BMI Calculator

Body Mass Index is a single number that places your weight into a category for your height. It was designed in the 19th century as a population statistic and adopted by the NHS and WHO as a quick screening tool — useful as a starting point, not as a verdict on your health.

The formula

In metric, BMI is your weight in kilograms divided by your height in metres squared:
BMI = kg ÷ (m × m)

For imperial units, the formula multiplies by 703 to handle pounds and inches:
BMI = (lb ÷ (in × in)) × 703

A worked example

Someone 175cm tall weighing 78kg has a BMI of 78 ÷ (1.75 × 1.75) = 78 ÷ 3.0625 = 25.5. That sits just inside the overweight band — the kind of borderline number where a waist measurement or a body-fat estimate is more useful than the BMI alone.

What the categories mean

• Under 18.5 — underweight. May indicate undernutrition or an underlying condition worth checking with a GP.
• 18.5 to 24.9 — healthy range. Associated with the lowest population-level risk of weight-related illness.
• 25 to 29.9 — overweight. Slightly raised long-term risk of type 2 diabetes, high blood pressure and heart disease.
• 30 and over — obese. Risk rises more steeply; the NHS typically recommends a structured weight-management plan.

The NHS lowers the overweight and obese thresholds (to 23 and 27.5) for people of South Asian, Chinese, Middle Eastern, Black African and African Caribbean background, who tend to develop weight-related health issues at lower BMIs.

Why this matters

Knowing where you sit on the BMI scale is most useful as a prompt. A figure inside the healthy band is reassurance you can park; a borderline overweight result is a signal to look at your waist measurement and overall habits before it drifts further; a result in the obese range is a reason to talk to your GP about a proper plan rather than another short-term diet. The number itself doesn't change anything — what you do with it can.

Where BMI falls short

BMI only knows two things — your weight and your height — so it can't see muscle, bone density, fat distribution or age. Rugby players and serious lifters routinely score as overweight or obese with very low body fat. Older adults can score in the healthy range while carrying enough visceral fat to push their cardiovascular risk up. And a pregnant woman's BMI tells you almost nothing useful. Pair the number with a waist measurement, a body-fat estimate, and how you actually feel.

Common mistakes

• Mixing units — entering height in cm but weight in pounds, or vice versa. Pick metric or imperial and stick with it.
• Using clothing-tag height rather than a measured value. A 2cm difference can shift you a whole category at the boundaries.
• Treating BMI as a body-fat percentage. It isn't — see the Body Fat Calculator for that.
• Re-weighing daily and worrying about tiny shifts. Weigh weekly at the same time, ideally first thing.

Related tools: Ideal Weight Calculator · Body Fat Calculator · Calorie Needs Calculator