BMI Calculator — Check Your Body Mass Index
Calculate your Body Mass Index (BMI) from height and weight, in metric or imperial units.
How to use the BMI Calculator
- Choose Metric (cm/kg) or Imperial (ft/lb).
- Enter your height and weight.
- Read your BMI and the category it falls into — underweight, normal weight, overweight, or obese.
About the BMI Calculator
Body Mass Index (BMI) is a screening measurement calculated from height and weight. It's used by doctors and health organizations as a quick, low-cost way to categorize a person's weight as underweight, normal weight, overweight, or obese, using the formula BMI = weight (kg) ÷ height (m)².
This calculator accepts both metric (kilograms and centimeters) and imperial (pounds, feet and inches) units, so you don't need to convert anything yourself. Enter your height and weight and it instantly returns your BMI value along with the standard World Health Organization weight category it falls into.
Worked example
Example: a person who is 1.75 m tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, which falls in the 'normal weight' range of 18.5–24.9.
How the calculation works in each unit system
In metric units, BMI is weight in kilograms divided by height in meters squared. In imperial units the calculator uses BMI = 703 × weight (lb) ÷ height (in)², where 703 converts pounds and inches into the metric result. Someone who is 5 ft 9 in (69 inches) and 154 lb has a BMI of 703 × 154 ÷ 69² ≈ 22.7 — the same answer you'd get converting to 1.75 m and 69.9 kg first.
Turning BMI into a weight range for your height
Because BMI is weight ÷ height², you can run the formula backwards to see the weight range each category covers at a given height: weight = BMI × height². For someone 1.75 m tall, the 18.5–24.9 'normal' band works out to roughly 56.7–76.3 kg (125–168 lb). This is often more useful than the single number, since it shows how much room the healthy range really allows.
Where BMI needs extra context
BMI was designed to describe populations, and it can mislead for individuals in predictable ways. Beyond muscular builds, a few situations call for a different yardstick:
- Children and teens: their BMI is compared against age- and sex-specific growth percentiles, not the adult categories shown here.
- Ethnicity: some health bodies use lower cut-offs for people of Asian descent, who tend to face higher metabolic risk at the same BMI — flagging increased risk from a BMI of about 23 rather than 25.
- Where fat is carried: waist circumference adds information BMI can't. A waist above about half your height is a common warning sign even when BMI is in the normal range.
- Older adults: a slightly higher BMI is not necessarily a concern later in life, and loss of muscle can hide behind a 'normal' number.
Frequently asked questions
- What is a healthy BMI range?
- For most adults, the World Health Organization defines 18.5–24.9 as the 'normal weight' range, below 18.5 as underweight, 25–29.9 as overweight, and 30+ as obese.
- Is BMI accurate for everyone?
- BMI is a useful population-level screening tool, but it doesn't distinguish between muscle and fat mass. It can misclassify very muscular people as overweight and may be less accurate for older adults, children, or pregnant women. It should be one data point, not a standalone diagnosis.
- Does BMI differ for men and women?
- The standard BMI formula and category ranges are the same for adult men and women, though body composition naturally differs between sexes, which is part of why BMI is only a screening estimate.
- How often should I check my BMI?
- There's no strict rule — many people check every few months or after a significant weight change. BMI is meant to track general trends over time rather than day-to-day fluctuations.