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BMI Calculator
Body mass index in metric or imperial with the category shown.
What it does. BMI is weight in kilograms divided by height in meters squared. It is a population-level screening measure, not a diagnosis. This calculator works in metric or imperial, reports the WHO category, and shows the weight range that would put you in the healthy band for your height.
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How to use BMI Calculator
- Switch between metric (kg, cm) and imperial (lb, ft/in).
- Enter your height and weight.
- Read the BMI value, its category, and the healthy weight range for your height.
How is BMI calculated?
Metric: BMI = kg ÷ m². Someone 1.75 m and 70 kg has a BMI of 70 ÷ 3.0625 = 22.9.
Imperial: BMI = (lb ÷ in²) × 703. Someone 5'9" (69 inches) and 154 lb has 154 ÷ 4761 × 703 = 22.7. The 703 factor converts between the unit systems.
The formula dates to the 1830s, when Adolphe Quetelet devised it as a way to describe the distribution of body types across a population. It was never intended as an individual diagnostic, and was only renamed "body mass index" and adopted for clinical screening in 1972.
| Category | BMI range | Notes |
|---|---|---|
| Underweight | Below 18.5 | Increased risk from low body mass |
| Healthy weight | 18.5 – 24.9 | WHO reference range |
| Overweight | 25.0 – 29.9 | Elevated population-level risk |
| Obesity class I | 30.0 – 34.9 | |
| Obesity class II | 35.0 – 39.9 | |
| Obesity class III | 40.0 and above | Formerly "morbid obesity" |
What BMI does not measure
BMI knows your height and your weight. It does not know anything else about you, and four specific blind spots matter.
Body composition. Muscle is denser than fat, so a muscular person can register as overweight or obese while carrying very little fat. This is well documented in athletes — a large share of professional rugby and NFL players have a BMI over 30.
Fat distribution. Visceral fat around the organs carries substantially more metabolic risk than subcutaneous fat on the hips and thighs. Two people with identical BMI can have very different risk profiles. Waist-to-height ratio captures this better, and the rule of thumb — keep your waist under half your height — is simple and better evidenced than BMI for cardiovascular risk.
Ethnicity. Health risks associated with a given BMI appear at lower thresholds in South Asian, Chinese and other Asian populations. NICE guidance in the UK recommends using 23 rather than 25 as the overweight threshold for these groups.
Age and sex. The standard adult ranges do not apply to children, who are assessed on age-and-sex percentiles, or to adults over 65, where a slightly higher BMI is associated with better outcomes.
What should I use instead?
BMI is a reasonable first filter and a poor final answer. Three measures add more than they cost.
Waist circumference is the single most useful addition. Above 94 cm (37 in) for men or 80 cm (31.5 in) for women indicates increased risk; above 102 cm (40 in) and 88 cm (34.6 in) indicates substantially increased risk. These are the thresholds used by the NHS and the US CDC.
Waist-to-height ratio is simpler still and needs no tables: keep your waist measurement below half your height. It performs better than BMI as a predictor of cardiometabolic risk across every ethnic group studied.
Body fat percentage, measured by DEXA, hydrostatic weighing or reasonable-quality bioimpedance, is the direct measurement BMI is proxying for. It is more expensive and less available, which is the entire reason BMI persists.
Frequently asked questions
What is a healthy BMI?
18.5 to 24.9 by the WHO reference range. NICE recommends 18.5 to 22.9 for people of South Asian, Chinese, Black African and African-Caribbean family background, because risk appears at lower thresholds.
Is BMI accurate for athletes?
No. It cannot distinguish muscle from fat, so muscular people routinely register as overweight or obese despite very low body fat.
Does BMI work for children?
Not with adult ranges. Children are assessed against age-and-sex-specific percentile charts, because body composition changes substantially through growth.
Why is my BMI high when I do not look overweight?
Most often muscle mass, or a build with a large frame or short limbs relative to height. Check waist circumference, which correlates far better with actual health risk.
Should I use BMI at all?
As a rough screening number, yes. As a diagnosis or a target, no. Waist-to-height ratio is a better single measure and needs nothing but a tape measure.
Is BMI different for men and women?
The formula and the categories are identical, though women naturally carry a higher body fat percentage at the same BMI. Waist thresholds are sex-specific for this reason.
Are my measurements sent anywhere?
No. The calculation runs entirely in this page. Health information is not transmitted or stored.
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