Body Mass Index has been the NHS's go-to weight indicator for 50 years because it's cheap, fast and roughly correlates with health risk at a population level. For individuals it's far less useful — a 90kg rugby player and a 90kg sedentary office worker of the same height have the same BMI but very different health profiles. This guide explains what BMI actually measures, where it goes wrong and which other numbers are worth tracking.
BMI = weight (kg) ÷ height (m)². Devised by Belgian astronomer Adolphe Quetelet in the 1830s as a population-level statistical tool, never as a clinical measure for individuals. NHS thresholds: under 18.5 underweight, 18.5-24.9 healthy, 25-29.9 overweight, 30-34.9 obese class I, 35-39.9 class II, 40+ class III.
For people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African Caribbean backgrounds, NICE recommends lower thresholds: 23 instead of 25 for overweight, 27.5 instead of 30 for obese — because cardiometabolic risk rises at lower BMIs in these groups.
Athletes and very muscular people are routinely classified as overweight or obese despite having low body fat. Older adults often have low BMI despite high body fat (sarcopenic obesity) because they've lost muscle.
BMI also says nothing about fat distribution. Two people with BMI 27 — one carrying weight evenly, one carrying it all around the abdomen — have meaningfully different cardiometabolic risk. The visceral (organ-surrounding) fat is the dangerous kind.
NICE's 2022 guidance promotes waist-to-height ratio as a complementary measure for adults with BMI under 35. Aim for waist circumference (measured at the navel, exhaled) less than half your height.
A 175cm adult should keep waist under 87.5cm. Above 0.5 raises cardiometabolic risk independently of BMI. Above 0.6 is high risk regardless of weight.
Healthy ranges: 14-24% for men, 21-31% for women, depending on age. Athletes 6-13% and 14-20%. Below 6% (men) or 14% (women) is the essential-fat floor and risks hormonal disruption.
Cheap home scales using bioelectrical impedance can be 5-8% off — useful only for tracking trends, not absolute measurement. DEXA scans (around £100 privately) and skinfold calipers in expert hands are accurate. For most people, waist measurement and progress photos beat trying to pin down body fat %.
BMI 25+ AND waist over half your height = lifestyle changes worth prioritising. BMI 30+ or 27.5+ in higher-risk ethnic groups = ask your GP about NHS weight management services or eligibility for new GLP-1 weight loss medications (semaglutide, tirzepatide) which are now available on the NHS for people meeting strict criteria.
Don't get fixated on a specific scale weight. A 5% reduction in body weight has been shown in multiple trials to meaningfully reduce blood pressure, cholesterol and type 2 diabetes risk, even if BMI is still 'overweight' afterwards.
On its own, no — many people with BMI 25-27 have excellent metabolic health. Risk rises if waist circumference, blood pressure, cholesterol or HbA1c are also abnormal.
Cost and time. BMI takes 30 seconds with no equipment. Accurate body fat measurement needs DEXA or hydrostatic weighing.
Yes, but using age- and sex-specific centile charts (UK90 or WHO 2007), not adult thresholds. Speak to your GP or school nurse rather than self-interpreting.
It's a population tool. NICE and the WHO are reviewing whether to add waist-to-height ratio and ethnicity-adjusted thresholds as standard. The 2022 NICE guidance already does this.