A $25,000 homestead exemption at 60 mills saves 25,000 × 0.060 = $1,500 a year. Its cash value is the exemption multiplied by the mill rate — nothing to do with the assessment ratio, which has already been applied.
Working through the formula
Assessed value = market value × assessment ratio. Read this number off your assessment notice rather than deriving it if you can — some states apply caps that hold assessed value below the ratio for long-held homes, and a few assess land and improvements on different bases.
Taxable value = assessed value − exemptions. Homestead exemptions for owner-occupiers, and additional exemptions for seniors, veterans and people with disabilities, reduce the base. Some are flat dollar amounts, some are a percentage of value, and some cap the annual increase in assessed value rather than reducing it — a cap is not an exemption and does not belong in that field.
Tax = taxable value × mills ÷ 1,000. The division by a thousand is the whole meaning of a mill. If your bill quotes a rate per $100 of value instead, multiply it by ten to get mills.
Special assessments are added afterwards. A sewer connection charge, a paving assessment or a refuse fee is a charge for a specific benefit rather than a tax on value. It appears on the same bill but it is not ad valorem, and it is generally not deductible as a real estate tax on a federal return.
Effective rate = tax ÷ market value. This is the figure to use when comparing towns, states or two properties. Dividing by assessed value instead produces a number that depends on the assessment convention and cannot be compared to anything.
Worked example: a $300,000 home at 40 percent assessment and 60 mills
A homeowner in a state that assesses at 40 percent of market value owns a home appraised at $300,000. The combined millage from the county, city and school district is 60 mills, and a $25,000 homestead exemption applies.
- Assessed value. 300,000 × 0.40 = $120,000.
- Taxable value. 120,000 − 25,000 = $95,000.
- Annual tax. 95,000 × 60 ÷ 1,000 = 95,000 × 0.060 = $5,700.00.
- Monthly escrow. 5,700 ÷ 12 = $475.00.
- Effective rate on market value. 5,700 ÷ 300,000 = 1.90 percent.
- Rate on assessed value. 5,700 ÷ 120,000 = 4.75 percent — a number that looks alarming and means nothing outside this state's convention.
Value the homestead exemption on its own: 25,000 × 0.060 = $1,500 a year. Without it the bill would be 120,000 × 0.060 = $7,200, and the effective rate would be 2.40 percent. That is why the first thing to check on a new home is whether the homestead exemption has actually been filed — in most states it is not automatic and it is not applied retroactively for more than a year or two.
Now test a levy. A school bond that adds 5 mills raises the tax to 95,000 × 0.065 = $6,175, an increase of $475 a year on this property, or $39.58 a month in escrow. Millage changes are the lever voters actually control; assessments are not.
How to read the result
Compare on the effective rate and nothing else. Effective rates on market value across the United States run from roughly a quarter of a percent to well over two percent, and a property tax bill of the same dollar size can represent very different burdens. If a quoted rate is above 4 percent of market value, the figure is almost certainly a rate on a fractional assessment.
The monthly escrow figure is what your lender will collect, not what you owe monthly. Lenders normally collect one twelfth of the annual bill each month plus a cushion of up to two months, so the actual escrow line is usually a little higher. It also lags: an escrow account set from last year's bill produces a shortfall the year after a reassessment.
An assessment increase and a millage increase are different problems with different remedies. An assessment you believe is wrong is appealable, usually within a short window after the notice, on evidence of comparable sales or of errors in the recorded characteristics. A millage increase is a political decision by the taxing bodies and is not appealable at all.
Watch what happens when you buy. Several states reassess to the sale price on transfer, so the seller's tax bill can badly understate yours. Others cap annual increases for long-held homes, which means the previous owner's low bill was a function of tenure, not of the property.
Deductibility is limited. Real estate taxes are an itemised deduction and are inside the combined state and local tax cap, so a large property tax bill may produce no federal benefit at all if state income tax has already used the cap. Check with the taxable income calculator before assuming a deduction.
The same $350,000 property under different assessment conventions
| Assessment ratio | Mill rate | Assessed value | Annual tax | Effective rate on market value |
|---|---|---|---|---|
| 100% | 10 | $350,000.000000 | $3,500.00 | 1.000% |
| 100% | 15 | $350,000.000000 | $5,250.00 | 1.500% |
| 100% | 22.5 | $350,000.000000 | $7,875.00 | 2.250% |
| 100% | 30 | $350,000.000000 | $10,500.00 | 3.000% |
| 50% | 30 | $175,000.000000 | $5,250.00 | 1.500% |
| 40% | 60 | $140,000.000000 | $8,400.00 | 2.400% |
| 25% | 90 | $87,500.000000 | $7,875.00 | 2.250% |
| 10% | 200 | $35,000.000000 | $7,000.00 | 2.000% |
Every row is market value times the ratio times the millage divided by 1,000. Note the last four rows: 30 mills at 50 percent, 60 at 40 percent, 90 at 25 percent and 200 at 10 percent all produce different bills, so a high mill rate tells you nothing until you know the ratio.
Mistakes that produce the wrong bill
- Applying the mill rate to market value in a fractional-assessment state. At a 40 percent ratio this overstates the tax by two and a half times.
- Using one taxing body's millage. The county, city, school district and any special districts each levy separately and the bill is their sum. School districts are usually the largest single component.
- Treating the exemption as a reduction in tax. It reduces taxable value, so it is worth the exemption times the mill rate — $1,500 on a $25,000 exemption at 60 mills, not $25,000.
- Forgetting to file for the homestead exemption. In most states it must be claimed and is not transferred automatically when a property changes hands.
- Budgeting from the seller's tax bill. A sale can trigger a reassessment or reset a capped value, so the previous bill may be far below what you will pay.
- Including special assessments in the deductible amount. Charges for a specific local benefit are generally not deductible as real estate taxes, even though they sit on the same bill.
- Comparing mill rates between states. Meaningless without the assessment ratio. Compare effective rates on market value.
Why the mill rate moves when values rise
Most jurisdictions budget first and set the rate second. The taxing body decides how much revenue it needs, divides by the total taxable value in the district, and the quotient is the millage. That means a general rise in property values across a district lowers the mill rate rather than raising everyone's bill — many states formalise this with a rollback or truth-in-taxation rule that requires the rate to fall so revenue stays flat, with any increase above that needing a public hearing or a vote. The practical consequence for you is that your bill rises only if your assessment rises faster than the district average, or if the taxing bodies vote for more revenue. A reassessment that raises everyone equally raises nobody's tax.
Key terms
- Mill
- One dollar of tax per $1,000 of taxable value — one tenth of one percent.
- Assessment ratio
- The statutory fraction of market value that is subject to tax. Also called the assessment level or the classification percentage where different property classes carry different ratios.
- Ad valorem
- Latin for “according to value” — a tax proportional to the value of the property, as distinct from a flat special assessment.
- Homestead exemption
- A reduction in taxable value for an owner-occupied principal residence. Amounts, eligibility and filing deadlines are set by state law.
- Levy
- The total revenue a taxing body raises. Dividing the levy by the district's total taxable value gives the mill rate.
Where property tax fits in the wider picture
Property tax is the main revenue source for local government in the United States and is the principal funding mechanism for public schools, which is why school district millage is usually the largest line on the bill. It is levied on value rather than on income or transactions, so it does not fall when your income does — the reason exemptions and circuit-breaker relief for older and lower-income owners exist at all.
For a buyer, the annual bill divided by twelve belongs in the housing payment alongside principal, interest and insurance. The mortgage payment calculator takes the annual figure directly, and the home affordability calculator shows how much a two-point difference in effective rate changes what you can borrow.
For an investor, property tax is an operating expense that comes straight off net operating income, so a millage increase reduces value at the capitalisation rate — a $500 rise in annual tax at a 6 percent cap rate costs about $8,300 of property value. Model it with the net operating income calculator and the cap rate calculator.
On the federal return, real estate tax on a personal residence is an itemised deduction inside the combined state and local tax cap. On a rental or business property there is no cap: property tax is an ordinary and necessary business expense on Schedule E or Schedule C, deducted in full alongside depreciation.