BMI Calculator

Body Mass Index (BMI) is a simple screening measurement that relates your weight to your height, giving a quick indication of whether you fall into an underweight, healthy weight, overweight, or obese range. It doesn't measure body fat directly, but for most adults it correlates closely enough with body fat to be useful as a fast first check.

This calculator uses the World Health Organization's standard BMI formula: weight in kilograms divided by height in meters squared (kg/m²). The WHO's classification is also what sets the category thresholds shown with your result — under 18.5 is underweight, 18.5 to 24.9 is a normal weight, 25 to 29.9 is overweight, and 30 and above is obese.

BMI is a population-level screening tool, not a diagnosis. It doesn't distinguish between muscle and fat, so a very muscular person can register as "overweight" despite having low body fat, and an older adult with reduced muscle mass can register as "normal" despite having a higher proportion of fat. It also doesn't account for where fat is distributed on the body, which matters for health risk. If your BMI falls outside the normal range, or you have concerns about your weight more broadly, talk to a doctor — they can look at the full picture, not just one number.

A worked example

Say you weigh 70 kg and stand 175 cm tall. Convert height to meters: 175 cm = 1.75 m. Square it: 1.75 × 1.75 = 3.0625. Divide weight by that: 70 ÷ 3.0625 = 22.86, which rounds to a BMI of 22.9 — inside the WHO's 18.5–24.9 "normal weight" band. That's the same three-step calculation — convert, square, divide — this calculator runs on your own numbers.

Common mistakes when reading your result

The most common misreading is treating BMI as a diagnosis rather than a screening number — the CDC is explicit that BMI is a screening tool, not a direct measure of body fat or a determination of individual health. A "normal" BMI doesn't guarantee good metabolic health, and an "overweight" BMI doesn't automatically mean poor health; both are starting points for a fuller conversation, not conclusions on their own. It's also easy to over-read small movements: because the formula only uses weight and height, day-to-day fluctuations from water retention or food volume can nudge the number without reflecting any real change in body composition.

When the standard formula doesn't fit well

BMI can't tell fat and muscle apart — a pound of muscle and a pound of fat weigh the same, so the formula treats them identically even though muscle is denser and takes up less space. This is why very muscular people and trained athletes often land in the "overweight" or "obese" range despite having low body fat; the CDC lists this directly as one of BMI's known limitations. The reverse also happens: older adults who have lost muscle mass with age can score "normal" while carrying a higher proportion of body fat than a younger person at the same BMI. Population matters too — a 2004 WHO expert consultation, published in The Lancet, found that Asian populations tend to face higher health risks at lower BMI values than the standard cutoffs suggest, and WHO added supplementary reference points (23.0, 27.5, 32.5, and 37.5 kg/m²) for public health use in those populations alongside the general classification.

What to do with your number

Use a single BMI reading as one input, not the whole picture. It's most useful tracked as a trend over months — alongside how your clothes fit and how you feel — rather than treated as a single precise figure. If your BMI is outside the normal range, or you have questions about what it means for you personally, that's a conversation for a doctor, who can weigh it against your muscle mass, waist circumference, family history, and overall health rather than the number in isolation.

BMI vs. body fat percentage

BMI and body fat percentage measure different things. BMI is a ratio of weight to height that's fast to calculate from two numbers anyone can measure at home; body fat percentage measures what portion of your weight is actually fat, typically via skinfold calipers, bioelectrical impedance scales, or a DEXA scan. BMI is a reasonable population-level screening tool precisely because it's so easy to obtain, but body fat percentage is the more direct measurement when muscle mass is unusually high or low — exactly the situation where BMI is least reliable, per the special cases above.