BMI Calculator
Work out your body mass index from your height and weight. Pick the units you think in — the calculation is the same either way, and switching keeps whatever you have already entered.
Calculate your BMI
Height
Weight
- Your BMI
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- Category
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- Healthy range
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- BMI Prime
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- Ponderal index
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BMI is a screening figure for adults, not a diagnosis, and it does not measure body fat. Talk to a doctor before acting on it.
How BMI is calculated
Body mass index divides your weight by the square of your height. The result is measured in kilograms per square metre, which is why the metric form of the calculation is the real one and every other version is a conversion of it.
From metric
BMI = kilograms ÷ (metres × metres)
From pounds and inches
BMI = 703 × pounds ÷ (inches × inches)
Someone 178 cm tall weighing 75 kg is 1.78 metres, and 1.78 × 1.78 is 3.1684. Dividing 75 by that gives a BMI of 23.7, which falls in the healthy band. The squaring is the part worth noticing: it means height affects the result far more than weight does, so a small error in your height moves the answer about twice as much as the same error in your weight.
The 703 in the imperial version is not a rounded-off version of the metric calculation — it is the unit conversion itself, folded into one number. This page converts your entry to centimetres and kilograms and works from those, so the figure you see is the metric result whichever boxes you typed in.
BMI categories
These are the World Health Organization cut-offs for adults, and the same ones the NHS and the CDC publish. The calculator above reports the first four; the obesity classes are here for reference.
| Category | BMI |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 to 24.9 |
| Overweight | 25.0 to 29.9 |
| Obesity, class I | 30.0 to 34.9 |
| Obesity, class II | 35.0 to 39.9 |
| Obesity, class III | 40.0 and above |
The figures ending in .9 are a rounding convention, not the real boundary. Each band runs up to but not including the next one's starting figure, so a BMI of exactly 25 is overweight rather than the top of the healthy range, and 24.96 is healthy even though it displays as 25.0 at one decimal place.
Healthy weight by height
The weight range that puts an adult of each height between BMI 18.5 and 25. Read across from your height rather than working backwards from a target BMI — the range widens with height, because it scales with the square.
| Height | Feet & inches | Healthy weight | In stone |
|---|---|---|---|
| 150 cm | 4′ 11.1″ | 41.6 – 56.3 kg | 6 st 7.8 lb – 8 st 12 lb |
| 152 cm | 4′ 11.8″ | 42.7 – 57.8 kg | 6 st 10.2 lb – 9 st 1.3 lb |
| 154 cm | 5′ 0.6″ | 43.9 – 59.3 kg | 6 st 12.7 lb – 9 st 4.7 lb |
| 156 cm | 5′ 1.4″ | 45 – 60.8 kg | 7 st 1.3 lb – 9 st 8.1 lb |
| 158 cm | 5′ 2.2″ | 46.2 – 62.4 kg | 7 st 3.8 lb – 9 st 11.6 lb |
| 160 cm | 5′ 3″ | 47.4 – 64 kg | 7 st 6.4 lb – 10 st 1.1 lb |
| 162 cm | 5′ 3.8″ | 48.6 – 65.6 kg | 7 st 9 lb – 10 st 4.6 lb |
| 164 cm | 5′ 4.6″ | 49.8 – 67.2 kg | 7 st 11.7 lb – 10 st 8.2 lb |
| 166 cm | 5′ 5.4″ | 51 – 68.9 kg | 8 st 0.4 lb – 10 st 11.9 lb |
| 168 cm | 5′ 6.1″ | 52.2 – 70.6 kg | 8 st 3.1 lb – 11 st 1.6 lb |
| 170 cm | 5′ 6.9″ | 53.5 – 72.2 kg | 8 st 5.9 lb – 11 st 5.3 lb |
| 172 cm | 5′ 7.7″ | 54.7 – 74 kg | 8 st 8.7 lb – 11 st 9.1 lb |
| 174 cm | 5′ 8.5″ | 56 – 75.7 kg | 8 st 11.5 lb – 11 st 12.9 lb |
| 176 cm | 5′ 9.3″ | 57.3 – 77.4 kg | 9 st 0.3 lb – 12 st 2.7 lb |
| 178 cm | 5′ 10.1″ | 58.6 – 79.2 kg | 9 st 3.2 lb – 12 st 6.6 lb |
| 180 cm | 5′ 10.9″ | 59.9 – 81 kg | 9 st 6.1 lb – 12 st 10.6 lb |
| 182 cm | 5′ 11.7″ | 61.3 – 82.8 kg | 9 st 9.1 lb – 13 st 0.6 lb |
| 184 cm | 6′ 0.4″ | 62.6 – 84.6 kg | 9 st 12.1 lb – 13 st 4.6 lb |
| 186 cm | 6′ 1.2″ | 64 – 86.5 kg | 10 st 1.1 lb – 13 st 8.7 lb |
| 188 cm | 6′ 2″ | 65.4 – 88.4 kg | 10 st 4.2 lb – 13 st 12.8 lb |
| 190 cm | 6′ 2.8″ | 66.8 – 90.3 kg | 10 st 7.2 lb – 14 st 3 lb |
| 192 cm | 6′ 3.6″ | 68.2 – 92.2 kg | 10 st 10.4 lb – 14 st 7.2 lb |
| 194 cm | 6′ 4.4″ | 69.6 – 94.1 kg | 10 st 13.5 lb – 14 st 11.4 lb |
| 196 cm | 6′ 5.2″ | 71.1 – 96 kg | 11 st 2.7 lb – 15 st 1.7 lb |
| 198 cm | 6′ 6″ | 72.5 – 98 kg | 11 st 5.9 lb – 15 st 6.1 lb |
| 200 cm | 6′ 6.7″ | 74 – 100 kg | 11 st 9.1 lb – 15 st 10.5 lb |
What BMI does not tell you
BMI was devised in the 1830s by Adolphe Quetelet as a way of describing populations, not individuals, and it has one input for your body: total mass. It cannot tell muscle from fat, or where on your body that mass sits.
Muscle is denser than fat, so a lean, heavily built athlete reads high. Someone 185 cm and 100 kg has a BMI of 29.2, which the table above calls overweight, whether that 100 kg is mostly muscle or not. The reverse happens too: it is possible to sit inside the healthy band with very little muscle and a high proportion of body fat.
The squaring is disputed for the same reason. Bodies do not scale as a square, so BMI reads high for tall people and low for short ones on build alone. The ponderal index in the results divides by height cubed instead, which holds steadier across heights — a 200 cm and a 150 cm adult of similar build get closer ponderal figures than BMI figures. It has no agreed categories, so it is a comparison rather than a verdict.
Where fat is carried matters more to health than how much a scale reports, and BMI is blind to it. Waist measurement, or waist compared against height, captures some of what BMI misses, which is why health services increasingly ask for both. BMI is a cheap first screen, useful for spotting who might benefit from a closer look — not an assessment of anyone's health on its own.
Frequently asked questions
- Does BMI work for children and teenagers?
- No, and using the adult categories on a child gives a misleading answer. Under 18, BMI is read against percentile charts for age and sex, because the healthy amount of body fat changes a great deal during growth. A BMI of 19 is squarely healthy for an adult, but for a seven-year-old the same figure sits near the overweight percentile. This calculator uses the adult thresholds only.
- Why does my result show 25.0 but say healthy weight?
- Because the category is worked out from the full value and only the display is rounded. A BMI of 24.97 shows as 25.0 at one decimal place, and 24.97 is genuinely below the 25 threshold. Raising the Decimals control above the result will show the figure the category was based on. Classifying the rounded number instead would mean your category could change when you changed a display setting, which would be worse.
- Are the thresholds the same everywhere in the world?
- Not in practice. A 2004 WHO expert consultation kept the international cut-offs but added lower public health action points of 23 and 27.5 for Asian populations, because the risk of type 2 diabetes and cardiovascular disease starts rising at a lower BMI. Japan treats 25 as the obesity threshold rather than the overweight one. This calculator uses the international figures, which is what most sources you will compare against also use.
- Where does the 703 in the imperial formula come from?
- It converts pounds per square inch into kilograms per square metre. A pound is 0.45359237 kg and an inch is 0.0254 m, and dividing the first by the square of the second gives 703.0696, which everyone rounds to 703. This calculator never uses it: entering feet, inches, stone or pounds converts those to centimetres and kilograms first, so the answer does not carry that rounding.
- Does BMI account for age or sex?
- No. The formula and the thresholds are identical for a 25-year-old woman and an 80-year-old man. At the same BMI, women typically carry more body fat than men, and body fat generally rises with age while muscle falls, so the same number can mean quite different things. Some research suggests a BMI slightly above the healthy band carries no extra risk in older adults.
Related tools
Need one measurement on its own? The height converter handles feet and inches to centimetres, and the weight converter does kilograms to stone and pounds. The full list of tools is on the homepage.