Both BMI and body fat percentage try to answer "is my weight healthy?" — but they measure different things, and knowing the difference keeps you from drawing the wrong conclusion.
What BMI measures
Body mass index is simply weight divided by height squared. It's fast, needs no special tools, and works well as a population-level screen. Its weakness is that it can't tell muscle from fat, so a lean, muscular person can read as "overweight" while someone with low muscle mass can read as "normal" despite high body fat.
Check yours with the BMI Calculator — but treat it as a starting point, not a verdict.
What body fat percentage measures
Body fat percentage estimates the share of your weight that's fat versus everything else. It's a better indicator of fitness because it reflects composition directly. The Body Fat Calculator uses the U.S. Navy tape method, which needs only a measuring tape and is accurate within a few points for most people.
Use them together
For most people the smart move is to glance at BMI, then confirm with body fat percentage and a waist measurement. If BMI says "overweight" but your body fat is in the athletic range, the muscle is the explanation.