Body mass index, or BMI, compares your weight with your height. It’s quick to work out, which is why doctors and health surveys use it as a first screen.
It’s a screening number, nothing more. The page gives you the figure and the standard category, then explains what BMI can’t tell you.
How to use the BMI Calculator#
- Choose your units. Metric or imperial.
- Enter height and weight. Use your current height and weight without shoes or heavy clothing.
- Read the result. You get your BMI, the category and a healthy range for your height.
The BMI formula#
In imperial units, 5 ft 9 in is 69 inches and 154 lb gives 703 × 154 ÷ 4,761 = 22.7.
Adult BMI categories#
| BMI | Category (WHO) |
|---|---|
| Under 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obesity class I |
| 35 to 39.9 | Obesity class II |
| 40 or more | Obesity class III |
For people of Asian descent, the WHO has suggested lower action points, with overweight starting at 23 and obesity at 27.5, because health risks rise at lower BMIs in some Asian populations. The calculator lets you switch to those cut-offs.
A worked example#
A person who is 175 cm tall and weighs 70 kg has a BMI of 70 ÷ 1.75² = 70 ÷ 3.0625 = 22.9. That is in the healthy range of 18.5 to 24.9. For their height, the range of weights with a BMI in that band is 56.7 kg to 76.3 kg. In imperial units the formula is 703 × weight in pounds ÷ height in inches squared, and the calculator handles either system.
Different cut-offs for different populations#
Studies of Asian populations found that the risk of type 2 diabetes and heart disease rises at lower BMIs than in European populations. The WHO has therefore suggested lower action points of about 23 for overweight and 27.5 for obesity for people of Asian descent, and the calculator has a switch for that. For children and teenagers, BMI is read against age and sex percentile charts, not against fixed numbers, so this tool is for adults aged 18 and over.
What BMI can and cannot tell you#
BMI is a quick screening tool that uses only height and weight. It doesn’t measure body fat or where fat is carried, so a muscular person can be classed as overweight, and an older person can have a normal BMI with low muscle. Fat around the waist carries more risk than fat on the hips, and a waist measurement under half your height is a simple second check.
It’s designed for adults aged 18 and over. Children’s results are compared with growth charts, and pregnancy and some illnesses change how to read the figure. Use BMI as one number alongside blood pressure, blood tests, fitness, family history and a talk with your doctor. This page is information only and does not replace medical advice. If you want to plan food intake, the fat intake calculator is a good next step.
Waist measurements as a second check#
Fat stored around the waist carries more health risk than fat on the hips and thighs, and BMI cannot see where it sits. The WHO gives waist cut-offs: risk is raised above 94 cm for men and 80 cm for women, and substantially raised above 102 cm and 88 cm. A simpler rule is to keep your waist under half your height, so a person who is 170 cm tall should aim for a waist under 85 cm. Measure halfway between the bottom rib and the top of the hip bone, after breathing out. Used together, BMI and waist give a better picture than either alone.
Frequently asked questions#
Divide your weight in kilograms by your height in metres squared.
For most adults, 18.5 to 24.9 is classed as a healthy weight.
Not always. Muscle weighs more than fat, so very muscular people can have a high BMI without excess body fat.
No. Children and teenagers are assessed against growth charts for their age and sex.
For children and teenagers, BMI is compared with growth charts for age and sex, so the adult categories do not apply. For older adults, a slightly higher BMI is often not linked to higher risk, and losing weight unintentionally can be a concern. A doctor will look at the full picture.