BMI Calculator (NHS Categories)
This BMI calculator uses the NHS adult categories: a BMI of 18.5 to 24.9 is a healthy weight, so someone 170 cm tall and 70 kg has a BMI of 24.2. If your family background is South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean, NICE advises lower thresholds, with overweight starting at 23.
Enter your height and weight in metric or imperial. You can also add your waist size to get your waist-to-height ratio.
- Healthy BMI18.5 – 24.9
- Lower threshold groups18.5 – 22.9
- Healthy waistunder half your height
Updated November 2026 · Uses NHS and NICE BMI categories
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BMI chart for adults UK
The NHS uses the same adult BMI categories as NICE. These apply to adults aged 18 and over, whatever their sex.
| Category | Standard BMI | Lower threshold groups |
|---|---|---|
| Underweight | Below 18.5 | Below 18.5 |
| Healthy weight | 18.5 – 24.9 | 18.5 – 22.9 |
| Overweight | 25 – 29.9 | 23 – 27.4 |
| Obesity class 1 | 30 – 34.9 | 27.5 – 32.4 |
| Obesity class 2 | 35 – 39.9 | 32.5 – 37.4 |
| Obesity class 3 | 40 or more | 37.5 or more |
The lower thresholds are for people with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, who have a higher risk of type 2 diabetes and other conditions at a lower BMI. NICE sets obesity classes 2 and 3 for these groups by taking 2.5 off the standard thresholds.
How to calculate BMI
BMI is your weight in kilograms divided by your height in metres squared. For someone 1.70 m tall weighing 70 kg: 1.70 × 1.70 = 2.89, and 70 ÷ 2.89 = 24.2.
If you use stone and pounds, multiply your total pounds by 0.4536 to get kilograms, and your total inches by 2.54 to get centimetres. The calculator does this for you.
Waist-to-height ratio
NICE recommends measuring your waist as well as BMI, because fat around the middle raises the risk of type 2 diabetes and heart disease. Aim to keep your waist to less than half your height.
| Waist-to-height ratio | Meaning |
|---|---|
| 0.4 – 0.49 | Healthy |
| 0.5 – 0.59 | Increased central fat |
| 0.6 or more | High central fat |
When BMI is not the right measure
- Children and teenagers. Under-18s need a BMI centile that takes age and sex into account, not the adult categories.
- Pregnancy. Adult BMI should not be used during pregnancy. Your midwife will use your BMI from early pregnancy.
- Muscular builds. Muscle is heavy, so athletes can show as overweight with little body fat. Waist size helps here.
- Older adults. BMI can hide muscle loss, so a GP may look at other measures too.
Frequently asked questions
What is a healthy BMI for a woman?
For adult women and men the NHS healthy range is 18.5 to 24.9. For people from a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background, NICE uses 18.5 to 22.9.
Is a BMI of 25 overweight?
Yes. In the standard categories a BMI of 25 to 29.9 is overweight and 30 or more is obesity. With NICE’s lower thresholds, overweight starts at 23 and obesity at 27.5.
How do I work out my BMI in stone and pounds?
Convert your weight to kg (pounds × 0.4536) and your height to metres (inches × 0.0254), then divide weight by height squared. Enter stone and pounds above and the calculator converts for you.
Does BMI work for children?
No. Adult BMI categories do not apply to anyone under 18. Children’s BMI is plotted as a centile for their age and sex.
Can I use a BMI calculator when pregnant?
No. Adult BMI is not used during pregnancy because weight gain is expected. Your midwife will use your BMI from your booking appointment.
Sources and method
We use official UK guidance and recognised formulas, and test each calculator against worked examples. Results are estimates to help you plan, not professional advice. See how we work.