BMI vs body fat: what each one actually tells you

Body mass index is your weight in kilograms divided by your height in metres squared. It was devised in the 1830s by a Belgian statistician who was explicitly trying to describe the average characteristics of populations, and who said it should not be used to judge individuals.

Two centuries later it is printed on your GP record.

What BMI is good at

It is cheap, needs no equipment beyond scales and a tape measure, and at a population level it tracks health risk well. If you want to know whether a country’s health is deteriorating, BMI will tell you.

For an individual it works as a first filter. For most adults who are not unusually muscular, a BMI of 35 tells you something real.

What it cannot do

It cannot tell muscle from fat. A prop forward and a sedentary man of the same height and weight get the same number.

It cannot tell you where fat is. Two people with a BMI of 29 — one carrying it around the hips, one around the middle — have meaningfully different cardiometabolic risk. Abdominal fat is the type most associated with blood pressure, blood glucose and cholesterol problems.

It does not adjust for age. Older adults lose muscle and gain fat at an unchanged BMI, so the number understates fat mass.

It was built on European populations. Risk starts to rise at lower BMI values in some groups, which is why UK guidance uses lower thresholds of 23 and 27.5 for people of South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds ^[1].

What to use alongside it

Waist-to-height ratio

The single most useful addition, and NICE now recommends it. Keep your waist to less than half your height. A ratio of 0.5 to 0.59 indicates increased central adiposity; 0.6 or above, further increased ^[1].

The measurement: halfway between the lowest rib and the top of the hip bone, after breathing out normally. Not where your trousers sit.

NICE advises using it only when BMI is under 35 — above that, BMI is already telling you what you need to know ^[1].

Waist circumference alone

Cruder, but still informative. Increased risk is generally taken as 94 cm or more in men and 80 cm or more in women.

The things that actually get measured in a clinic

Blood pressure, HbA1c, cholesterol, liver function. These describe what your body is doing, which is the question BMI is a proxy for.

Body fat percentage

More directly relevant than BMI — and much harder to measure well.

Home scales are worth having if you treat the number as a trend under consistent conditions — same time, same hydration state — and worthless if you treat any single reading as fact.

The more useful framing

A 2025 international commission proposed separating pre-clinical obesity — excess fat without organ dysfunction — from clinical obesity, where excess fat is measurably affecting how organs work ^[3]. It is a better question than “what is my BMI”, because it asks what the fat is doing rather than how much there is.

Practically, for an individual, the useful set is: BMI as a filter, waist-to-height ratio for distribution, and blood pressure, HbA1c and lipids for consequence.

When to speak to a clinician

Worth an appointment if you:

• have a BMI of 30 or above, or 27.5 or above with a South Asian, Chinese, Black African or African-Caribbean background
• have a waist measurement over 94 cm (men) or 80 cm (women)
• have not had blood pressure, HbA1c and cholesterol checked in the last few years
• are being told your BMI is fine but have symptoms — thirst, fatigue, snoring, breathlessness
• are very muscular and have been given advice based on a BMI that does not reflect your body composition

This article is for general information and isn’t a substitute for advice from your own clinician. If you feel unwell, contact your GP or NHS 111. In an emergency, call 999.