BMI Calculator

Worked example

165 lbs at 5'11" is a BMI of 23.0, inside the 18.5–24.9 healthy band. The band itself spans 133–178 lbs at that height, which is a 45 lb window — BMI is a screening tool, not a target weight.

How to work it out yourself

1. 1.Weigh yourself first thing in the morning, and measure height without shoes.
2. 2.Convert to metric: kilograms and meters. 1 lb is 0.4536 kg and 1 in is 2.54 cm.
3. 3.Divide the weight in kilograms by your height in meters squared.
4. 4.Compare against the CDC bands — under 18.5, 18.5–24.9, 25–29.9, 30 and above — and treat the result as a screen, not a target.

BMI at 5 ft 9 in by weight

At a fixed height of 175 cm. Roughly 7 lb moves BMI by one point at this height.

The formula

1. Body mass index83.914588 / pow(1.8034, 2)
2. Healthy-range weight, low end18.5 * pow(1.8034, 2)
3. Healthy-range weight, high end24.9 * pow(1.8034, 2)
4. BMI Prime — your BMI against the upper healthy limit25.801998 / 25
5. Ponderal index83.914588 / pow(1.8034, 3)
6. Weight worth one BMI point at your heightpow(1.8034, 2)
7. Weight where the overweight band starts25 * pow(1.8034, 2)
8. Weight where the obese band starts30 * pow(1.8034, 2)
9. Weight between you and the nearest edge of the healthy band83.914588 - 80.981064

Source: CDC — about adult BMI, NHLBI — body mass index and health risk, WHO — obesity and overweight, and the BMI classification

Questions people actually ask

What are the categories?

CDC adult cut-offs: under 18.5 underweight, 18.5–24.9 healthy, 25.0–29.9 overweight, 30.0 and above obese. They apply to adults 20 and older; children use age- and sex-specific percentiles instead.

Why does BMI misclassify athletes?

It uses total mass and cannot distinguish muscle from fat. A 6'0" 220 lb athlete at 10% body fat scores 29.8 — "overweight" — on the same number as a sedentary person at 30% body fat.

What is BMI Prime?

Your BMI divided by 25, the top of the healthy band. It turns the index into a plain ratio: 1.00 is exactly at the upper limit, 0.90 is 10% under it, 1.20 is 20% over. The same information as BMI, in the form people actually want it — how far from the line am I.

What is the ponderal index?

Weight divided by height cubed rather than squared. BMI’s squared term is an approximation that reads high for tall people and low for short ones; the ponderal index scales with volume instead, which is why paediatricians and studies of very tall or very short adults prefer it. Typical adult values run 11 to 15 kg/m³.

Does BMI work for children?

Not with these cut-offs. Children and teenagers aged 2 to 19 are read against age- and sex-specific percentile curves, because body composition changes through growth — the 85th percentile is where "overweight" begins, not a fixed number.

What should I use instead?

Waist circumference and waist-to-height ratio track metabolic risk more closely. A waist under half your height is the usual screening threshold, and it captures the visceral fat BMI is blind to.

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Worked out already

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