BMI: What It Measures and Its Limits
Body mass index, or BMI, is a single number calculated from height and weight, and it is the most widely used measure of healthy weight in the UK. GPs use it, the NHS uses it to guide who qualifies for weight management services, and insurers use it too. It is useful, but it is also crude, and understanding both what it does and what it does not tell you is the key to using it sensibly rather than being ruled by it.
How BMI is calculated
BMI is your weight in kilograms divided by your height in metres squared. For example, a person who is 1.70 metres tall and weighs 75 kilograms has a BMI of roughly 26, which sits just inside the overweight range. You do not need to do the maths yourself; NHS calculators and apps will do it, and your GP will work it out at a check-up. The beauty of the number is that it is standardised, so it gives health professionals a common starting point in a way that raw weight alone cannot.
What the numbers mean
| BMI range | Category | What it usually means |
|---|---|---|
| Below 18.5 | Underweight | May need support to gain weight healthily |
| 18.5 to 24.9 | Healthy range | Lower risk of weight-related conditions |
| 25 to 29.9 | Overweight | Health risks start to rise |
| 30 to 39.9 | Obese | Higher risk of heart, diabetes and joint issues |
| 40 and above | Severely obese | May qualify for specialist support |
These bands are the standard NHS categories, and they are genuinely useful for population screening. For an individual, though, the picture is more nuanced, which is why the number should be a conversation starter, not a verdict.
Where BMI falls short
BMI measures weight, not body composition, so it cannot tell fat from muscle,. A fit rugby player may have a BMI in the obese range while carrying very little body fat, while an older adult can have a healthy BMI with low muscle mass. It also tends to underestimate risk in some ethnic groups, who may face higher risks at lower BMIs, and it says nothing about fitness, blood pressure or cholesterol. Our article on body fat versus weight goes deeper into why composition matters more than the number on the scales.
When BMI is still useful
None of this makes BMI useless. As a screening tool it flags people who might benefit from a closer look, and it remains the entry point for most NHS weight management services and for referral pathways, including those that consider medical treatment. It is also useful at a population level for tracking trends. If your BMI puts you in a higher band, it is a prompt to talk to your GP, not a reason to panic.
Better ways to track progress
If you are losing weight, BMI is a slow and unhelpful yardstick, because it moves in tiny increments and ignores what the weight is made of. Waist circumference is more meaningful, since fat carried around the middle carries more risk than fat elsewhere. How clothes fit, how you feel climbing stairs, and any changes in blood pressure or blood results all give a fuller picture. Our article on how a calorie deficit works is the practical engine behind the change, and our guide to healthy weight loss rates explains the pace to aim for.
Using BMI well
Treat BMI as a starting point: know your number, understand roughly where it sits, and use it as a reason to have a fuller conversation rather than a private judgement. Pair it with the measures that actually track change, like waist size and how you feel, and you will have a far more honest view of your health. Your weight is affected by more than calories alone, and our guide to weight and hormones covers some of the things BMI simply cannot see.