BMI Calculator

A screening number, not a verdict

Body mass index is a century-old ratio of weight to height. Clinics and insurers still use it because it is cheap, repeatable, and correlated with some population-level risks. It is also blunt. This BMI calculator gives you the same number a nurse’s office would compute from a stadiometer and scale, plus the usual adult category labels, with a clear disclaimer that the label is not a medical exam.

You can enter metric or imperial units. The math is converted to a single BMI so 170 cm / 68 kg and the equivalent feet-pounds land in the same place. Nothing is uploaded. If you are tracking a change over time, write the number down yourself; KindCalc does not keep a history.

The formula

In metric units, BMI = kg ÷ (m × m). Height must be in meters, so 170 cm is 1.70 m. 68 ÷ (1.70 × 1.70) = 23.5. In imperial units, BMI = 703 × lb ÷ (in × in). The 703 constant converts the pound-inch system to the same scale as the metric definition. 5 ft 10 in is 70 inches; 185 lb gives 703 × 185 ÷ 4,900 ≈ 26.5.

Adult categories used here follow common WHO / CDC style cutoffs: below 18.5 underweight, 18.5 to 24.9 healthy weight, 25 to 29.9 overweight, and 30 or higher obesity. Some organizations split obesity into classes. This page stays with the four-band version so the result stays readable.

How to measure inputs that are not lying

Weigh yourself at a consistent time, ideally in the morning, without heavy clothes. Height should be current; people lose height with age and the formula is sensitive to small errors because height is squared. A one-inch mistake on a 5'7" adult moves BMI by roughly a point.

Do not use this tool for pregnancy, under-18s, or as a substitute for eating-disorder care. If numbers on a scale are a trigger, skip this page. A clinician can interpret weight in context with blood pressure, lipids, glucose, medications, and how you actually feel.

Examples

170 cm and 68 kg: BMI 23.5, healthy range. 160 cm and 50 kg: BMI 19.5, still healthy range but closer to the lower bound. 5 ft 10 in and 185 lb: BMI 26.5, overweight range on the adult chart. Two of those people could have identical bloodwork; one of them could be a powerlifter.

If you only have stones and pounds, convert weight to pounds first, or use the unit converter on KindCalc and then return here.

What to do with the result

Treat BMI as one snapshot. If you are changing habits, pair it with measurements that match your goal: strength, walk time, waist, sleep, or labs your clinician ordered. A falling BMI from illness is not a success story. A rising BMI from added muscle after a training block may be fine.

This site is not a clinic. The calculator exists because “BMI calculator” is a practical query and the formula should be transparent. For advice about your body, talk to a licensed professional who can see more than two numbers.

Typical examples

Frequently asked questions

How is BMI calculated?

Metric BMI is weight in kilograms divided by height in meters squared. Imperial BMI is 703 × weight in pounds divided by height in inches squared. Both describe the same ratio.

What are the standard BMI categories?

Adult WHO-style bands commonly used in the U.S. are: underweight below 18.5, healthy 18.5–24.9, overweight 25–29.9, and obesity 30 or above. Categories do not equal a diagnosis.

Why can BMI be misleading for athletes?

Muscle is denser than fat. A muscular person can land in the overweight band with a low body-fat percentage. Older adults can show a “healthy” BMI while losing muscle. Waist, labs, and performance matter more than a single ratio.

Is this calculator for children?

No. Pediatric BMI is plotted on age-and-sex growth charts. Do not apply adult cutoffs to kids or teens.

Do you store height and weight?

No. The calculation never leaves your device.