How to use the BMI calculator
Choose US or metric units, enter your height and weight, and the calculator shows your BMI, the category it falls in, and the weight range that would put you in the healthy category for your height. The colored bar shows where you sit across the whole scale.
The formula
BMI = weight (kg) ÷ height (m)²
BMI = 703 × weight (lb) ÷ height (in)²A person who is 5 ft 9 in (69 inches) and 160 lb has a BMI of 703 × 160 ÷ 69² = 23.6, in the healthy range. The healthy weight range for that height is roughly 125–169 lb.
BMI categories for adults
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40 and above | Obesity class III |
These are the World Health Organization cut-offs, also used by the US CDC and the UK NHS. Some countries use lower thresholds for people of Asian descent (overweight from 23, obese from 27.5) because health risks appear at lower BMIs in those populations.
What BMI can and can’t tell you
BMI is a screening tool, not a diagnosis. It’s useful at the population level because it correlates with body fat for most people and is trivially easy to measure. At the individual level it has known blind spots:
- Muscle mass — bodybuilders and many athletes register as overweight.
- Age — older adults tend to lose muscle, so a “healthy” BMI can hide low muscle and high fat.
- Fat distribution — abdominal fat carries more risk than fat on hips and thighs, and BMI can’t see the difference.
- Sex and ethnicity — the same BMI corresponds to different body-fat percentages across groups.
If your BMI is outside the healthy range, or in it but you’re concerned, waist circumference is a good second check: risk rises above 40 in (102 cm) for men and 35 in (88 cm) for women. For a proper assessment, talk to a doctor.
Healthy weight range
The range shown is what a BMI of 18.5 to 24.9 works out to at your height. It’s deliberately wide — a 5 ft 9 in adult can be anywhere from about 125 to 169 lb and be in the healthy category. Where you feel and function best within that range is individual.
Frequently asked questions
How is BMI calculated?
Weight in kilograms divided by height in metres squared. In US units it's weight in pounds × 703 ÷ height in inches squared. Both give the same number.
What are the BMI categories?
Under 18.5 is underweight, 18.5–24.9 is the healthy range, 25–29.9 is overweight, and 30 or above is obese, using the World Health Organization thresholds for adults.
Is BMI accurate for athletes?
Not very. BMI can't tell muscle from fat, so muscular people often score as overweight while carrying little body fat. Waist circumference or body-fat percentage are better measures for them.
Does BMI work for children?
Children and teens use the same formula but are assessed by age- and sex-specific percentiles rather than fixed cut-offs. This calculator is intended for adults aged 20 and over.
Last updated August 26, 2026.