BMI is the most used — and most argued about — health number in the world. Doctors screen with it, insurers price with it, and the internet loves to declare it useless. The truth is in the middle: BMI is a genuinely useful screening tool with well-known blind spots. Here's how to read yours intelligently.
The formula
BMI (Body Mass Index) is your weight divided by your height squared:
Metric: weight (kg) ÷ height² (m²)
Imperial: weight (lb) ÷ height² (in²) × 703
A person who is 1.75 m tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. You can get your own number instantly with our BMI calculator — it handles both unit systems and shows where you land on the scale.
The official categories
The World Health Organization ranges for adults:
Under 18.5 — Underweight. Associated with its own health risks: weakened immunity, bone density loss, nutrient deficiencies.
18.5 to 24.9 — Healthy weight. The range where, statistically, weight-related disease risk is lowest for most people.
25 to 29.9 — Overweight. Elevated risk of type 2 diabetes, high blood pressure, and cardiovascular disease.
30 and above — Obesity. Risk rises substantially, and further categories (class I, II, III) track how far above 30 you are.
Two important nuances: these cutoffs were derived from population statistics, not individual physiology, and some countries use adjusted thresholds — several Asian health authorities, for example, treat 23+ as overweight because health risks appear at lower BMIs in those populations.
Where BMI misleads
BMI knows only two things about you: height and weight. It cannot tell muscle from fat, or belly fat from hip fat. The famous failure cases:
Muscular people score "overweight." A 90 kg lifter at 1.80 m has a BMI of 27.8 — technically overweight, possibly with abs. Muscle is denser than fat, and BMI can't see the difference.
"Skinny fat" scores healthy. Someone with low muscle mass and high body fat can sit at a normal BMI while carrying the metabolic risks of excess fat.
It ignores fat location. Visceral fat around the organs is far more dangerous than fat stored on hips and thighs, and BMI treats them identically.
Age shifts the picture. Older adults naturally lose muscle; a "healthy" BMI in a 75-year-old can hide significant muscle loss.
What to check alongside BMI
If your BMI reads high (or suspiciously fine), one or two extra numbers give a much sharper picture:
Body fat percentage — the number BMI is trying to approximate. Estimate it with our body fat calculator using simple tape measurements.
Waist-to-height ratio — a quick visceral fat proxy: your waist should be less than half your height. That one rule catches most of the "skinny fat" cases BMI misses.
Trend over time — a single reading matters far less than the direction it's moving over months.
For a target to aim at, the ideal weight calculator compares several established formulas (Devine, Robinson, Hamwi) rather than relying on one, and the lean body mass calculator estimates how much of your weight isn't fat at all.
So is BMI worth using?
Yes — for what it's actually for. Across large populations, BMI correlates strongly with body fat and disease risk, which is why it survives as a screening tool. For an individual, treat it as a first signal, not a verdict: a BMI of 31 in someone who hasn't trained in years means something very different from the same number in a rugby player.
The bottom line
Calculate your BMI in ten seconds, see which range you're in, and read it with context — your muscle mass, your waistline, your trend. If it flags something, pair it with a body fat estimate before drawing conclusions. And if you're planning to change the number, the path runs through diet and training, not the formula: our guide to the calorie deficit covers exactly how the scale actually moves.