This will generate a table of bodyfat percentages and your corresponding abdomen measurements for men, or waist + hip sizes for women, with corresponding descriptions from this table from the American Council on Exercise:
The output is calculated using the U.S. Military (DoD) bodyfat formulas.
Neck measurements are assumed constant. If this assumption distresses you, enter your neck measurement as 0, and the resutls for men will be abdomen - neck, and for women will be waist + hip - neck.
The creators of these formulas only use them for male bodyfat percentages from 9% to 37%, and female bodyfat percentages from 19% to 47%. These may be reasonable ranges within to expect some semblance of accuracy.
How to measure males.
How to measure females.