BMI is one of the most widely used — and most misunderstood — health metrics. GPs use it. Insurers use it. It takes 10 seconds to calculate. But it doesn't tell you what most people think it does.
How to calculate BMI
Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = (weight in lbs × 703) ÷ height (inches)²
Worked examples:
- 80kg, 1.78m: 80 ÷ (1.78²) = 80 ÷ 3.1684 = 25.2 (overweight)
- 62kg, 1.65m: 62 ÷ (1.65²) = 62 ÷ 2.7225 = 22.8 (healthy)
- 95kg, 1.80m: 95 ÷ (1.80²) = 95 ÷ 3.24 = 29.3 (overweight)
BMI categories (WHO classification)
- Below 18.5 — Underweight
- 18.5–24.9 — Healthy weight
- 25.0–29.9 — Overweight
- 30.0–34.9 — Obese (Class 1)
- 35.0–39.9 — Obese (Class 2)
- 40.0 and above — Severely obese (Class 3)
The NHS uses lower thresholds for people of South Asian, Chinese, Black African, African-Caribbean, and other non-white ethnic backgrounds, as research shows higher health risks at lower BMI levels in these populations. For these groups, overweight begins at 23 and obese at 27.5.
Why BMI has real limitations
BMI was developed in the 1830s by Belgian mathematician Adolphe Quetelet as a population statistics tool — not as a medical diagnostic for individuals. It has several well-documented flaws:
It doesn't measure body fat. BMI measures weight relative to height. Muscle weighs more than fat. A highly muscular person — a professional athlete, a weightlifter — will often have a BMI in the overweight range despite being extremely lean. Conversely, a sedentary person with low muscle mass might have a "healthy" BMI while carrying a high proportion of body fat.
It ignores fat distribution. Where you carry fat matters more than how much you carry. Visceral fat — stored around internal organs in the abdomen — is strongly associated with type 2 diabetes, heart disease, and metabolic syndrome. A person with a "healthy" BMI but a large waist circumference may be at higher risk than someone with a BMI of 27 but a lean midsection.
It varies by age. Older adults typically have lower muscle mass and higher body fat at the same BMI as younger adults. Children use a separate BMI-for-age chart rather than absolute thresholds.
Better measures to use alongside BMI
Waist circumference: A waist measurement above 94cm (men) or 80cm (women) indicates increased health risk. Above 102cm (men) or 88cm (women) indicates high risk. This is a stronger predictor of cardiovascular disease than BMI alone.
Waist-to-height ratio: Simply keep your waist circumference to less than half your height. A person 180cm tall should aim for a waist below 90cm. This ratio works across ethnicities and age groups better than BMI.
Body fat percentage: Measured by DEXA scan (most accurate), underwater weighing, or bioelectrical impedance (gym scales). Healthy ranges: women 20–35%, men 8–24%.
What should you actually do with your BMI?
Use BMI as a starting point, not a verdict. If your BMI is in the healthy range and your waist circumference is within target, you're likely in good metabolic health regardless of exact BMI value.
If your BMI is in the overweight or obese range, it's worth discussing with a GP — not because the BMI number itself causes problems, but because it flags a higher statistical risk of conditions worth screening for.
Don't obsess over the number. What matters is the trajectory — is it stable, improving, or worsening? — and the behaviours driving it.
Frequently asked questions
What is a healthy BMI?
The World Health Organisation (WHO) classifies BMI for adults as: underweight (below 18.5), healthy weight (18.5–24.9), overweight (25–29.9), and obese (30 and above). A BMI of 18.5–24.9 is considered the healthy range. However, BMI is a screening tool, not a diagnostic measure — it doesn't account for muscle mass, bone density, age, or ethnic differences in body composition.
How do you calculate BMI?
BMI = weight in kilograms ÷ (height in metres)². For example: a person who is 75kg and 1.75m tall has a BMI of 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5. In imperial: BMI = (weight in pounds × 703) ÷ (height in inches)². A 165lb person who is 5'9" (69 inches): (165 × 703) ÷ (69 × 69) = 116,000 ÷ 4,761 = 24.4.
What BMI is considered obese?
A BMI of 30 or above is classified as obese. This is further divided into: Class 1 obesity (30–34.9), Class 2 obesity (35–39.9), and Class 3/morbid obesity (40 and above). For people of Asian descent, the NHS uses lower thresholds — increased health risk begins at 23, and obesity is classified at 27.5. This is because the same BMI corresponds to a higher body fat percentage in many Asian populations.
Is BMI accurate?
BMI is a useful population-level screening tool but has well-documented limitations. It overestimates body fat in athletes and muscular individuals (a professional rugby player might have a BMI of 29 with very low body fat). It underestimates body fat in older adults who have lost muscle mass. It also doesn't account for where fat is distributed — visceral fat around the abdomen carries higher health risks than subcutaneous fat. Waist circumference and waist-to-hip ratio are better indicators of metabolic risk.
What is a healthy BMI for women vs men?
The WHO BMI classifications are the same for men and women. However, women naturally have a higher body fat percentage than men at the same BMI due to physiological differences (breast tissue, reproductive organs, hormonal differences). A healthy body fat percentage is approximately 20–35% for women and 8–24% for men. BMI does not measure body fat directly, so two people with identical BMIs can have very different body compositions.