How to Calculate BMI — Formula, Ranges & What the Number Means
What is BMI?
BMI (Body Mass Index) is a screening number that relates your weight to your height. It was designed as a quick population-level tool to flag underweight, healthy, overweight and obese weight categories — not as a measure of body fat or health on its own.
The BMI formula
Imperial: BMI = 703 × weight (lb) ÷ height (in)²
Worked examples
Metric — 70 kg at 1.75 m
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 → healthy weight range
Imperial — 154 lb at 5 ft 9 in (69 in)
BMI = 703 × 154 ÷ (69 × 69) = 703 × 154 ÷ 4761 = 108,262 ÷ 4761 ≈ 22.7 → healthy weight range
BMI ranges (WHO classification)
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal / healthy weight |
| 25 – 29.9 | Overweight |
| 30 and above | Obese |
The honest limitations of BMI
- Muscle vs fat: BMI cannot tell them apart, so muscular athletes often score "overweight" despite low body fat.
- Age and sex: the same BMI can mean different things for different bodies.
- Distribution matters: where fat sits (e.g. waist circumference) is often more predictive of risk than BMI alone.
- It is a screen, not a diagnosis: BMI is a starting point — talk to a healthcare professional for a real assessment.
Working the formula backwards: your healthy weight range
The most useful way to use BMI is in reverse — instead of scoring your current weight, compute the weight range your height implies. Rearranged: healthy weight = BMI × height², using the range boundaries. For the 1.75 m example above:
Lower bound: 18.5 × 3.0625 = 56.7 kg · Upper bound: 24.9 × 3.0625 = 76.3 kg
So 1.75 m corresponds to a healthy range of roughly 57–76 kg — a 19 kg spread, which is why a single "target weight" is less meaningful than the range.
The same rearrangement answers "what would I weigh at BMI 22?": 22 × 3.0625 ≈ 67.4 kg. This is exactly how the ideal weight calculator works, except it draws from several published formulas that often disagree with each other — the disagreement itself is informative.
Two refinements worth knowing
- BMI Prime — your BMI divided by 25 (the upper healthy limit). The 70 kg example scores 22.9 ÷ 25 = 0.92; anything above 1.00 is by definition outside the WHO healthy band. It's the same information in a friendlier scale: how far above or below the ceiling you sit.
- Adjusted cut-offs for Asian populations. Several health authorities apply lower thresholds for some Asian populations — commonly overweight from 23 and obesity from 27.5 — following WHO expert consultations that found elevated health risk at lower BMIs in those groups. Check which standard your own health provider uses; the number 24.9 is not a universal constant.
One more formula exists for extremes of height: the Ponderal index (mass ÷ height³) scales differently and is sometimes preferred for neonates and very tall or short adults — the 70 kg / 1.75 m example scores 13.06. For most adults BMI remains the standard first screen; the alternatives exist because height² is an approximation, not a law.
Try it instantly
Calculate your BMI with the BMI Calculator — it works in metric or imperial and shows your category. Related tools:
Worked example: Ideal Body Weight at its default inputs
Open Ideal Body Weight and press calculate without editing a field. What comes back is the table and result below, so the bmi numbers here are checkable in one click.
| Input | Default |
|---|---|
| Height (cm) | 175 |
Result: ~70.5 kg
The steps behind that answer
- Height in inches: 68.9
- Hamwi: 72.0 kg | Devine: 70.5 kg
- Robinson: 68.9 kg | Average ≈ 70.5 kg
Sensitivity: Height (cm) from half to one-and-a-half times
Hold every other field still and move only height (cm).
| Height (cm) | Result |
|---|---|
| 87.5 | ~-8.8 kg |
| 175 | ~70.5 kg |
| 262.5 | ~149.7 kg |
The middle row is the default. The outer two show how far the answer drifts when height (cm) is wrong by half or by a half in the other direction.
Related guides
BMI screens populations, not people
BMI is weight divided by height squared, and its cut-offs come from population mortality curves, not from an individual risk test. That is why a muscular athlete lands in "overweight" while an older adult with little muscle can read "normal" while carrying a high body-fat percentage.
Read it as the first question, not the answer: it tells you whether height and weight are proportionate, and nothing about where the weight sits or what it is made of.
The cut-offs are not the same everywhere
The World Health Organization puts overweight at 25 and obesity at 30. Many Asian-Pacific guidelines use 23 and 27.5, because cardiometabolic risk rises at a lower BMI in South and East Asian populations. The same body can be labelled differently by two clinics using two published tables.
If you are comparing your number to a chart, check which table the chart uses. A 24 is "normal" on one and "overweight" on the other, and neither is wrong for its own population.
Pair it with a waist measurement
Waist circumference catches the risk BMI misses: fat around the organs behaves differently from fat under the skin. Waist-to-height ratio is the simplest version of that check — keep your waist under about half your height.
A trend over months is more useful than any single reading. Weight fluctuates one to two kilograms a day with food, salt and hydration, so compare weekly averages rather than individual mornings.
Frequently asked questions
What is the BMI formula?
Metric: BMI = weight (kg) ÷ height (m)². Imperial: BMI = 703 × weight (lb) ÷ height (in)².
What are the BMI ranges?
Below 18.5 is underweight, 18.5–24.9 is normal/healthy weight, 25–29.9 is overweight, and 30 or above is obese, according to the WHO classification used by most health authorities.
Is BMI accurate for athletes?
No. BMI cannot distinguish muscle from fat, so muscular athletes often score as "overweight". It is a population-level screening tool, not a body-composition measurement.
What is a healthy BMI?
The commonly used healthy range is 18.5 to 24.9. Ranges can differ slightly for children (percentile-based) and some regional guidelines.
Last updated