The categories at a glance
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity, class 1 |
| 35.0 – 39.9 | Obesity, class 2 |
| 40.0 + | Obesity, class 3 |
The math (both unit systems)
Metric: BMI = kg ÷ m². A 70 kg person at 1.75 m: 70 ÷ 3.0625 = 22.9 — healthy range.
Imperial: BMI = 703 × lb ÷ in². A 154 lb person at 5'9" (69 in): 703 × 154 ÷ 4,761 = 22.7 — same person, same answer.
Get your number and category in one tap — metric or imperial.BMI calculator →
Three things the number cannot see
- Muscle. An 85 kg rugby player at 1.78 m scores 26.8 — "overweight" — at 12% body fat. Same BMI as a sedentary person at 30% body fat. Identical number, opposite health stories.
- Fat location. Visceral fat around the organs drives metabolic risk far more than fat under the skin. Two people at BMI 24 can carry very different waistlines — which is why waist circumference is the standard companion check.
- Age. Muscle declines ~3–8% per decade after 30. An older adult can sit at BMI 23 while carrying more fat than their younger self did at 25 — and large studies suggest 23–27 associates with the best outcomes after 65.
So is it useless? No —
Across whole populations, BMI tracks disease risk well and it costs nothing to compute, which is exactly why clinics use it as a first filter. The mistake is treating a screening number as a verdict. Pair it with a tape measure and, if you're planning changes, an honest calorie baseline — those together tell you far more than any single figure.
Quick self-check: keep your waist below half your height. It takes ten seconds, needs no chart, and catches the central-fat risk BMI misses.
Run your own numbers
FAQ
What is the healthy BMI range for adults?
18.5 to 24.9 for most adults per WHO and CDC guidance. Below 18.5 is classed underweight; 25–29.9 overweight; 30+ obese. These are population screening bands, not individual diagnoses.
Does the healthy range change with age?
The formula doesn't, but interpretation should. Research on older adults (65+) links slightly higher BMIs — roughly 23–27 — with better outcomes, partly because some reserve protects during illness and because muscle loss can mask fat gain at a 'normal' BMI.
Why does BMI flag muscular people as overweight?
BMI only knows weight and height. Muscle is denser than fat, so a lean, muscular person carries 'extra' weight the formula can't distinguish from fat. Athletes routinely score 26–29 with low body-fat percentages.
Is BMI calculated differently for men and women?
No — same formula and categories. Women naturally carry more body fat than men at the same BMI, which is one of the known blind spots of a single unisex scale.
What should I check alongside BMI?
Waist circumference (risk rises above ~40in/102cm for men, ~35in/88cm for women), waist-to-height ratio (keep waist under half your height), blood pressure, and blood work. Those capture the fat-distribution risk BMI can't see.
Sources
Primary references used for the figures and rules on this page.
- Adult BMI Calculator — CDC
- Body mass index (BMI) — World Health Organization