Calculator
Navy body fat calculator
The US Navy circumference method — a tape measure, three numbers, and an honest error range.
How it is calculated
The method fits body density from circumferences and height, then converts to a percentage. It is free, repeatable and carries roughly three to four percentage points of error against a DEXA scan.
That error matters less than it sounds, because the error is fairly consistent for one person over time. Measured the same way each month, the direction of travel is reliable even when the absolute number is not.