BMI Calculator

How to Use the BMI Calculator

Enter two numbers and read the result — there is nothing to submit and nothing is stored. The calculator updates as you type, so you can drag a slider and watch the category change in real time.

1. Pick your units. The metric side asks for centimetres and kilograms; the imperial side splits height into feet and inches and takes weight in pounds. Switching units keeps the calculation identical — only the labels change.
2. Enter your height. Measure standing straight against a wall with shoes off, heels together, looking straight ahead. Height matters more than weight here: it is squared in the formula, so a two-centimetre error moves BMI by roughly 0.6 points at average adult sizes.
3. Enter your weight. Weigh yourself at the same time of day, ideally before breakfast. Day-to-day swings of one to two kilograms are water, not fat, and they will move your BMI by about half a point.
4. Read the four supporting tiles. They show the healthy weight window for your exact height, how far outside it you are, your BMI prime, and the Asia-Pacific category, which uses lower cut-offs than the WHO international ones.
5. Open the weight table. It recalculates BMI at three steps either side of your current weight so you can see what a realistic change actually does.

The Share link button copies a URL with your height and weight already in it, which is useful if you are tracking numbers with a coach or a family member. Print / PDF produces a clean one-page sheet with the inputs, the result and nothing else — no navigation, no ads.

BMI Formula

Body mass index divides weight by the square of height. Adolphe Quetelet published the ratio in 1832 as a way to describe populations; Ancel Keys renamed it BMI in 1972 after finding it tracked body fatness better than the other simple indices available at the time.

BMI prime is the same number expressed as a fraction of the upper healthy limit. A prime of 1.00 sits exactly on 25; 1.20 means you are 20% above the top of the healthy band. It travels well between the WHO and Asia-Pacific scales because it is a ratio rather than a raw value.

Example

Two worked cases, both computed by hand so you can check the arithmetic against the calculator.

Metric: 72 kg at 1.68 m

1. Convert height to metres: 168 cm ÷ 100 = 1.68 m.
2. Square it: 1.68 × 1.68 = 2.8224 m².
3. Divide weight by that: 72 ÷ 2.8224 = 25.51, which rounds to 25.5.
4. Compare with the cut-offs: 25.5 is above 25, so this falls in the Overweight band — by half a point.
5. Healthy window for 1.68 m: 18.5 × 2.8224 = 52.2 kg and 24.9 × 2.8224 = 70.3 kg. Losing 1.7 kg would move this person back inside it.

Imperial: 185 lb at 5 ft 10 in

1. Convert height to inches: (5 × 12) + 10 = 70 in.
2. Square it: 70 × 70 = 4,900 in².
3. Multiply weight by the constant: 703 × 185 = 130,055.
4. Divide: 130,055 ÷ 4,900 = 26.54, rounding to 26.5 — also Overweight.
5. Healthy window for 70 in: (18.5 × 4,900) ÷ 703 = 128.9 lb up to (24.9 × 4,900) ÷ 703 = 173.6 lb, so this is 11.4 lb above the top.

The two cases land close together for a reason: 185 lb at 5 ft 10 in is 83.9 kg at 1.778 m, and 83.9 ÷ 1.778² = 26.5. The imperial formula is the metric one wearing a different hat.

BMI Categories and What They Mean

The World Health Organization's international cut-offs are the ones most clinics print on a chart. They apply to adults of 20 and over, and they are the same for men and women.

The WHO also publishes lower thresholds for many Asian populations, because cardiometabolic risk tends to appear at a lower BMI. On that Asia-Pacific scale the healthy band ends at 22.9, overweight runs from 23.0 to 24.9, and obesity starts at 25.0. Someone at 1.72 m and 69.2 kg has a BMI of 23.4: comfortably healthy on the international chart, already overweight on the Asia-Pacific one. That is why the calculator shows both.

Children and teenagers are a separate case entirely. Their BMI is compared against age-and-sex percentile charts rather than fixed numbers, because a healthy 12-year-old and a healthy 17-year-old have very different body compositions. Use a paediatric growth chart for anyone under 20, not the table above.

Where BMI Falls Down — and What to Use Instead

BMI is a screening ratio, not a measurement of body fat. It cannot tell muscle from fat, it ignores where fat sits, and it was built from population data rather than individual physiology. Four situations where it misleads:

• Muscular builds. A rugby forward or a competitive powerlifter can sit at BMI 30 with body fat in the low teens. Muscle is denser than fat, so the numerator climbs while the denominator does not.
• Older adults. After roughly 60, people lose muscle and bone while gaining fat. BMI can stay flat while body composition worsens — the number hides the change.
• Very tall and very short people. Squaring height slightly over-corrects for tall frames and under-corrects for short ones, so BMI reads a little high for short adults and a little low for tall ones.
• Pregnancy and recent illness. Fluid retention, oedema and rapid weight change all move BMI without saying anything useful about health.

Two cheap measurements fix most of this. Waist circumference — measured at the midpoint between the lowest rib and the top of the hip bone — is a better single predictor of metabolic risk than BMI, with common warning thresholds around 94 cm for men and 80 cm for women. Waist-to-height ratio is even simpler: keep your waist under half your height. Pair either with BMI and you get a far more honest picture than BMI alone.

The most common mistakes we see are entering height in centimetres while the metric toggle expects metres elsewhere, weighing at wildly different times of day, and treating a one-point move as progress. BMI is a slow number. Compare it month to month, not morning to morning, and use the body fat calculator alongside it if you want a composition estimate rather than a weight-for-height ratio.