BMI Calculator

Enter your height and weight to calculate your Body Mass Index (BMI) and see which weight category you fall into. Works with both metric (cm/kg) and imperial (ft-in/lb) units.

What is BMI?

BMI, or Body Mass Index, is a single number that compares your weight to your height. It was designed as a quick way to screen large populations for underweight, healthy weight, overweight and obesity — not as a diagnosis for any one person.

Because it needs only two measurements, BMI is the most widely used weight indicator in the world. Health services, insurers and researchers all use the same thresholds, which makes results easy to compare over time.

How to calculate BMI

BMI is your weight in kilograms divided by the square of your height in metres. If you measure in pounds and inches, multiply the result by 703 to get the same number.

BMI = weight (kg) ÷ height (m)²

Imperial version: BMI = 703 × weight (lb) ÷ height (in)²

BMI formula explained

Squaring the height is what makes the number comparable between a short person and a tall person. Body mass grows roughly with the square of height in adults, so dividing by height² cancels out most of the effect of being tall or short and leaves a figure that reflects body size.

The calculator above converts whatever units you enter into kilograms and metres first, so you always get the same BMI whether you work in metric or imperial.

BMI categories

World Health Organization BMI categories for adults
BMI range	Category	What it suggests
Below 18.5	Underweight	Body mass is low for your height
18.5 – 24.9	Normal weight	Weight is in the healthy range
25.0 – 29.9	Overweight	Raised risk of weight-related conditions
30.0 – 34.9	Obese class I	Moderately increased health risk
35.0 – 39.9	Obese class II	Severely increased health risk
40.0 and above	Obese class III	Very severely increased health risk

Example calculation

Take an adult who is 175 cm tall and weighs 70 kg. First convert the height to metres: 175 cm = 1.75 m. Squaring it gives 1.75 × 1.75 = 3.0625.

Now divide the weight by that figure: 70 ÷ 3.0625 = 22.9. A BMI of 22.9 sits inside the 18.5 – 24.9 band, so this person is in the normal weight category.

70 kg ÷ (1.75 m)² = 70 ÷ 3.0625 = 22.9

When BMI is misleading

BMI measures body mass, not body composition, so it cannot tell muscle from fat. A well-trained athlete can be classed as overweight while carrying very little fat, and an inactive person with low muscle mass can sit in the normal range while carrying too much of it.

Athletes and heavily muscled people often read high
Older adults lose muscle, so a normal BMI can hide excess fat
Children and teenagers need age- and sex-specific BMI percentiles
It is not validated for pregnancy
It says nothing about where fat is stored, and waist fat matters most

Waist circumference and waist-to-height ratio are useful companions to BMI.

Frequently Asked Questions

What is a healthy BMI?

For most adults a BMI between 18.5 and 24.9 is considered healthy. It is a range rather than a target, and where you sit inside it matters far less than your overall diet, activity and blood markers.

Is BMI accurate?

It is accurate as a population screening tool but rough for individuals. BMI cannot distinguish muscle from fat or show where fat is stored, so treat it as a first indicator rather than a verdict on your health.

Does BMI differ for men and women?

The formula and the adult categories are identical for both. Women typically carry a higher percentage of body fat at the same BMI, which is one reason BMI is best read alongside a body fat estimate.

Can I use this BMI calculator for children?

No. Children and teenagers under 18 should be assessed with age- and sex-specific BMI percentile charts, because healthy body composition changes as they grow. Ask a paediatrician or use a dedicated child BMI percentile tool.

What should I do if my BMI is outside the healthy range?

Treat it as a prompt to look further, not as a diagnosis. A BMI outside 18.5 – 24.9 is worth discussing with a doctor, who can look at your waist measurement, blood pressure, blood sugar and lipids to build a fuller picture.