How this works
BMI (Body Mass Index) is a simple measure of body fat based on height and weight. It is used by healthcare professionals as a quick screening tool, though it does not directly measure body fat percentage or muscle mass.
The formula was devised by the Belgian astronomer and statistician Adolphe Quetelet in the 1830s as a population-level measure, not an individual diagnostic — Quetelet himself called it the "social average" weight for height. The fact that it ended up routinely applied to individuals in clinical settings is partly an accident of being simple and cheap to compute. It works well enough as a first-pass screen for population health (the BMI–disease-risk relationship is real and well documented), but at the individual level it has well-known blind spots: it conflates fat with muscle, ignores where body fat sits (visceral vs subcutaneous), and uses thresholds derived predominantly from white European populations of the mid-20th century.
For most adults checking in on a new health goal, the BMI number is a useful starting point — but it should never be the only one. Pair it with waist circumference (a better predictor of cardiovascular and metabolic risk), body fat percentage if you can measure it, and how you actually feel and function day to day. A BMI that classifies a fit, muscular athlete as "obese" is telling you about the BMI scale's limitations, not about the athlete.
The formula
In imperial: BMI = (weight (lbs) ÷ height (in)²) × 703
Example calculation
- Convert height: 5′10″ = 70 inches = 1.778 metres
- Weight: 180 lbs = 81.6 kg
- Apply formula: 81.6 ÷ (1.778 × 1.778) = 81.6 ÷ 3.16 = 25.8
- Result: BMI 25.8 → Overweight (25–29.9 range)
Frequently asked questions
Is BMI an accurate measure of health?
BMI is a useful population-level screening tool, but it has limitations. It does not distinguish between fat and muscle mass, so very muscular individuals may be classified as overweight. It also does not account for body fat distribution, age, or sex differences in fat distribution.
What is a healthy BMI range?
The WHO defines healthy BMI as 18.5–24.9 for adults. Below 18.5 is underweight; 25–29.9 is overweight; 30 and above is obese. However, these thresholds were derived primarily from studies on European populations, and some health organisations recommend lower thresholds for Asian populations.
How is BMI calculated for children?
For children and teens aged 2–19, BMI is calculated the same way but then compared to age and sex-specific percentile charts rather than fixed thresholds, because children's body composition changes as they grow.
I lift weights — is BMI useless for me?
It overestimates risk for muscular people, but it's not useless — just incomplete. Lean muscle is denser than fat, so a powerlifter at 6'0" / 220 lb with 12% body fat has a BMI of 30 (technically obese) without carrying excess fat. If you train consistently and your BMI lands in overweight or obese territory, supplement with a body-fat measurement (skinfold calipers, BIA scale, or DEXA) and a tape measure on your waist. A waist of 40+ inches (men) or 35+ inches (women) is a meaningful warning regardless of BMI; a waist below those numbers paired with a high BMI usually just means extra muscle, not extra risk.
What's a better measurement than BMI?
Three options, in roughly this order of accuracy. (1) Waist-to-hip ratio: divide waist circumference by hip circumference; healthy is roughly <0.9 for men and <0.85 for women. Tracks visceral fat (the metabolically dangerous kind) better than BMI. (2) Body fat percentage: a direct measurement of fat mass. Methods range from DEXA scan (gold standard, ~$100+) to 7-site skinfold calipers (cheap, decent in trained hands), to bioelectrical impedance scales (cheap, noisy). (3) Waist circumference alone: less precise than the ratio but surprisingly predictive — >40 in / 102 cm for men or >35 in / 88 cm for women flags elevated cardiovascular risk regardless of BMI. For a one-number snapshot you can do in 30 seconds with a tape measure, waist circumference is hard to beat.
I've heard Asian BMI thresholds are different. Why?
At equivalent BMIs, people of South and East Asian descent tend to carry more visceral fat and have higher rates of type 2 diabetes and cardiovascular disease. Studies in the early 2000s showed the WHO standard 25 (overweight) and 30 (obese) thresholds underestimated risk in these populations. The WHO Expert Consultation in 2004 recommended additional public-health action points at BMI 23 (overweight risk) and 27.5 (high risk) for Asian populations. Some countries — Japan, China, India, Singapore — use these adjusted thresholds in clinical practice. If your ancestry skews Asian and your BMI is in the high-normal or low-overweight range, take it slightly more seriously than the standard chart suggests.
Does the healthy BMI range change as I get older?
Yes — slightly. Several large studies have found that mortality risk in adults 65+ is lowest at BMIs in the 25–30 range, slightly higher than the standard 18.5–24.9 healthy range for younger adults. The reasons aren't fully settled, but include: muscle mass naturally declines with age (sarcopenia), so a slightly higher BMI may reflect preserved muscle rather than excess fat; underweight in older adults is associated with frailty and worse outcomes from illness; some "obese" BMIs in older adults are protective during periods of recovery (e.g., after surgery). The takeaway: don't aggressively try to lose weight to hit BMI 22 if you're 70 and otherwise healthy. "Moderate plumpness" with active muscle and mobility is generally a better goal at that age than thinness.