The mill levy is a factor applied to the value of each piece of property in the county to generate taxes. The mill is calculated by dividing the total assessed valuation of an area divided by 1000.
Jefferson County Example:
$146,501,279 (assessed valuation) / 1000 = $146,501 (value of one mill)
A property's tax rate is determined by multiplying the assessed value by the mill rate.
Residential property is assessed at 11.5% of its value, so for a house valued at $100,000, the assessed value is $11,500 ($100,000 x .115)
Each entity collecting tax dollars follows the same method, then the total mill rate for a tax unit is calculated by summing the mill rates for all entities involved. The total mill rate is the number called "Mill Levy" in the tax information search
More information: Computing a Mill
Jefferson County Mill Rates: 2004-2020