BMI Calculator
Body mass index in metric or imperial, with what the number does and doesn't mean.
About this BMI calculator
Enter your height and weight in either metric or imperial units and you get your body mass index, the category it falls in, and the weight range that would put you in the healthy band at your height. The scale underneath shows where the number sits rather than presenting it as a verdict.
The formula is simply weight in kilograms divided by height in metres squared. Imperial figures are converted to metric first, which is why the two modes agree exactly rather than drifting apart the way rounded imperial formulas do.
The standard categories
- Under 18.5 — underweight
- 18.5 to 24.9 — healthy weight
- 25 to 29.9 — overweight
- 30 to 34.9 — obese, class I
- 35 to 39.9 — obese, class II
- 40 and over — obese, class III
These are the World Health Organization's adult thresholds, and they are the ones most health services use.
What BMI cannot tell you
BMI compares weight to height and nothing else. It has no way of knowing what that weight is made of, which produces two well-known failure modes: muscular people are routinely classified as overweight despite low body fat, and someone sedentary can sit in the healthy range while carrying a lot of it. Where fat is carried matters too — around the abdomen is associated with higher risk than around the hips — and BMI is blind to that.
The thresholds were also derived largely from European populations. Health bodies in a number of countries use lower cut-offs for people of South Asian, Chinese and other Asian descent, where the risks associated with a given BMI appear at a lower number. BMI is not valid for children, during pregnancy, or for older adults without adjustment.
None of that makes it useless — it is a quick, free population-level screen, which is what it was designed to be. It is simply one indicator among several. A doctor reading it alongside a waist measurement, blood pressure, blood work and your history is doing something a calculator cannot.
Frequently asked questions
How is BMI calculated?
Weight in kilograms divided by height in metres squared. Someone 1.75 m tall weighing 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. Imperial measurements are converted to metric first, so both modes give identical results.
What is a healthy BMI?
18.5 to 24.9 is the standard healthy range for adults. Below that is classed as underweight, 25 to 29.9 as overweight, and 30 or above as obese.
Is BMI accurate for athletes?
Often not. Muscle is denser than fat, so a muscular person can register as overweight or obese while carrying very little body fat. BMI cannot distinguish the two, which is its best-known limitation.
Does BMI work the same for everyone?
No. The thresholds came largely from European populations, and several countries apply lower cut-offs for people of South Asian and East Asian descent, where health risks appear at a lower BMI. It also is not valid for children, pregnancy, or older adults without adjustment.
Should I be worried about my BMI?
It is a screening number, not a diagnosis. If it falls outside the healthy range it is worth raising with a doctor, who can interpret it alongside waist measurement, blood work and your history — none of which a calculator has access to.