BMI and body metrics calculator

The thresholds are not universal, and that matters

The World Health Organization's familiar cut-offs — overweight at 25, obese at 30 — were derived largely from European populations. They understate risk in South and East Asian populations, where cardiometabolic problems appear at a lower BMI and at a lower body weight. India, China, Japan and Singapore all use lowered thresholds where overweight begins at 23 and obesity at 25.

The gap is not academic. Someone 170 cm and 70 kg has a BMI of 24.2: comfortably in the healthy band under the WHO scale, and overweight under the Asian one. Most calculators only offer the WHO numbers, which quietly gives the wrong answer to a very large share of the world. Both scales are here and you choose which applies to you.

What BMI can and cannot tell you

BMI is your weight divided by the square of your height. That is all it is. It cannot distinguish muscle from fat, so it classifies a great many athletes as overweight and some sedentary people with little muscle as healthy. It says nothing about where weight sits, which is the part most strongly linked to metabolic risk. It is not valid for children, who need age-and-sex percentile charts, nor during pregnancy, nor for people with substantially different limb proportions.

It survives because it takes two measurements anyone can make and correlates well enough across large groups. It is a screening indicator for populations that has been pressed into service as a personal diagnosis, and it does not carry that weight.

Waist-to-height is the better single number

If you are willing to take one more measurement, it is worth more than the BMI. Waist circumference divided by height predicts cardiometabolic risk better than BMI does, precisely because it captures where the weight is rather than only how much of it there is. The usual guidance is to keep your waist under half your height, which is easy to remember and easy to check. Enter a waist measurement and the ratio appears alongside everything else.

Energy: BMR and daily needs

Basal metabolic rate is what your body uses at complete rest. This uses the Mifflin-St Jeor equation, which replaced the older Harris-Benedict as the recommended estimate because it is more accurate in modern populations — though "more accurate" still means roughly plus or minus ten per cent for any individual. Multiplying by an activity factor gives a daily total. Treat both as a starting point to adjust from, not as a target to hit.

The equation was derived with separate male and female forms, which is why it asks; you can skip it and take the BMI alone.

A healthy weight is a range

The result gives you the actual weight range that corresponds to a healthy BMI at your height, on whichever scale you chose, and how far outside it you are if you are outside it. That is more useful than a category label, because it is expressed in the unit you can act on.