What each one actually measures
BMI (Body Mass Index) is weight divided by height squared — a simple ratio that says nothing about what that weight is made of. Body fat percentage, estimated here via circumference measurements (the U.S. Navy method — neck, waist, and for women, hip), is an attempt to measure body composition directly: how much of your weight is fat versus lean mass (muscle, bone, organs, water).
BMI is fast (just height and weight) and was originally designed for population-level health screening, not individual diagnosis. Body fat percentage needs a few tape measurements but reflects what actually drives most health outcomes associated with excess weight — the fat itself, not total body mass.
Where BMI gets it wrong — a worked example
Consider someone 175 cm tall weighing 82 kg with a muscular build (a 40 cm neck, 85 cm waist). Their BMI is 26.8 — classified "Obese" under the WHO Asia-Pacific bands commonly used for Indian populations (≥25 is Obese, 23-24.9 is Overweight). But their estimated body fat percentage, from the same measurements, is just 15.4% — comfortably in the athletic-to-fit range for men, nowhere near what "obese" implies.
The mismatch happens because BMI can't distinguish muscle (which is denser than fat) from fat — a heavily muscled person weighs more per unit of height without being unhealthily fat, and BMI has no way to see that difference.
Where BMI still works well
For most people who aren't unusually muscular or unusually lean, BMI correlates reasonably well with body fat and is a legitimate, low-effort first screen — it's why doctors and public health surveys still use it widely. The mismatch shown above is a real limitation, but it mainly affects people at the edges: bodybuilders, strength athletes, and very elderly or very sedentary people (whose issue runs the other way — normal BMI but higher-than-expected body fat from muscle loss).
Which should you actually track?
If you're not doing serious strength training and don't have an unusually muscular or unusually frail build, BMI is a reasonable quick check. If you lift seriously, are very lean, or BMI results seem inconsistent with how you look and feel, body fat percentage (or better still, a proper method like DEXA scanning, if accuracy really matters) gives a truer picture. Many people find it most useful to track both over time rather than relying on either alone — a rising BMI with a stable or falling body fat percentage usually means you're gaining muscle, not gaining fat.