body fat %

You really can't calculate it from a website. It must be measured. See if there's a BodPod or DEXA in your city if you really want the measurement. Caliper-based readings have a larger margin of error, and don't trust a single device that claims to measure BF % via bio-impedence. My bio-impedence scale continued to tell me I was 16% body fat on the same day the DEXA pegged me at 8%.

If you are man, suspect you are < 20% bf, and have pictures online of your abs both flexed and unflexed, I'm willing to bet I can make a guess at least as accurate as calipers.

Men and women carry different amounts of fat in different locations. It's hard to guess either gender's fat % until abs are visible when flexed at the least, but it's still much harder to guess by pictures with women simply because y'all aren't as lean.

As far as what's 'acceptable' -- that's up to you, of course. But some rules of thumb are: A chick is fairly lean once she's under 20%. For a guy, call it 12%. A ridiculously lean (perhaps unhealthy) female might be closer to 15%. Less than that is too lean to healthily sustain, but female bodybuilders get down to between 10-15% before getting on stage. A ridiculously lean male might be closer to 8%, but male bodybuilders often get down to 4-6% for shows.

TL;DR: Don't compare yourself to bodybuilders. It's up to you to decide what's 'acceptable' for you. As a male I'm happiest at 10-12%, though I've been lower as well as much higher. My girlfriend is about 20% and she's very lean looking.