Each year, the County Board of Assessors (BOA) uses mass appraisal technology to determine the market value of all properties based on factors such as use type and neighborhood. Market value is defined as “the amount a knowledgeable buyer would pay for the property and a willing seller would accept for the property at an arm's length, bona fide sale.”

The BOA then multiplies your property's market value by the assessment ratio to calculate your property's taxable value. The assessment ratio for all counties in Georgia is 40%. The result of multiplying the assessment ratio by the market value is the taxable value of your property.

In the final step, the BOA multiplies the taxable value by the tax rate. The property tax rate varies from county to county, and even differs between municipalities within each county. To calculate effective tax rate, some counties use millage to express their tax rates. A mill is one one-thousandth of a dollar, and in property tax terms is equal to $1.00 of tax for each $1,000 of assessment. 15 mills, therefore, is equal to $15 for every $1,000 of assessed value, or 1.5%. Some counties, such as Gwinnett, show the mill rate already divided by 1,000. The millage rate can be confusing and not every county expresses their tax rate in mills.