BMI Calculator

How the BMI Calculator Works

BMI is your weight in kilograms divided by your height in metres squared. Someone weighing 70kg at 1.70m tall has a BMI of 70 ÷ (1.70 x 1.70) = 24.2, which sits at top of healthy range.

Enter your measurements in whichever units you know them and calculator handles conversion. It is same formula used by the NHS, World Health Organization and GPs across the UK as a population level screening tool.

NHS BMI Categories

For most adults the NHS classifies BMI as follows:

• Below 18.5: underweight
• 18.5 to 24.9: healthy weight
• 25 to 29.9: overweight
• 30 to 34.9: obese class I
• 35 to 39.9: obese class II
• 40 and above: obese class III

These thresholds apply equally to men and women aged 18 and over. Children and teenagers are assessed completely differently, using age and sex specific centile charts rather than fixed cut offs, so adult BMI figures should never be applied to anyone under 18.

Why Ethnicity Changes the Thresholds

The standard categories were developed mainly from data on White European populations. NICE and NHS recommend lower thresholds for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African Caribbean family background, because health risks such as type 2 diabetes rise at lower BMI values in these groups.

For these backgrounds, increased risk starts at a BMI of 23 rather than 25 and high risk starts at 27.5 rather than 30. If that applies to you, read your result against the adjusted thresholds rather than standard bands.

What BMI Cannot Tell You

BMI is a useful screen but a blunt one and knowing its limits stops it misleading you.

It cannot distinguish muscle from fat, so a muscular rugby player can register as overweight while carrying very little body fat. It says nothing about where fat is stored, yet fat around middle carries more health risk than fat elsewhere. And it does not adjust for the natural shift from muscle to fat that comes with age.

That is why the NHS pairs BMI with a waist measurement. Wrap a tape midway between bottom of your ribs and top of your hips. A waist above 88cm for women or 102cm for men signals increased risk even when BMI reads healthy. Our Waist to Height Ratio Calculator adds another useful check.

What to Do With Your Result

If your BMI sits outside healthy range in either direction, result is a prompt rather than a diagnosis. A BMI below 18.5 without intentional weight loss is worth raising with your GP. For a result in overweight or obese range, NHS recommends gradual change through a balanced diet and regular activity at a rate of around 0.5 to 1kg per week rather than crash dieting.

BMI is one data point. Blood pressure, waist size, fitness, diet quality, family history all matter alongside it and a GP can put your number in the context of the full picture.