BMI Calculator — Body Mass Index for Adults

BMI is your weight divided by your height squared. In US units the formula is 703 × weight (lb) ÷ height (in)², and for adults the NHLBI puts the healthy range at 18.5 to 24.9. Below that is underweight; above it, overweight or obesity. Enter your height and weight:

The formula, worked out

Someone 5 ft 9 in and 180 lb: 5 ft 9 in is 69 inches, so BMI = 703 × 180 ÷ (69 × 69) = 26.6 — the overweight range. Someone 5 ft 5 in and 130 lb: 703 × 130 ÷ (65 × 65) = 21.6, inside the healthy range.

Now the part that surprises people. Keep the weight at 130 lb and drop the height to 5 ft 0 in: BMI = 703 × 130 ÷ (60 × 60) = 25.4, which is overweight. Same number on the scale, different category — because height is squared in the formula and weight is not. That single detail explains most of the “but I weigh the same as my friend” confusion.

If you think in kilograms and meters, the formula drops the 703: BMI = kg ÷ m². The metric BMI calculator does it that way.

Adult BMI categories

BMI	Category
Below 18.5	Underweight
18.5 – 24.9	Healthy weight
25.0 – 29.9	Overweight
30.0 and above	Obesity
40.0 and above	Severe obesity

Ranges as published by the NHLBI; the NIDDK uses the same cutoffs and is where the 40.0 line for severe obesity comes from. Note that severe obesity is a subset of obesity, not a separate band after it. The finer split of obesity into classes 1, 2 and 3 is common in clinical writing, but we only publish numbers we could read in an accessible official source, and the CDC page that carries that table blocks automated access — so it is not in the table above.

The full BMI chart gives the weight range for each height, if you would rather look it up than calculate.

What BMI is bad at

BMI was built in the 19th century by the Belgian statistician Adolphe Quetelet to describe populations, not individuals, and that origin still shows. It does not tell muscle from fat, so a lifter and a sedentary person of the same height and weight get the same number. It ignores where fat sits, and abdominal fat carries more cardiovascular risk than the same amount on the hips. It uses one formula for nearly every adult, which is a simplification by design.

It is also the wrong tool during pregnancy, when expected weight gain changes week by week and is tracked in prenatal care instead.

Read BMI as a first screen that tells you whether a closer look is worth it — never as a verdict. A number just past a line, 25.1 instead of 24.8, is inside the measurement error of a bathroom scale.

Measure before you conclude

Small measurement errors move BMI more than people expect, and they matter most right at a boundary:

• Weighing yourself dressed, or right after a meal, can add 1 to 3 lb. Morning, before breakfast, in light clothes, is the comparable condition.
• Measuring height without heels against the wall shortchanges you by half an inch to an inch — enough to shift BMI by a few tenths.
• Rounding before you calculate: turning 5 ft 8.5 in into “5 ft 9 in” changes the result by about a third of a point.

If your result landed within a few tenths of a boundary, measure again before drawing any conclusion from which side it fell on.

This content is informational and does not replace evaluation by a health professional.