Guide · Updated 2026-10-08
BMI: What the Number Means and What It Misses
How body mass index is calculated, the standard adult ranges, why the NHS uses lower cut-offs for some ethnic backgrounds, and where BMI is unreliable.
Body mass index is a quick way of relating weight to height. It is useful for looking at groups of people, and less reliable for judging one person. This guide explains how it works, using the BMI Calculator. It gives general information, not medical advice.
The calculation
BMI = weight in kilograms ÷ (height in metres)². A person who is 175 cm tall and weighs 70 kg has a BMI of 70 ÷ 1.75² = 22.9. In imperial units, 69 inches and 154 pounds give 22.7.
The standard adult ranges
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 and above | Obese |
The calculator also shows the weight range that corresponds to a healthy BMI for your height. At 175 cm that is about 56.7 to 76.3 kg.
Different cut-offs for some backgrounds
The NHS says that people with Asian, Middle Eastern, Black African or African-Caribbean backgrounds have a higher risk of conditions such as type 2 diabetes at a lower BMI, so a lower threshold is used: 18.5 to 22.9 is the healthy range, 23 to 27.4 is overweight and 27.5 or above is obese. The calculator applies the standard ranges only, so use the NHS calculator if this applies to you.
What BMI does not measure
- Body fat and muscle. A muscular person can have a high BMI with little excess fat.
- Where fat is carried. Weight around the waist carries more health risk.
- Age. Older adults can have a normal BMI and low muscle mass.
- Children and pregnancy. Adult ranges do not apply to children, and BMI is unreliable in pregnancy.
Other things to look at
Waist measurement, blood pressure, diet, activity and family history give a fuller picture. If you are working out how much to eat, the Calorie Calculator estimates daily needs, with its own limits. Speak to a GP, pharmacist or dietitian about your weight and health.
Sources
Frequently asked questions
Is a BMI of 25 bad? It is the start of the overweight range, but it does not say how healthy you are.
Can athletes use BMI? It often over-states their weight problem, because muscle weighs more than fat.