BMI is a screening tool, not a diagnosis. It cannot tell muscle from fat and says nothing about where weight is carried. Nothing on this page is medical advice — if you are worried about your weight, a GP or pharmacist can see what BMI cannot.
What is the BMI formula?
Weight in kilograms, divided by height in metres, squared.
The working
BMI = weight (kg) ÷ height (m)² 70 kg, 1.75 m tall: 1.75 × 1.75 = 3.0625 70 ÷ 3.0625 = 22.9
In imperial the formula is pounds ÷ inches², multiplied by 703. The 703 is simply the conversion factor folded into the sum.
The working
BMI = weight (lb) ÷ height (in)² × 703 11 st 0 lb (154 lb), 5 ft 9 in (69 in): 69 × 69 = 4761 154 ÷ 4761 × 703 = 22.7
What is a healthy BMI?
| BMI | Category | Lowered threshold * |
|---|---|---|
| Under 18.5 | Underweight | Under 18.5 |
| 18.5 – 24.9 | Healthy weight | 18.5 – 22.9 |
| 25 – 29.9 | Overweight | 23 – 27.4 |
| 30 – 39.9 | Obese | 27.5 and over |
| 40 and over | Severely obese | — |
* The NHS uses the lower thresholds for people of Black African, African-Caribbean and South Asian background, because the risk of type 2 diabetes and heart disease begins at a lower BMI in these groups. A calculator offering only the 25 and 30 cut-offs will tell a great many people they are in the clear when their GP would not.
Why BMI is a blunt instrument
- It cannot distinguish muscle from fat. A rugby player or a serious weightlifter can be in excellent health and read as obese, because muscle is denser than fat.
- It ignores where the weight sits. Weight around the middle carries more risk than the same weight elsewhere — the NHS suggests keeping your waist under half your height as a companion check.
- It does not apply to children. Under 18, BMI is read against age-and-sex growth charts, because a healthy BMI changes throughout childhood.
- It is not meaningful in pregnancy, and becomes a weaker guide in older adults, where a slightly higher reading is often not a problem.
None of that makes BMI useless. It works reasonably well across a population and it is free, instant and requires no equipment beyond scales and a tape measure. It is one number among several, and it is most useful as a prompt for a conversation rather than an answer.
Common questions
What is a healthy weight for my height?
Multiply your height in metres squared by 18.5 and by 25 — that gives the bottom and top of the healthy range in kilograms. At 1.75 m: 3.0625 × 18.5 = 56.7 kg and 3.0625 × 25 = 76.6 kg, so roughly 8 st 13 lb to 12 st 1 lb.
Why are the BMI thresholds lower for some ethnic groups?
Because the health risk starts sooner. At the same BMI, people of South Asian, Black African and African-Caribbean background have a higher risk of type 2 diabetes and heart disease, so the NHS lowered the overweight threshold to 23 and the obese threshold to 27.5 for those groups.
Is BMI accurate?
It is accurate arithmetic and a crude health measure — two different things. It works across populations and poorly for individuals at the extremes. Treat it as a screening number rather than a verdict.
Does BMI work for children?
Not with the adult categories. A child’s BMI is compared against centile charts for their exact age and sex. For a child, use the NHS healthy weight calculator or ask a GP or school nurse.
My BMI is in the overweight band — what should I do?
Nothing a website tells you. BMI is a number that suggests a conversation might be worth having. A GP or pharmacist can look at your waist, blood pressure, family history and everything else BMI cannot see, and that is free.
Check this before you rely on it. A free guide to everyday arithmetic — not tax, accounting, financial, legal or medical advice. The working is shown above so you can verify it against your own figures. See our terms of use.