That's pretty inaccurate.
if you actually want to get an accurate number, go to a gym or something related to that, they should have equipment you can use to get that number.
Calculating body fat % is difficult w/o specialized equipment b/c concentration of fat varies throughout your body, so measuring general areas doesn't work. There's also the factor that every person is built differently in terms of height, stockiness, muscle mass, e.t.c. Equipment makes it easier, trust, you do not want to waste your time trying to accurately calculate it.
~How I know, I took gym in school