BMI Calculator
Calculate body mass index from your height and weight.
Unit: cm (e.g. 170)
Unit: kg
BMI calculator
Computes body mass index from height and weight.
Uses BMI = weight(kg) ÷ height(m)² to show whether you fall in underweight, normal, overweight, or obese (WHO ranges).
- WHO: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30+ obese.E.g. 170 cm, 70 kg → BMI ≈ 24.2 (normal range).
- BMI (body mass index)
- BMI is weight in kg divided by height in metres squared. It is a simple screening tool for weight category but does not reflect muscle mass or body shape.
- WHO weight categories
- WHO classifies BMI ≥ 25 as overweight and ≥ 30 as obese. Some Asian guidelines use lower cut-offs (e.g. 23 for overweight, 25 for obese).
Understanding BMI
BMI estimates whether your weight sits in a healthy range using only height and weight. The arithmetic is trivial; reading the result correctly is where most people go wrong.
The formula
BMI = weight in kilograms divided by height in metres, squared. Height goes in as metres, not centimetres, and it is the height that gets squared, not the weight.
For someone 170 cm tall, convert to 1.7 m, then divide by 1.7 × 1.7 = 2.89. Mixing up those two steps is the most common mistake when calculating by hand.
Worked examples
170 cm · 70 kg → 70 ÷ 2.89 = 24.2
160 cm · 50 kg → 50 ÷ 2.56 = 19.5
180 cm · 95 kg → 95 ÷ 3.24 = 29.3
Take the first example again: the same 70 kg gives a BMI of 27.3 at 160 cm. BMI measures weight relative to height, not weight on its own.
Reading the number
The WHO classifies BMI below 18.5 as underweight, 18.5 to 24.9 as normal, 25 to 29.9 as overweight, and 30 or above as obese. Obesity is further split into class I (30–34.9), class II (35–39.9) and class III (40 and above).
These are international cutoffs. Several countries in Asia use lower thresholds, treating 23 as the start of the at-risk range and 25 as obese, because metabolic risk rises at a lower BMI in East and South Asian populations. If a local health check gave you a different category than this page, that is usually why.
What BMI cannot tell you
BMI does not distinguish muscle from fat. A trained athlete can land in the obese range with very low body fat, while someone with little muscle and high body fat can sit squarely in the normal range.
It also ignores where fat is stored. At an identical BMI, weight carried around the abdomen carries more metabolic risk than weight carried on the hips and thighs.
And it does not transfer cleanly across life stages. Children and teenagers need age- and sex-specific percentiles rather than adult cutoffs, and the standard ranges are a poor fit during pregnancy or for older adults who have lost muscle mass.
Measurements worth pairing it with
Waist circumference is the simplest proxy for visceral fat. A common threshold is 94 cm for men and 80 cm for women, with higher risk above 102 cm and 88 cm respectively.
Waist-to-height ratio is another quick check: keeping your waist under half your height is the usual guidance, and it works across a wide range of body sizes.
If your BMI sits near a boundary, the number on its own is not worth agonising over. Waist measurement, body composition, and blood pressure, glucose and lipid readings together describe your health far better than a single ratio.
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese class I |
| 35.0 – 39.9 | Obese class II |
| 40.0 and above | Obese class III |
Frequently asked questions
- Why do sources disagree about the normal range?
- They apply different cutoffs. The WHO international standard treats anything below 25 as normal, while several Asian guidelines set the at-risk boundary at 23, reflecting evidence that metabolic risk rises at a lower BMI in those populations.
- Does muscle make my BMI higher?
- Yes. Muscle is denser than fat, so a muscular build produces a higher BMI even at low body fat. If you strength train regularly, body fat percentage or waist measurement will describe you better than BMI.
- Does BMI work the same way for children?
- No. Healthy ranges shift with age and sex during growth, so children and adolescents are assessed with BMI-for-age percentiles instead of adult cutoffs. Above the 85th percentile is generally considered overweight and above the 95th obese.
- My BMI is normal but I carry weight around my middle. Is that fine?
- Not necessarily. A normal BMI with high visceral fat still raises metabolic risk, a pattern sometimes called normal-weight obesity. Checking your waist circumference alongside BMI gives a fuller picture.
- How much weight changes BMI by one point?
- It equals your height in metres, squared. At 170 cm that is about 2.9 kg per BMI point; at 180 cm it is about 3.2 kg. Gradual loss of 5–10% of current body weight over six months is the usual recommendation rather than rapid change.
- Is this a diagnosis?
- No. BMI is a screening indicator, not a diagnostic tool. If your result falls outside the normal range or you have health concerns, speak with a healthcare professional.