How to Calculate Your BMI (and Why the Number Alone Can Mislead You)

BMI has one job: compare your weight to your height so the same weight counts differently for a tall person and a short one. There are two versions of the same equation depending on the units you use.

Metric is the cleaner one. Take your weight in kilograms and divide it by your height in metres, squared. If you only know your height in centimetres, divide that by 100 first (170 cm becomes 1.70 m).

Imperial needs a conversion factor because pounds and inches do not divide neatly. Multiply your weight in pounds by 703, then divide by your height in inches squared. The 703 is what rescales the answer so both systems land on the same BMI number for the same person.

Take a metric example: someone who weighs 70 kg and stands 1.75 m tall. First square the height: 1.75 × 1.75 = 3.0625. Then divide the weight by that: 70 ÷ 3.0625 = 22.86. Round to one decimal and the BMI is 22.9. That falls inside the healthy range.

Now the same person in imperial: roughly 154 lb and 5 feet 9 inches, which is 69 inches (multiply feet by 12 and add the extra inches). Square the height: 69 × 69 = 4,761. Multiply weight by 703: 154 × 703 = 108,262. Divide: 108,262 ÷ 4,761 = 22.7. The tiny gap from the metric answer is just rounding in the unit conversions.

One more, to see the categories move. At 90 kg and 1.75 m: 90 ÷ 3.0625 = 29.4, which lands in the overweight band. Add or lose a few kilograms and you can watch the number cross a threshold — a reminder that the boundaries are lines drawn on a smooth scale, not cliffs where your health changes overnight.

The World Health Organization sorts adult BMI into the bands below. These are the same cut-offs most national health services use, and they apply to men and women aged roughly 18 to 65 who are not pregnant.

The obesity range is often split into three classes because the health risks tend to rise as you move up it. But read the far-right column as tendencies across large populations, not a personal diagnosis — plenty of people sit outside the healthy band and have normal blood pressure, blood sugar and cholesterol, and plenty inside it do not.

The short answer is 18.5 to 24.9. The longer answer is that this range comes from population studies where, on average, people in that band had the lowest rates of weight-related illness and death. Averages hide a lot of individual variation.

Several large observational studies have found the lowest mortality clustered somewhere around a BMI of 22 to 25, with risk ticking up at both ends — being underweight carries its own risks, from weaker bones to a struggling immune system, not just the obvious ones at the high end. So healthy is a range, not a single target, and the bottom of the scale is not automatically better.

It also helps to know which direction your number is trending and why. A BMI of 26 in a lifelong athlete means something very different from a BMI of 26 in someone who has quietly gained 10 kg over two years. BMI gives you the snapshot; only you and, ideally, a clinician can supply the story behind it.

BMI is a ratio of total weight to height. It has no way of knowing what that weight is made of or where it sits. That blind spot trips it up for whole groups of people.

If you fall into any of the categories below, your BMI number is a weak signal and should not be read literally.

Because BMI ignores fat distribution, it is best paired with a measure of where your weight sits — specifically around the middle, since fat packed around the organs (visceral fat) is more strongly linked to metabolic risk than fat on the hips or thighs.

Two simple additions cost nothing but a tape measure. Waist circumference: measured at the belly button, commonly cited high-risk thresholds are about 40 inches (102 cm) for men and 35 inches (88 cm) for women, with lower cut-offs used for some Asian populations. Waist-to-height ratio: a handy rule of thumb is to keep your waist under half your height — for a 70-inch person, that means under 35 inches.

If you want more precision, body-fat measurements (from calipers, bioelectrical scales, or a DEXA scan) tell you the fat-versus-muscle split that BMI cannot. None of these are perfect either, but together they turn a single flag into a fuller picture.

Calculate your BMI, note which band it falls in, and then ask the more useful questions: is it changing over time, how does my waist measurement look, and how do I actually feel and function day to day? A number on its own does not tell you to do anything.

If your BMI sits well below 18.5 or in the obesity range, or if it has shifted noticeably without you trying, that is a reasonable prompt to talk to a doctor — not to panic, but to get context BMI cannot provide, like blood pressure, blood sugar and cholesterol. Those markers often matter more than the ratio itself.

None of this is medical advice, and guidelines vary by country and by person. BMI is a quick, free, decent screening tool used sensibly, and a misleading one used as a scorecard. Know the formula, know its limits, and let it start a conversation rather than end one.