Working out your BMI: formula, bands and limits

You type your weight and your height into something, a number comes out, and then what? Either it is absurd (0.0023), or it is plausible but you have no idea whether 24.4 and 25.6 really tell two different stories. Both problems have the same root: BMI is a single division that people happily perform in the wrong units, and then read as a verdict when it is only a screening indicator.

The formula, and the two ways of getting it wrong

Body mass index is weight in kilograms divided by height in metres squared. For 70 kg and 1.75 m: 1.75 × 1.75 = 3.0625, then 70 ÷ 3.0625 = 22.9.

The first mistake, by a wide margin the most common, is entering the height in centimetres. 70 ÷ 175² gives 0.0023. That result is so far off the scale that you spot it, but the reverse is nastier: typing 1.75 into a field that expects centimetres produces a six-figure BMI (228,571 for 70 kg), and some pages will display it without blinking.

The second mistake is mixing the two systems. In imperial units the formula changes shape and becomes 703 × weight in pounds ÷ height in inches squared. There is nothing mysterious about the 703: it is simply the conversion (one pound = 0.45359237 kg, one inch = 2.54 cm) folded into a single constant. For 154 lb and 69 in: 703 × 154 ÷ 4,761 = 22.7. The same body in metric, 69.9 kg at 1.753 m, gives the same 22.7.

The BMI calculator handles both systems and converts your entries from one to the other without shifting the result on screen. It also turns down implausible values instead of doing the sum anyway: a height of 1.75 in the centimetres field brings up an error message telling you the range that field accepts, not a six-figure BMI.

Reading the number: the WHO's seven bands

The WHO adult scale, in the simplified version most calculators use, reads as seven bands — the WHO additionally splits thinness into moderate (16–17) and mild (17–18.5). Upper bounds are exclusive: 24.9 is still a healthy weight, 25.0 tips over.

These boundaries are statistical conventions drawn from large populations, not biological switches. Nothing happens inside a body between 24.9 and 25.1. What counts is the order of magnitude, and above all the direction the figure moves in from one year to the next.

The healthy weight range, and why height matters more than you think

The formula inverts: weight = BMI × height². At 1.75 m, just under 5 ft 9 in, the 18.5–24.9 band works out at 56.7 kg to 76.3 kg — close to 20 kg of spread, a touch under 9 st to a touch over 12 st. It is a range, not a target.

Height enters the calculation squared, though, so a rough measurement costs you. Take the same 70 kg: at 1.73 m the BMI is 23.4, at 1.77 m it is 22.3. Four centimetres of uncertainty, an inch and a half, shifts the result by more than a full point. Worse, near a boundary a single centimetre is enough to change category: 79 kg at 1.78 m gives 24.9, so a healthy weight, while the same 79 kg at 1.77 m gives 25.2, so overweight.

So, a few precautions when measuring, worth more than the calculator's decimal place:

• Barefoot, with heels, bottom and shoulder blades against a wall, looking straight ahead.
• In the morning: you lose 1 to 2 cm over the course of a day as the intervertebral discs compress.
• Don't reuse the height you once wrote on a form — a gym sign-up, an insurance questionnaire. It was self-reported and never updated, and past 50 people genuinely do lose centimetres.

What BMI does not measure

BMI ignores body composition entirely. It cannot tell a kilo of muscle from a kilo of fat, nor where that fat sits. The cases where it misleads are well documented, and there are plenty of them:

• High muscle mass. A prop forward or a competitive powerlifter clears 25 easily with no excess body fat.
• Older people. Age-related muscle loss can hold BMI inside the healthy band while fat mass has risen and lean mass has fallen.
• Children and teenagers. Adult cut-offs do not apply at all. A child's BMI is read off a centile chart, by sex and by age to the month — the age calculator gives that exact age in years and months if the chart calls for it.
• Pregnancy and breastfeeding. The index means nothing during that period; the pre-pregnancy BMI is the reference.
• Amputation, oedema, heavy fluid retention. The weight on the scales no longer represents what the formula assumes.

The simplest complement is waist circumference, measured midway between the lowest rib and the top of the hip bone, at the end of a normal breath out. The WHO uses 94 cm in men and 80 cm in women, about 37 in and 31.5 in, as the first threshold worth noting, and 102 cm and 88 cm, about 40 in and 34.6 in, as the high-risk threshold. Two people with the same BMI can be ten centimetres apart at that level.

A BMI outside the range is not a diagnosis. It is a signal worth raising with your GP, who has measurements available that the formula ignores.

Tracking the trend without being fooled by the scales

One BMI point is about 2.6 kg for someone 1.60 m tall, 3.1 kg at 1.75 m and 3.6 kg at 1.90 m — roughly half a stone in the middle of that range. Which means the day-to-day swings in weight, 1 to 2 kg between getting up and going to bed depending on meals, hydration and where you are in your cycle, already amount to 0.3 to 0.7 of a BMI point. Recalculating every day measures nothing but that noise.

A method that holds up: weigh yourself once a week, always on the same day, on waking, after a trip to the loo, before breakfast, on the same scales and on a hard floor (carpet throws the reading out by several hundred grams). Then compare monthly averages rather than isolated points. To express the gap as a percentage rather than in kilos, the percentage calculator handles the change from a starting value to a final value directly.

Converting stones, pounds, feet and inches before you calculate

British bathroom scales talk in stones and pounds, and the stone is the one unit no version of the formula accepts: one stone is 14 pounds, or 6.35 kg. A weight given as 11 st 4 lb is 158 pounds, so 71.7 kg. For height, a measurement written 5'9" is 5 × 12 + 9 = 69 inches, or 175.3 cm.

You can enter those converted values in the BMI calculator by choosing the imperial system: the weight in pounds (158), then the height split across the two fields for feet and inches (5 and 9). Or use the unit converter if you need the plain equivalents for other purposes — it covers pounds, ounces, stones, inches, feet and yards.

One last remark: a BMI is health data. Convertu's calculator runs entirely in your browser, with no account and nothing sent to a server — your weight and your height never leave your device.