BMI for Adults: What Your Number Means (and Why It Isn't the Whole Story)
BMI is the UK's go-to weight metric — but it was invented in 1832 and ignores muscle, frame size and where you carry fat. Here's how to use BMI properly, what it gets right, and the two extra measurements you should track alongside it.
BMI — Body Mass Index — is the number your GP uses, the number your gym signs you up with, and the number every UK weight-loss programme starts with. It's also nearly 200 years old, was never designed for individuals, and has some serious blind spots. Here's how to use it without being misled by it.
The formula
BMI = your weight in kilograms ÷ (your height in metres)². So an 80 kg adult who's 1.75 m tall has a BMI of 80 ÷ (1.75 × 1.75) = 26.1.
The standard categories (NHS)
| BMI | Category |
|---|---|
| below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25 – 29.9 | Overweight |
| 30 – 39.9 | Obese |
| 40 + | Severely obese |
Note: thresholds are slightly lower for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family backgrounds because cardiometabolic risk rises at lower BMIs.
What BMI gets right
- It's free, fast, and only needs a tape and a scale.
- For populations, it correlates well with risk of heart disease, type 2 diabetes and several cancers.
- Tracking changes in your own BMI over time is genuinely useful — your trend is meaningful even when the absolute number isn't.
What BMI gets wrong
- It can't tell muscle from fat. A 110 kg rugby player and a 110 kg sedentary office worker can have the same BMI of 31 and very different health risks.
- It ignores fat distribution. Fat carried around the middle ("visceral fat") is metabolically dangerous; fat carried on the hips and thighs is largely benign. BMI sees both as identical.
- It doesn't scale linearly with height. Very tall and very short people get systematically misclassified.
- It's a snapshot, not a trajectory. Someone at BMI 28 who's been losing 0.5 kg/week for three months is in a very different health position from someone at 28 who's been gaining.
The two measurements to track alongside
If you want a fuller picture in under sixty seconds:
- Waist circumference. Wrap a tape around your middle at the level of your belly button. Under 94 cm (men) or 80 cm (women) = low risk. Over 102 cm or 88 cm respectively = increased risk regardless of BMI.
- Waist-to-height ratio. Your waist in cm divided by your height in cm. Aim for under 0.5. ("Keep your waist to less than half your height" — the rule even Public Health England has started using.)
How to use BMI sensibly
Treat BMI like a bathroom scale: useful for direction, not for absolutes. The single most informative thing you can do is combine three numbers — BMI, waist circumference, and the trend of both over the last 8–12 weeks. That's a real picture. A single BMI reading is just a starting point.