How BMI is calculated
Body mass index is your weight divided by the square of your height:
- Metric: BMI = weight (kg) ÷ height (m)²
- Imperial: BMI = 703 × weight (lb) ÷ height (in)²
For example, someone 5 ft 9 in (69 inches) tall who weighs 180 lb has a BMI of 703 × 180 ÷ 69² = 26.6. That puts them in the overweight range. For their height, a healthy BMI corresponds to a weight of about 125 to 169 lb. The BMI Calculator works this out for you in either unit system and shows the healthy range for your height.
The adult BMI ranges
These categories are used by the World Health Organization and the US Centers for Disease Control and Prevention for adults aged 20 and over:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obesity class 1 |
| 35 to 39.9 | Obesity class 2 |
| 40 or above | Obesity class 3 (severe) |
The cut-offs come from large population studies that link BMI to the risk of conditions such as type 2 diabetes, high blood pressure and heart disease. Risk doesn't jump at 25 or 30; it rises gradually, and the lines are drawn at round numbers for convenience.
Healthy weight by height
Turning the 18.5–24.9 range back into weights gives these healthy ranges:
| Height | Healthy weight range |
|---|---|
| 5 ft 2 in (157 cm) | 101–136 lb |
| 5 ft 5 in (165 cm) | 111–150 lb |
| 5 ft 8 in (173 cm) | 122–164 lb |
| 5 ft 11 in (180 cm) | 133–179 lb |
| 6 ft 2 in (188 cm) | 144–194 lb |
For a single target figure, the Ideal Weight Calculator compares this range with the older Devine, Robinson, Miller and Hamwi formulas.
Lower cut-offs for Asian populations
People of South Asian, Chinese and other Asian backgrounds tend to develop type 2 diabetes and heart disease at a lower BMI. A WHO expert consultation in 2004 suggested lower action points, and guidance such as the UK's NICE uses 23 for overweight and 27.5 for obesity for these groups. The difference matters: at 175 cm and 85 kg, a BMI of 27.8 is classed as "overweight" on the standard scale but "obese" on the Asian one. Our BMI Calculator has an option for these cut-offs.
What BMI gets wrong
BMI is cheap, quick and good at spotting trends across a population. For an individual it has real blind spots:
- It can't tell muscle from fat. A muscular athlete can have a BMI of 28 with little body fat, and be labeled overweight.
- It ignores where fat sits. Fat around the abdomen is more strongly linked with heart disease and diabetes than fat on the hips and thighs, but two people with the same BMI can carry it very differently.
- It misses "normal-weight" risk. Someone with a healthy BMI can still have a high body fat percentage and little muscle, particularly with age.
- It varies by sex, age and ethnicity. At the same BMI, women generally carry more fat than men, older adults more than younger ones, and risk differs between ethnic groups.
In 2023 the American Medical Association adopted a policy calling BMI an imperfect measure on its own and encouraging doctors to use it alongside other measures such as waist circumference and body composition.
Better measures to use alongside BMI
Waist-to-height ratio
Divide your waist measurement by your height, in the same units. Keeping your waist to less than half your height (a ratio under 0.5) is a simple target, and it's what UK NICE guidance recommends alongside BMI. Our example person at 175 cm with an 88 cm waist has a ratio of 0.50: "increased risk". Measure around your middle, roughly halfway between your lowest rib and the top of your hip bone, after breathing out normally. Add your waist to the BMI Calculator and it shows the ratio too.
Body fat percentage
Body fat is what BMI tries to stand in for. A tape-measure estimate using the US Navy equations needs only your height, neck and waist (and hips for women); the Body Fat Calculator does the math. It can be a few points off for an individual, but it does respond to muscle in a way BMI can't. A DEXA scan is more accurate, but costs more and isn't widely available.
Health markers
Blood pressure, blood sugar (HbA1c or fasting glucose) and cholesterol tell you about the health risks BMI is meant to flag. A routine check-up covers them.
How to use your BMI
- Work out your BMI and note where it sits in the ranges above.
- Measure your waist and check your waist-to-height ratio.
- If both are in the healthy range, you're in good shape on these measures. If either isn't, look at the trend over months rather than a single reading.
- If you're outside the healthy range or worried about your weight, talk to a doctor, who can look at the whole picture.
If you're planning to change your weight, our health calculators include TDEE and calorie tools to set a realistic target.
This guide is general information, not medical advice. BMI ranges here apply to adults who are not pregnant.