Tape measure body fat measurements are generally considered to be the least reliable of all the ways to test body fat. The problem is that we're trying to determine body composition - how much of us is fat and how much is lean body mass - and it's difficult to do that by measuring your surface. Are your inches made up of inches of muscle? Or of fat? A tape measure can't tell you that. What we need a way to figure out what's beneath the surface.
I haven't found a on-line body fat calculator that's anywhere close to where I test with calipers and electric impedance. Partly it's because they're biased against a woman in her 50's being in the teens in body fat. I can pretend my waist measurement is 15 inches and they'll still tells me that I'm 22% or something, and i know that's not right!!
Baffled had a good suggestion about the Y or a gym. Any personal trainer can measure your body fat, either with electric impedance or calipers.