BMI Calculator

What is BMI?

Body mass index (BMI) is a simple calculation that compares your weight to your height: weight in kilograms divided by height in metres squared. It's the same method the NHS BMI calculator uses, and it remains the standard first check for whether someone's weight is likely to be affecting their health — not because it's perfect, but because it's quick, free, and right often enough to be useful.

The BMI chart: what the ranges mean

BMI	Category	What it suggests
Below 18.5	Underweight	You may be under the weight that's healthiest for you — worth discussing with a GP or pharmacist, especially if you've lost weight without trying.
18.5 – 24.9	Healthy weight	The range linked with the lowest risk of weight-related health problems for most adults.
25 – 29.9	Overweight	Extra weight that may begin to raise the risk of type 2 diabetes, high blood pressure and joint problems.
30 and above	Obese	A level where weight is likely to be affecting health now, and where support to lose weight has the clearest benefit.

If your background is South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean, health risks rise at lower BMIs: NICE advises treating 23 and above as increased risk and 27.5 and above as high risk in these groups.

What is a healthy BMI?

For most adults, a healthy BMI sits between 18.5 and 24.9. Within that band there's no "perfect" number — a BMI of 20 isn't automatically better than 24. What matters more is whether your weight is stable, how it's distributed (see waist size, below), and the habits behind it: what you eat, how you move, how you sleep.

BMI for women and men: is it different?

The calculation and the ranges are identical for women and men — there's no separate scale. What differs is body composition: women naturally carry more body fat than men at the same BMI, and men more muscle. That's one of BMI's known blind spots, which is why the same number can mean slightly different things for different people — and why waist measurement makes a useful second check for everyone.

Where BMI gets it wrong

• Muscle: BMI can't tell muscle from fat. Rugby players and regular weight-trainers often score "overweight" while carrying very little fat.
• Age: older adults tend to carry less muscle, so a "healthy" BMI can hide low muscle mass; slightly higher BMIs are often acceptable later in life.
• Pregnancy: BMI doesn't apply — use pre-pregnancy weight and speak to your midwife about healthy weight gain instead.
• Under-18s: children and teenagers need age-and-sex-adjusted centile charts, not the adult scale. The NHS provides a dedicated child BMI calculator — this one is for adults.

The second number worth knowing: your waist

Fat carried around the middle is more strongly linked to type 2 diabetes and heart disease than fat carried elsewhere — and BMI can't see the difference. As a rule of thumb, try to keep your waist to less than half your height. Measure across the belly button, breathing out gently, without pulling the tape tight.

What to do with your result

A single BMI reading is a conversation starter, not a diagnosis. If your result is outside the healthy range and you'd like to act on it, our Health Advice articles cover sleep, eating habits and everyday health, our calorie calculator will show you what your body actually needs each day, and you can always talk to our pharmacist — free, without an appointment. For BMIs under 18.5 or over 40, or any unexplained weight change, your GP is the right first stop.

Frequently asked questions

How do I calculate BMI manually?

Divide your weight in kilograms by your height in metres, then divide the answer by your height again. Example: 70kg ÷ 1.75m ÷ 1.75m = BMI 22.9.

Is BMI accurate?

For most adults it's a reasonable screening tool, and large studies show clear links between BMI ranges and health outcomes. For individuals it can mislead — especially for muscular, older or Asian-background adults — which is why waist size and overall habits belong in the picture.

What BMI should I aim for?

Anywhere in the 18.5–24.9 band, reached through habits you can keep. If you're above it, even losing 5–10% of your body weight measurably improves blood pressure, blood sugar and joint pain — you don't have to reach a "perfect" number to benefit.