Body Mass Index (BMI) is the most widely used screening tool for weight, because it needs only two measurements and a calculator. Doctors, insurers and public-health researchers use it to spot possible risk at a glance. But BMI was designed in the 19th century to describe populations, not to diagnose individuals, and it has well-known blind spots. Here’s how to calculate it, how to read it, and what to look at alongside it.
This guide is general information, not medical advice. If you’re concerned about your weight or health, talk to a GP or other qualified health professional.
The BMI formula
Metric: BMI = weight (kg) ÷ height (m)²
Example: 70 kg and 1.75 m tall. 1.75² = 3.0625. 70 ÷ 3.0625 = 22.9.
Imperial: BMI = 703 × weight (lb) ÷ height (in)²
Example: 154 lb and 5 ft 9 in (69 in). 69² = 4,761. 703 × 154 = 108,262. 108,262 ÷ 4,761 = 22.7.
(The small difference between the two examples is because 154 lb and 69 in aren’t exactly 70 kg and 1.75 m.) Our BMI calculator handles both unit systems and shows where you fall in each category.
WHO categories for adults
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight (pre-obesity) |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40 and above | Obesity class III |
For someone 1.75 m tall, the healthy range corresponds to roughly 56.7 kg to 76.3 kg (18.5 × 3.0625 and 24.9 × 3.0625).
Different thresholds for some groups
Risk of type 2 diabetes and cardiovascular disease rises at lower BMIs in some populations. In the UK, NICE advises using lower thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background: overweight from 23 and obesity from 27.5.
The limits of BMI
BMI treats every kilogram the same, whether it’s muscle, fat, bone or water, and ignores where fat is stored. That leads to predictable errors:
- Athletes and muscular people: a rugby player at 1.85 m and 100 kg has a BMI of 29.2, “overweight”, while possibly having low body fat.
- Older adults: muscle mass declines with age, so an older person can have a “normal” BMI with relatively high body fat. Some research also suggests slightly higher BMIs are not linked to worse outcomes in older age, so clinicians interpret it differently.
- Children and teenagers: they’re still growing, so BMI is compared with age- and sex-specific growth charts and given as a centile, not judged on adult cut-offs.
- Pregnancy: BMI isn’t meaningful during pregnancy; pre-pregnancy BMI is used for guidance instead.
- Body shape and fat distribution: two people with the same BMI can have very different health risks depending on how much fat is stored around the abdomen.
Recognising these limits, an international Lancet commission in 2025 recommended that obesity should be diagnosed using additional measures, such as waist size or direct body-fat measurement, plus assessment of health effects, rather than BMI alone.
Waist measurements: a useful second number
Fat stored around the organs (visceral fat) is closely linked to metabolic risk. Two simple measures:
Waist circumference
Measure halfway between the bottom of your ribs and the top of your hip bones, after breathing out naturally. NHS guidance:
| Increased risk | Very high risk | |
|---|---|---|
| Men | 94 cm (37 in) or more | 102 cm (40 in) or more |
| Women | 80 cm (31.5 in) or more | 88 cm (34.6 in) or more |
Waist-to-height ratio
Divide your waist by your height in the same units. NICE recommends keeping it below 0.5: your waist should be less than half your height.
Example: height 175 cm, waist 84 cm → 84 ÷ 175 = 0.48: within the healthy range. A ratio of 0.5–0.59 indicates increased risk; 0.6 or above, high risk.
Ideal weight formulas
Several formulas estimate an “ideal” body weight from height. They were developed largely for medication dosing and give a single figure, so treat them as rough references, not targets. For a man 175 cm tall (about 68.9 in, so 8.9 in over 5 feet):
| Formula | Men | Women |
|---|---|---|
| Devine (1974) | 50 kg + 2.3 kg per inch over 5 ft | 45.5 kg + 2.3 kg per inch over 5 ft |
| Robinson (1983) | 52 kg + 1.9 kg per inch | 49 kg + 1.7 kg per inch |
| Miller (1983) | 56.2 kg + 1.41 kg per inch | 53.1 kg + 1.36 kg per inch |
Devine for that man: 50 + 2.3 × 8.9 = 70.5 kg. Robinson: 52 + 1.9 × 8.9 = 68.9 kg. Both sit comfortably inside the BMI healthy range of 56.7–76.3 kg. Our ideal weight calculator shows several formulas side by side.
Using BMI sensibly
- Treat it as a starting point, not a verdict.
- Add your waist measurement or waist-to-height ratio.
- Consider your context: age, muscle mass, ethnicity, family history, blood pressure, blood sugar and cholesterol.
- Watch the trend over months rather than one reading.
- Focus on habits: activity, sleep, diet quality. If you want to change your weight gradually, a calorie needs calculator estimates your daily energy requirement as a starting point.
Common misunderstandings
- “BMI 25 means unhealthy”: it means higher risk on average, not a diagnosis.
- “A normal BMI means I’m healthy”: metabolic risk can be present at any BMI.
- Using adult BMI categories for children.
- Comparing BMI calculated with clothes, shoes or a rough height estimate. Measure accurately.
Checklist
- Calculate BMI with accurate height and weight.
- Compare with the WHO ranges, or lower thresholds if they apply to you.
- Measure your waist and check your waist-to-height ratio.
- Discuss results with a health professional if you’re outside the healthy range or have other risk factors.