What your BMI means — and what it cannot tell you

Body mass index is one number built from two measurements. It is quick and useful for spotting trends across large groups of people, and much blunter when applied to one person. Knowing both sides stops it from misleading you.

How BMI is calculated

BMI = weight (kg) ÷ height (m)²

Someone 1.75 m tall weighing 72 kg: 1.75 × 1.75 = 3.0625, and 72 ÷ 3.0625 = 23.5. In pounds and inches the equivalent is 703 × weight ÷ height², and the BMI Calculator accepts either.

The imperial version worked through: someone 5 ft 9 in tall is 69 inches, and 69 × 69 = 4,761. At 160 lb, 703 × 160 = 112,480, and 112,480 ÷ 4,761 = 23.6. If your measurements are in a mixture of units, convert them first with the Weight Converter or the Length Converter.

What the categories mean

The World Health Organization's adult categories are:

• below 18.5 — underweight
• 18.5 to 24.9 — healthy weight
• 25 to 29.9 — overweight
• 30 and above — obese

The obese range is further divided into class I (30 to 34.9), class II (35 to 39.9) and class III (40 and above). The categories are the same for men and women.

Working backwards gives a healthy weight range for a height. At 1.75 m that is roughly 56.7–76.3 kg — the weights that put BMI between 18.5 and 24.9. The table shows the weight at each end of that range for a few heights.

Notice how wide each range is — about 16 to 21 kg. BMI does not name an ideal weight; it describes a broad band.

Where the number came from

The formula was devised in the 1830s by the Belgian statistician Adolphe Quetelet, who was studying the proportions of populations, not the health of patients. The name “body mass index” came much later, from a 1972 paper by the physiologist Ancel Keys, who recommended it as a convenient measure for studies of large groups. That history explains both its strength and its weakness: it was built to describe populations cheaply, and it still does that well.

What BMI cannot tell you

• Muscle and fat look the same to it. BMI only knows weight, so muscular people often read as overweight.
• It ignores where fat sits. Fat around the waist matters more for health than fat elsewhere, and BMI cannot see the difference.
• It does not account for age, sex or ethnicity, all of which change what a healthy body composition looks like.
• It does not apply to children (who are assessed against growth charts), during pregnancy, or at very short or very tall statures.

The ethnicity point has a practical consequence. Research reviewed by a WHO expert group found that health risks rise at lower BMI values in many Asian populations, and it suggested 23 and 27.5 as additional points at which to take action. Several countries use lower thresholds in their own guidance for that reason.

Height is a quieter problem. Because the formula squares height, it tends to read a little high for tall people and a little low for short people of the same build. At average heights the effect is small; at the extremes it is one more reason not to over-read a single number.

Other useful measures

Waist circumference and waist-to-height ratio say more about where fat is carried, which is why clinicians often use them alongside BMI. A tape measure and a doctor's appointment tell you more than any calculator.

• Waist-to-height ratio. A widely used rule of thumb is to keep your waist measurement below half your height. It needs no chart and applies to both sexes.
• Waist circumference. WHO guidance associates increased risk with a waist above 94 cm for men and 80 cm for women, and substantially increased risk above 102 cm and 88 cm. As with BMI, the thresholds were drawn mainly from European populations.

To measure your waist, stand up, breathe out normally, and pass the tape around your middle midway between the bottom of your ribs and the top of your hip bones — usually just above the navel, not where your trousers sit.

Getting a reliable reading

Small measuring errors move the result more than you might expect, because height is squared. Measure height without shoes, standing against a wall, and do not rely on a figure from years ago. Weigh yourself at the same time of day, ideally in the morning, since body weight varies by a kilogram or more over a day. If you are tracking change, the trend over several weeks means far more than any single reading.

A note on using it

BMI is a screening measure, not a diagnosis or an assessment of your health. If your result worries you — in either direction — talk to a qualified healthcare professional, who can look at the whole picture rather than one number.