How to Use a BMI Calculator — and What the Number Cannot Tell You
2026-07-20
BMI is everywhere — insurance forms, gym intake sheets, health apps — and it is one number derived from two measurements. That simplicity is why it spread, and also why it gets over-interpreted.
Using it well means knowing what it was built to do, and what it was never built to do.
The formula
BMI is your weight in kilograms divided by your height in metres squared:
BMI = kg ÷ m²
Someone 1.75 m tall weighing 70 kg: 70 ÷ (1.75 × 1.75) = 22.9.
In pounds and inches, multiply by 703: (lb ÷ in²) × 703.
You can run it in the BMI Calculator, which handles both unit systems.
The standard categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal range |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
These cut-offs come from population-level research linking BMI to health outcomes across large groups. That origin matters, and it is where most misreading starts.
What BMI actually is
It is a screening tool for populations, not a diagnostic test for individuals.
It was designed to answer questions like "how is the distribution of weight-related risk shifting across this country?" — where averaging across millions of people smooths out individual variation. Applied to one person, that smoothing is exactly what goes missing.
The formula only knows two things: your height and your weight. It does not know what the weight is made of, where it sits on your body, how old you are, or how fit you are. Those are the things that actually drive health outcomes.
Where the number misleads
Muscular people. Muscle is denser than fat. A rugby player or a serious lifter routinely lands in the "overweight" or even "obese" band while carrying very little body fat. The BMI is arithmetically correct and clinically meaningless.
Older adults. People tend to lose muscle mass with age while gaining fat. Someone can hold the same BMI at 70 as at 30 with a substantially different body composition. There is also consistent evidence that a slightly higher BMI is associated with better outcomes in older adults, which the standard categories do not reflect.
Different ancestries. Health risk does not appear at the same BMI across populations. The WHO has noted that people of South Asian, Chinese and other Asian descent tend to develop metabolic risk at lower BMIs, and several countries use lowered thresholds — often around 23 for overweight — rather than the standard 25.
Where the fat sits. Fat around the abdomen carries considerably more metabolic risk than fat on the hips and thighs. Two people with identical BMIs can have meaningfully different risk profiles. This is why waist circumference, or waist-to-height ratio, is often more informative — a common rule of thumb is keeping your waist under half your height.
It does not apply to children. Children's BMI is read against age-and-sex percentile charts, not adult categories. The adult bands are simply wrong for anyone still growing.
It does not apply during pregnancy. Weight gain is expected and BMI categories are not meaningful.
Using it sensibly
Treat BMI as one rough indicator among several, useful mainly for noticing direction of change over time rather than for judging a single moment.
If your number sits well inside the normal range and you feel well, it is not telling you much you did not know. If it sits near a boundary, the boundary itself is not a cliff edge — the underlying risk changes gradually, not at 24.9.
If it sits well outside the range, that is worth a conversation with a doctor rather than a conclusion drawn from a calculator. A clinician can look at waist measurement, blood pressure, blood glucose and lipids — the things BMI is standing in for, measured directly.
And if you are athletic and the number looks alarming, it is very likely measuring your muscle.
Related figures
BMR — your basal metabolic rate, the energy your body uses at complete rest. It answers a different question from BMI and is calculated from height, weight, age and sex. See BMR Calculator.
Neither number is a verdict on anyone. They are inputs, and fairly crude ones. Calculations here run entirely in your browser, so nothing you enter about your body is sent anywhere or stored.