BMI and waist-to-height calculator

BMI is your weight in kilograms divided by your height in metres squared. It is a rough guide: it cannot see muscle or where fat sits. That is why the calculator also works out your waist-to-height ratio if you like. Rule of thumb: keep your waist to less than half your height.

Your BMI

Example:

24.1 BMI · Healthy weight

WHO categories for adults.

• Waist-to-height: 0.48 · no increased risk
• Half your height: 84 cm

A guide, not a diagnosis: muscle is heavy, and BMI cannot see where fat sits.

Your details

The calculator asks for:

• Height (cm)
• Weight (kg)
• Waist (cm, optional)
• Age (years)

The calculation runs in your browser. Your details are not saved and not sent anywhere.

BMI chart (WHO)

Source: World Health Organization (WHO), Regional Office for Europe, 2025

Waist-to-height (NICE)

From this waist size the ratio reaches 0.5 (increased) or 0.6 (high).

Source: National Institute for Health and Care Excellence (NICE), 2025

How the calculator works

• BMI = weight in kg ÷ (height in m)², to one decimal place.
• WHO categories for adults: under 18.5 underweight, 18.5 to 24.9 healthy weight, 25 to 29.9 overweight, 30 and over obesity (classes I to III).
• Waist-to-height ratio = waist ÷ height, both in cm. NICE: 0.4 to 0.49 no increased health risk, 0.5 to 0.59 increased, 0.6 and over high.
• NICE recommends the ratio alongside BMI for everyone with a BMI under 35, including very muscular people.

Example: 168 cm, 68 kg, waist 80 cm:

• BMI = 68 ÷ (1.68 × 1.68) = 24.1
• Waist-to-height = 80 ÷ 168 = 0.48

Good to know

• For adults aged 18 and over. Children and teenagers use centiles by age and sex.
• Lots of muscle raises your BMI without it being a problem. Older people often have less muscle at the same BMI.
• For people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, NICE uses lower thresholds: overweight from 23, obesity from 27.5.
• BMI does not apply in pregnancy.
• If food and weight weigh heavily on you, talk to your GP or an eating disorder support service.

Frequently asked questions

What is a good BMI?

The WHO calls 18.5 to 24.9 a healthy weight for adults. But BMI is only a rough guide: it cannot see muscle or where fat sits.

Why does my waist matter?

Fat around the belly is more closely linked with type 2 diabetes, high blood pressure and heart disease than fat on hips and legs. The waist-to-height ratio shows this better than BMI. NICE’s rule of thumb: keep your waist to less than half your height.

How do I measure my waist?

Stand up straight, breathe out normally and place the tape halfway between your lowest rib and the top of your hip bone. Don’t pull your stomach in and don’t pull the tape tight.

What is my ideal weight?

There is no ideal weight for everyone. What suits you depends on your build, muscle and age. Your waist, how much you move and how fit you feel day to day say more than one number.

Is BMI the same for women and men?

Yes, the WHO categories are the same for both. At the same BMI, women have more body fat than men on average.

When should I see a doctor?

With a BMI under 18.5 or 30 and over, with a waist-to-height ratio of 0.6 or more, and whenever you lose or gain weight without trying.

Are my details saved?

No. The calculator works in your browser and sends nothing.

Sources

1. A healthy lifestyle – WHO recommendations (Body mass index). World Health Organization (WHO), Regional Office for Europe, 2025
2. Obesity and overweight (fact sheet). World Health Organization (WHO), 2025
3. Overweight and obesity management (NG246): Identifying and assessing overweight, obesity and central adiposity. National Institute for Health and Care Excellence (NICE), 2025

Updated 22 October 2026