BMI
Calculate your Body Mass Index in metric or imperial units and see your category.
BMI (Body Mass Index) is a number that relates your weight to your height. It's an imperfect proxy for health, but a useful one: your doctor, your insurance form, or a gym has probably asked for it at some point.
Type in your height and weight. You'll get the number, the WHO category it falls into, and the weight range considered healthy for someone your height. The bottom of the page covers the cases where BMI breaks down (athletes, elderly, some ethnic groups).
How BMI is calculated
BMI is weight in kg divided by height in meters squared. The imperial version adds a conversion factor but the ratio underneath is the same.
The WHO groups the result into six categories: underweight (< 18.5), normal (18.5–24.9), overweight (25–29.9), obese class I (30–34.9), obese class II (35–39.9), obese class III (≥ 40).
A detail most people never notice: because height is squared in the denominator, taller people score slightly higher at the same proportions. It's a quirk of the formula, not a measurement problem.
BMI = weight (kg) / height (m)²
- weight
- Body weight in kilograms
- height
- Height in meters
Practical examples
Average adult
Setup: Height 1.75 m, weight 70 kg.
BMI = 70 / (1.75)² = 70 / 3.0625 ≈ 22.9
Takeaway: Falls comfortably in the normal range (18.5–24.9).
Athletic build
Setup: Height 1.80 m, weight 95 kg (muscular build).
BMI = 95 / (1.80)² ≈ 29.3
Takeaway: Classified as overweight, but BMI doesn't distinguish muscle from fat. A body composition scan would give a better picture for athletes.
Underweight
Setup: Height 1.65 m, weight 48 kg.
BMI = 48 / (1.65)² ≈ 17.6
Takeaway: Below the underweight threshold. Worth a conversation with a doctor to check for underlying causes.
How to interpret your number
- It's a screen, not a diagnosis. A BMI in the overweight bracket isn't proof that something is wrong. A BMI in the normal bracket isn't proof that everything is fine.
- Pair it with waist circumference. Abdominal fat correlates with cardiovascular risk more strongly than fat around the hips. A waist-to-height ratio under 0.5 is the usual healthy benchmark.
- Muscle throws off the number. BMI has no way to distinguish 80 kg of muscle from 80 kg of fat. Lifters and athletes often land in the overweight bracket with perfectly healthy body composition.
- Older adults get a wider range. Mortality data from studies of people over 65 tends to favor a BMI of 23–30, which would be labeled overweight for younger adults. The WHO thresholds have not been formally updated.
When BMI is misleading
BMI is not reliable for people under 18 (use age-and-sex-specific charts instead), pregnant or breastfeeding women, competitive athletes, bodybuilders, or people with significant muscle loss due to illness or aging.
It also has known accuracy issues across ethnic groups: some Asian populations show elevated cardiometabolic risk at lower BMI values, while some Pacific Islander populations show lower risk at higher values. Country-specific guidelines often adjust the thresholds.
For a fuller picture, pair BMI with waist circumference, blood pressure, cholesterol panel, and fasting glucose — not any single number in isolation.
Frequently asked questions
Is BMI accurate?▾
BMI correlates reasonably well with body-fat percentage for the general adult population, but it's imprecise at the individual level — especially for muscular, very tall, very short, or elderly people. Treat it as a starting point, not a verdict.
What's a healthy BMI?▾
The WHO defines 18.5–24.9 as the healthy range for most adults. Mortality data suggests the lowest-risk zone is roughly 20–25, but health is multi-factorial — BMI alone is not enough to judge.
Should children use the same categories?▾
No. For anyone under 18, pediatricians use age- and sex-adjusted BMI percentile charts. A 10-year-old and a 15-year-old with the same adult-BMI value can be in very different categories. This calculator is for adults.
Why does BMI classify muscular people as overweight?▾
Muscle is denser than fat. A person carrying 10 kg of extra muscle weighs more than a sedentary person of the same height but is often healthier. BMI can't tell the two apart. If you lift weights seriously, consider body-fat percentage instead.
Does BMI matter for longevity?▾
Large epidemiological studies show a U-shaped curve: both very low and very high BMI are associated with higher mortality. The flat bottom of the U covers roughly 20–28 for most populations. Movement, diet quality, sleep, and smoking status each have a larger effect than BMI within that range.
Sources
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