BMI: What It Misses, and What to Measure Instead

BMI is weight divided by height squared. It was devised in the nineteenth century by a Belgian statistician who was interested in describing populations, and who explicitly said it should not be used to judge individuals.

It is now used to decide who gets a knee replacement, who is eligible for fertility treatment, and who can be prescribed weight loss medication. That is a considerable amount of weight for one crude ratio to carry, and it is worth understanding where it holds up and where it does not.

What BMI is actually good at

Credit where it is due. Across large populations, BMI tracks health outcomes reasonably well, it costs nothing, it needs no equipment beyond scales and a tape measure, and anyone can calculate it. As a way of comparing the health of one country to another, it works.

The problem is not the measure. It is the leap from population to person.

Where it goes wrong

It cannot tell muscle from fat

The textbook example is the rugby player classed as obese. Real, but not the version that matters most.

The more common and more consequential error runs the other way. Someone can sit squarely in the "normal" BMI range while carrying very little muscle and a substantial amount of fat around the organs — sometimes called being thin on the outside, fat on the inside. Their BMI is reassuring, their metabolic risk is not, and nobody looks any further because the number was fine.

It says nothing about where the fat is

This is the central failing. Fat around the abdomen and liver behaves entirely differently from fat on the hips and thighs — it is metabolically active, drives insulin resistance, and is what links weight to type 2 diabetes, high blood pressure and heart disease.

Two people with identical BMIs can carry very different risks depending on where they store it. BMI cannot distinguish them; a tape measure can.

It is misleading with age

Muscle mass falls with age while fat rises. An older person's BMI can stay unchanged for twenty years while their body composition changes considerably underneath it. Stable weight is not the same as stable health.

The thresholds are not the same for everyone

This is the part least widely known, and it matters a great deal in Britain.

People of South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean family background develop type 2 diabetes and cardiovascular disease at lower BMIs than white European populations. The same number carries more risk.

UK guidance reflects this: for these groups, the overweight threshold drops to around 23, and the obesity threshold to around 27.5. Someone of South Asian background at a BMI of 25 is not comfortably in the middle of the range — they are already at meaningfully raised risk, and are frequently told the opposite.

The measure worth using instead

Keep your waist to less than half your height.

That is the whole rule. It is called waist-to-height ratio, it applies regardless of sex, ethnicity or age from about five years old upwards, and it predicts cardiometabolic risk better than BMI does. It is now recommended in UK guidance alongside BMI.

• If you are 5ft 10in (178cm), aim for a waist under 89cm, or about 35 inches
• If you are 5ft 4in (163cm), aim for a waist under 81cm, or about 32 inches

How to measure your waist properly

Most people measure the wrong place, or hold their breath.

1. Find the midpoint between the bottom of your lowest rib and the top of your hip bone. This is usually a little above the navel — it is not your trouser size, which is measured lower and generously
2. Stand up, tape flat against the skin, snug but not compressing
3. Breathe out normally and measure at the end of the breath. Do not hold it in
4. Measure at the same time of day, and record it

What to track instead of the scales

If the aim is health rather than a number, these tell you more:

• Waist measurement, monthly. It responds to change before the scales do, and it moves in the right direction with exercise even when weight does not
• Blood pressure
• HbA1c, cholesterol and liver function — fatty liver is common, silent, and one of the earliest signs that visceral fat is causing trouble
• What you can do. Whether you can climb stairs without stopping, carry shopping, or get off the floor unaided is a better health measure than any ratio

Someone whose weight has not changed but whose waist has reduced by 5cm and whose HbA1c has fallen has made real progress. The scales are the least informative instrument in the house.

When BMI is used as a gate

BMI thresholds are used to decide access to some treatments — certain operations, fertility treatment, weight loss medication. Sometimes there is a genuine clinical reason: surgical and anaesthetic risk does rise with weight, and that is a real consideration rather than a judgement.

Sometimes the threshold is doing less work than it appears to. If you feel a symptom has been attributed to your weight without being properly investigated, that is a reasonable thing to say out loud, and it is worth asking directly: what would you be considering if I were thinner, and can we rule that out? It is a fair question and a useful one.

The practical summary

• BMI is a population tool being used as a personal verdict
• Waist-to-height ratio under 0.5 is a better target, and easier to remember
• The thresholds are lower if your family background is South Asian, Chinese, Middle Eastern, Black African or African-Caribbean — and that is frequently not mentioned
• Track waist, blood pressure and blood results rather than weight alone