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BMI and waist measurement explained
BMI and waist size are quick checks used by the NHS to see whether your weight might affect your health. They are useful guides, with limits worth knowing.
What is BMI?
Body mass index compares your weight with your height. For adults, the NHS describes these ranges:
| BMI | NHS description |
|---|---|
| Under 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 39.9 | Obese |
| 40 or above | Severely obese (a medical term) |
The NHS has a free BMI calculator that does the sum for you. These words are medical categories, and not a judgement of you as a person.
Different thresholds for some groups
For people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds, the NHS uses lower cut-offs: a BMI of 23 to 27.4 is classed as overweight and 27.5 or more as obese. Ask a health professional how this applies to you.
Why BMI is only a guide
- It cannot tell muscle from fat, so very muscular people may score high.
- It does not show where fat is stored.
- It is less reliable for older adults, who lose muscle, and for people who are pregnant.
- For children and teenagers, BMI is compared with growth charts for their age and sex, not these adult ranges.
Waist measurement
Fat around the middle is linked to a higher risk of type 2 diabetes and heart disease. The NHS advises keeping your waist measurement to less than half your height. For example, if you are 170 cm tall, aim for a waist under 85 cm. The NHS also offers a waist-to-height ratio calculator, which it suggests for people whose BMI is under 35. Older guidance used fixed waist sizes by sex; we do not repeat those figures because the NHS now uses the half-your-height rule.
How to measure your waist
- Stand up straight and breathe out normally.
- Find the midpoint between the bottom of your ribs and the top of your hips.
- Wrap a tape measure around your body at this point, snug but not squeezing.
- Read it off after you breathe out.
What to do with your numbers
If your BMI or waist is in a higher range, it does not mean something is wrong now. It signals that small changes could help, and that a conversation with a GP may be worthwhile (when to see a GP). If your BMI is under 18.5, or you are losing weight without trying, see a GP rather than trying to lose more.
Do not track obsessively
Measuring every day is not helpful. Once a month is plenty, and other signs matter too: energy, sleep, fitness and how clothes fit. If measuring causes anxiety, skip it and read our page on getting support. When you are ready, see the NHS weight loss plan and the advice on pace.
A note on language
Words such as overweight and obese are clinical labels that help health services decide who may need support. They are not a judgement about you. Many people find it easier to talk about health, habits and how they feel, and you can ask a clinician to do the same.
This is general information based on NHS guidance, not medical advice. Speak to your GP or pharmacist before making big changes, especially if you have a health condition, take medicines, are pregnant or are under 18.
Frequently asked questions
What is a healthy BMI?
For most adults the NHS gives 18.5 to 24.9 as the healthy range. Lower cut-offs apply for some ethnic backgrounds.
Is BMI accurate for athletes?
Not always, as it cannot tell muscle from fat.
What waist size is a risk?
The NHS advises keeping your waist measurement to less than half your height.