What your BMI can tell you, and what it cannot

Someone is 1.70 metres tall and weighs 70 kilograms. Their BMI is 24.2. Is that good? The honest answer is: it is a starting point for a conversation, not a verdict on anyone’s health, and the bands attached to it are rougher than most charts make them look.

BMI stands for body mass index. It is a screening tool: cheap, quick and useful across large groups of people. Used on one individual it can mislead. This article is general information, not medical advice. For anything about your own health, a doctor looking at the whole picture is worth far more than any single number.

The formula is one line

The usual bands

The World Health Organization’s standard categories are the ones used on most charts and apps. They were set from studies of populations in which these ranges tracked health risk reasonably well on average.

Notice how arbitrary the edges are. A BMI of 24.9 is “normal” and 25.0 is “overweight”, but nobody’s body changes at that line. These are bands drawn on a smooth curve.

For many people of South Asian origin, the lines move

Research found that at the same BMI, many Asian populations tend to carry more body fat and a higher risk of diabetes and heart conditions than the standard bands imply. Because of that, many Asian and Indian guidelines use lower cut-offs: commonly 23 for overweight and 27.5 (or 25 in some Indian consensus guidance) for obesity. Which set your doctor uses can differ, so ask rather than assume.

Where BMI quietly gets it wrong

• It cannot tell muscle from fat. A strength athlete can land in the “overweight” band while being lean, because muscle is dense and weighs a lot.
• It ignores where the weight sits. Fat around the middle is linked to more risk than the same amount on hips and thighs, and BMI cannot see the difference.
• It changes with age. Older adults lose muscle, so the same BMI can hide more fat. Children and teenagers use age-specific charts instead of these adult bands.
• Pregnancy and some conditions change weight in ways BMI is not designed for.
• Height rounding matters. Being off by two centimetres can move you a full point on the scale.

A tape measure fills in the gaps

Many clinicians now pair BMI with waist size, or with waist-to-height ratio: your waist divided by your height, in the same units. A commonly used rule of thumb is to keep your waist under half your height, which for 170 cm means under 85 cm. It is not a substitute for medical advice, but it captures what BMI misses: where the weight is carried.

How to use the number sensibly

1. Treat it as a first glance, like a thermometer reading rather than a diagnosis.
2. If you are well outside the normal band, or the number changed a lot without reason, mention it to a doctor who can look at blood tests, family history and the rest.
3. If you train regularly and feel strong and well, a high BMI on its own is not a reason to panic. Waist size and fitness tell you more.
4. Do not chase a number. If a chart is making you anxious about food or your body, talk to a professional rather than adjusting on your own.