Your BMI
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Enter your height and weight to see your result.
How BMI is calculated
Body mass index is your weight divided by the square of your height. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet as a way of describing populations, not individuals — which is worth knowing, because it explains most of what the number can and cannot do.
BMI = weight (kg) ÷ height (m)²
Divide your weight in kilograms by your height in metres, then divide by your height in metres again.
Someone who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. In imperial units the same calculation needs a conversion factor:
BMI = (weight (lb) ÷ height (in)²) × 703
A 154 lb adult who is 69 in tall: (154 ÷ 4,761) × 703 = 22.7.
The formula is identical for men and women, and for every adult age group. Only the interpretation of the result changes.
BMI categories for adults
The World Health Organization’s international cut-offs are the ones most countries publish, and the ones the calculator uses by default.
| Category | BMI |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25 – 29.9 |
| Obesity | 30 and above |
Obesity is further split into class I (30 – 34.9), class II (35 – 39.9) and class III (40 and above). The calculator names the class in its result, because the health implications of a BMI of 31 and one of 45 are not comparable.
A 2004 WHO expert consultation concluded that these cut-offs understate risk in many Asian populations, which show raised rates of type 2 diabetes and cardiovascular disease at a lower BMI — driven partly by a tendency to carry more visceral fat at the same weight. It recommended additional public-health action points rather than a wholesale relabelling:
| Category | BMI |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 22.9 |
| Increased risk | 23 – 27.4 |
| High risk | 27.5 and above |
Switch the population standard above the result to use these thresholds. Several countries, among them Singapore, Japan, China and India, have adopted cut-offs in this range.
Healthy weight range by height
Working the formula backwards gives the weight range that puts a given height in the healthy band under the WHO standard. It is usually the more useful number, because it is actionable in a way a single index value is not.
| Height (cm) | Height (ft/in) | Healthy weight (kg) | Healthy weight (lb) |
|---|---|---|---|
| 145 | 4' 9" | 38.9 – 52.5 | 86 – 115 |
| 150 | 4' 11" | 41.7 – 56.2 | 92 – 124 |
| 155 | 5' 1" | 44.5 – 60 | 98 – 132 |
| 160 | 5' 3" | 47.4 – 63.9 | 105 – 141 |
| 165 | 5' 5" | 50.4 – 68 | 112 – 150 |
| 170 | 5' 7" | 53.5 – 72.2 | 118 – 159 |
| 175 | 5' 9" | 56.7 – 76.5 | 125 – 168 |
| 180 | 5' 11" | 60 – 80.9 | 133 – 178 |
| 185 | 6' 1" | 63.4 – 85.5 | 140 – 188 |
| 190 | 6' 3" | 66.8 – 90.2 | 148 – 198 |
| 195 | 6' 5" | 70.4 – 95 | 156 – 209 |
Enter your own height in the calculator above to see this range for you — it appears as soon as the height field is filled in, before you enter a weight.
What BMI doesn’t tell you
BMI knows two things about you: how tall you are and how much you weigh. Everything it appears to say about your health is inferred from those two numbers, and the inference holds up far better across a population than it does for any one person standing on a set of scales.
- It cannot tell muscle from fat. Muscle is denser than fat, so a lean, heavily trained athlete can land in the “obese” range while carrying single-digit body fat.
- It ignores where fat sits. Fat around the abdomen carries substantially more cardiometabolic risk than the same amount on the hips and thighs. BMI treats them identically.
- It doesn’t account for age. Adults lose muscle and bone density as they get older, so an unchanged BMI at 70 usually represents more body fat than the same BMI did at 30.
- It says nothing about health. Blood pressure, blood glucose, lipids and fitness are all measured directly and all predict outcomes better than BMI does.
Who BMI doesn’t apply to
- Children and teenagers. BMI under 20 is read against age- and sex-specific growth percentiles, not fixed cut-offs, because healthy body composition changes throughout growth.
- Pregnant and breastfeeding people. Weight gain is expected and the categories do not apply. Pre-pregnancy BMI is what clinicians use.
- Serious strength and power athletes. A body fat estimate or a skinfold assessment will describe them far more accurately.
- People with very short or very tall stature. Squaring the height systematically overstates BMI for short adults and understates it for tall ones, because human bodies do not scale as perfect cubes.
What to measure alongside BMI
BMI is most useful as one number among several. Three others are easy to collect and add real information:
- Waist-to-height ratio. Keep your waist measurement to less than half your height. It is a simple proxy for abdominal fat and needs nothing but a tape measure.
- Waist circumference. Risk rises above roughly 94 cm (37 in) for men and 80 cm (31.5 in) for women, with lower thresholds again for South Asian populations.
- Body fat percentage. It answers the question BMI is standing in for, and separates a heavy, muscular build from a heavy, fat-carrying one.
If your BMI falls outside the healthy range, treat it as a reason to look closer rather than as a conclusion. A doctor can read it alongside your blood pressure, blood work and history — which is the only context in which the number means much at all.
Frequently Asked Questions
What is a healthy BMI?
Under the WHO's international cut-offs, a BMI between 18.5 and 24.9 is classed as a healthy weight for adults. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obesity. The WHO also recommends lower action points of 23 and 27.5 for Asian populations, which you can switch to above.
How accurate is BMI?
BMI is a screening tool, not a diagnosis. It is a good indicator of health risk across large groups, which is why public-health bodies use it, but it can be wrong about any individual person because it only knows your height and weight. It cannot tell muscle from fat, and it says nothing about where body fat sits — and abdominal fat carries more cardiometabolic risk than fat elsewhere.
Is BMI accurate for athletes and muscular people?
Often not. Muscle is denser than fat, so a well-trained athlete can carry very little body fat and still land in the overweight or obese range by BMI alone. If you strength train seriously, a body fat percentage estimate or a waist measurement will tell you more than BMI will.
Why are the BMI cut-offs different for Asian populations?
A 2004 WHO expert consultation found that many Asian populations show raised risk of type 2 diabetes and cardiovascular disease at a lower BMI than the international cut-offs assume. It recommended additional public-health action points at 23 and 27.5 rather than 25 and 30. Several countries, including Singapore, China, Japan and India, use thresholds in this range.
Can I use this calculator for children?
No. This calculator applies to adults aged 20 and over. For children and teenagers, BMI is interpreted against age- and sex-specific growth percentiles rather than fixed cut-offs, because healthy body composition changes throughout growth. Ask a paediatrician or use a dedicated BMI-for-age percentile chart.
Does BMI work differently for men and women?
The formula and the adult cut-offs are the same for both. Women typically carry a higher proportion of body fat than men at the same BMI, which is one of the reasons BMI alone is a blunt measure of body composition — but the classification thresholds are not adjusted by sex.
How do I calculate BMI by hand?
In metric units, divide your weight in kilograms by your height in metres squared: 70 kg at 1.75 m is 70 / (1.75 x 1.75) = 22.9. In imperial units, divide your weight in pounds by your height in inches squared and multiply by 703: 154 lb at 69 in is (154 / (69 x 69)) x 703 = 22.7.
What should I do if my BMI is outside the healthy range?
Treat it as a prompt to look further, not as a verdict. A waist measurement, blood pressure, blood glucose and a lipid panel say far more about your health than BMI does. Bring the number to a doctor or dietitian, who can interpret it alongside your medical history rather than in isolation.
References
- World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894, 2000.
- WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. The Lancet, 2004; 363(9403):157-163.
- Centers for Disease Control and Prevention. About Adult BMI. National Center for Chronic Disease Prevention and Health Promotion.
- National Heart, Lung, and Blood Institute. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, 1998.
- Nuttall FQ. Body Mass Index: Obesity, BMI, and Health — A Critical Review. Nutrition Today, 2015; 50(3):117-128.