How BMI is calculated
Updated 31 October 2026 · 8 minute read
Body mass index is a single number that compares weight with height. It was designed in the 19th century by Adolphe Quetelet, not as a diagnosis but as a way to describe populations. Today it is used as a first screening step because it costs nothing, needs only a scale and a tape measure, and is consistent between clinics.
This guide gives the formula our BMI calculator uses, explains the two sets of cut-offs, and — just as importantly — lists the cases where the number should not be trusted on its own.
The formula
BMI = weight (kg) ÷ height (m)²At 170 cm and 65 kg, height in metres is 1.70, so height squared is 2.89, and 65 ÷ 2.89 = 22.5.
In imperial units the same formula needs a conversion factor:
BMI = 703 × weight (lb) ÷ height (in)²Someone 5 ft 7 in (67 in) who weighs 143 lb: 703 × 143 = 100,529, and 67² = 4,489, so BMI = 22.4. The two methods agree to within rounding — there is no separate “imperial BMI”.
Because height is squared, small changes in height matter. A 2 cm error at adult height moves BMI by roughly 0.5, which is enough to cross a category boundary near 25.
Two sets of categories
The original WHO bands were built on data from predominantly European populations. Studies across Asia found that type 2 diabetes and cardiovascular disease appear at lower BMI values, so the WHO expert consultation proposed lower action points for Asian populations. Both sets are in use; neither replaces a clinical assessment.
| BMI | WHO international | WHO Asian |
|---|---|---|
| Below 18.5 | Underweight | Underweight |
| 18.5 – 22.9 | Healthy | Healthy |
| 23.0 – 24.9 | Healthy | Overweight |
| 25.0 – 27.4 | Overweight | Overweight |
| 27.5 – 29.9 | Overweight | Obese class I |
| 30.0 – 34.9 | Obese class I | Obese class I |
| 35.0 – 37.4 | Obese class II | Obese class II |
| 37.5 – 39.9 | Obese class II | Obese class III |
| 40.0 and above | Obese class III | Obese class III |
The Asian scale is not a stricter opinion; it is a different threshold for the same risk. A BMI of 24 is “healthy” on the international scale and “overweight” on the Asian one, and for a South Asian adult the second label better reflects the average risk.
Healthy weight range for your height
Turn the formula around and it gives a weight range: multiply 18.5 and the upper healthy limit by height squared. Two examples per height, in kilograms:
| Height | WHO international | WHO Asian |
|---|---|---|
| 150 cm | 41.6 – 56.0 kg | 41.6 – 51.5 kg |
| 160 cm | 47.4 – 63.7 kg | 47.4 – 58.6 kg |
| 170 cm | 53.5 – 72.0 kg | 53.5 – 66.2 kg |
| 175 cm | 56.7 – 76.3 kg | 56.7 – 70.1 kg |
| 180 cm | 59.9 – 80.7 kg | 59.9 – 74.2 kg |
| 190 cm | 66.8 – 89.9 kg | 66.8 – 82.7 kg |
The calculator computes the exact range for your own height and shows the distance to it in kilograms or pounds.
BMI prime
BMI prime is BMI divided by the upper limit of the healthy range:
BMI prime = BMI ÷ 25 (international) or BMI ÷ 23 (Asian)A prime below 1.00 means you are inside the healthy band. At 1.20 you are 20% above the upper limit; at 0.85 you are 15% below it. It is useful because it scales with the standard: someone with a BMI of 23 has a prime of 0.92 internationally but 1.00 on the Asian scale.
Where BMI misleads
- Muscle mass. Muscle is denser than fat. A trained athlete can have a BMI of 28 with low body fat, while a person with the same BMI and little muscle may carry a high fat percentage.
- Fat distribution. Fat around the waist and organs carries more risk than fat on the hips and thighs. A waist-to-height ratio above 0.5 is a simple warning sign that BMI alone would miss.
- Age. Adults over 65 often do better with a slightly higher BMI, and muscle loss can hide excess fat. Children and teenagers need BMI-for-age percentile charts, not adult bands.
- Pregnancy. BMI is not meaningful during pregnancy or breastfeeding.
- Ethnicity and body build. This is the reason the Asian cut-offs exist. The same number can mean different things in different populations.
- Medical conditions. Fluid retention, amputation, dwarfism and other conditions change the relationship between weight and body composition.
What to use alongside it
If BMI puts you outside the healthy band, the next step is more information, not a diet:
- Waist circumference — measured at the navel, relaxed. A waist-to-height ratio over 0.5 suggests excess central fat.
- Blood pressure, blood sugar and cholesterol — these matter more than BMI for cardiovascular risk.
- Trend over time — a BMI that has risen 3 points in a year is more informative than a snapshot.
- Body composition — a DEXA scan or bioimpedance measurement gives fat and muscle separately.
For most people, losing 5–10% of body weight already improves blood pressure, blood sugar and sleep quality. That is a practical target that does not require reaching a “perfect” BMI.
Adults only
From age 2 to 18, the healthy band changes every month because children grow at different rates. Paediatric charts use percentiles: below the 5th is underweight, 5th to 85th is healthy, 85th to 95th is overweight, and 95th or above is obese. An adult BMI number should not be applied to a child. Our calculator is for adults aged 18 and over.
Worked example
A person 170 cm tall, weighing 80 kg, using the Asian standard:
| Item | Result |
|---|---|
| Height squared | 2.89 m² |
| BMI | 27.7 |
| Category (Asian) | Obese class I |
| Healthy weight range | 53.5 – 66.2 kg |
| Distance to range | 13.8 kg |
| BMI prime | 1.20 |
The same person on the international scale would be “overweight” rather than obese, with a prime of 1.11 and a healthy range up to 72.0 kg. Both are descriptive labels, not decisions.