BMI Calculator

How to use this calculator

1. Use the units you actually think inMost people in Britain know their height in feet and inches and their weight in stones, and converting in your head introduces errors. The calculator does it for you and shows the metric equivalents in the working.
2. Measure height without shoesStanding straight against a wall with heels together. An inch of error changes BMI by about 0.7, which is enough to cross a category boundary if you are near one.
3. Weigh consistentlyBody weight swings by two to four pounds across a day with food and fluid. Morning, after the bathroom, before eating is the standard comparison point.
4. Read the healthy range, not just the numberThe breakdown converts the category thresholds back into stones and pounds for your height, which is far more actionable than an index value.

How the calculation works

Weight in kilograms divided by height in metres squared. Working in imperial, the calculator converts first — stones to kilograms at 6.35029 and feet and inches to metres at 0.0254 per inch — then applies the same formula.

Someone 5 feet 9 inches and 12 stone 4 pounds is 1.7526 m and 78.02 kg, giving a BMI of 78.02 ÷ 3.0716 = 25.4.

The squaring is the formula's known weakness. Human bodies scale closer to the cube of height than the square, so BMI systematically over-reads for tall people and under-reads for short ones — by roughly a point at either extreme of normal adult height. A very tall person can be classified as overweight at a body composition that would be entirely healthy.

The index was devised by Adolphe Quetelet in the 1830s to describe the distribution of weight across a population, not to assess individuals. It was adopted as a clinical screening tool much later, and much of the criticism of it stems from that change of purpose rather than from any error in the arithmetic.

The categories and what they mean in stones

NHS guidance applies lower thresholds for people of Black African, African-Caribbean and South Asian family background, where health risks begin at a lower BMI. In those groups the overweight threshold is treated as 23 rather than 25, which is a meaningful difference and not widely known.

Worked example

Someone 5 feet 9 inches tall weighing 12 stone 4 pounds.

Height converts to 1.7526 m and weight to 78.02 kg. BMI is 78.02 ÷ (1.7526 × 1.7526) = 78.02 ÷ 3.0716 = 25.4, which sits just inside the overweight category.

The healthy range for that height runs from a BMI of 18.5 to 24.9, which is 56.8 kg to 76.5 kg — 8 stone 13 pounds to 12 stone 1 pound. So the figure is about 3 pounds above the top of the healthy range.

Three pounds is within normal daily fluctuation, which is exactly why treating 24.9 and 25.0 as meaningfully different is a mistake. Nothing changes about anyone's health across that boundary. The categories are statistical conventions drawn on a continuous distribution, and a figure sitting near a line should be read as near a line rather than as belonging firmly to one side.

What BMI cannot see

BMI knows two numbers about you and nothing else. It cannot distinguish muscle from fat, which is denser, so a well-trained athlete can register as overweight or obese while carrying very little body fat. Rugby players and weightlifters produce famously misleading figures.

The reverse case matters more and is discussed less. Someone with low muscle mass and a high proportion of body fat can sit comfortably in the healthy range while carrying genuine metabolic risk — a pattern common enough in older adults to be well documented.

Fat distribution matters more than total fat, and BMI is blind to it. Visceral fat around the organs drives insulin resistance and cardiovascular risk far more than subcutaneous fat elsewhere. Two people with identical BMI and very different waist measurements are in different situations.

The cheap and useful alternative is waist-to-height ratio: keep your waist measurement below half your height. It needs only a tape measure, captures the distribution question BMI ignores, and NHS guidance now recommends using it alongside BMI rather than after it.

BMI is also not valid during pregnancy, for children — who are assessed against age and sex percentiles — or where there is significant oedema or muscle wasting. In those cases it is not a weak indicator; it is the wrong one.

Assumptions and limitations

• Applies to adults only — children are assessed against age and sex percentile charts
• Uses the standard WHO thresholds; NHS guidance applies lower ones for some ethnic backgrounds
• Cannot distinguish muscle from fat, so it misclassifies muscular and low-muscle bodies in opposite directions
• Says nothing about fat distribution, which matters more for metabolic risk than total fat
• Not valid during pregnancy, or where there is significant oedema or muscle wasting
• A screening indicator, not a diagnosis, and no substitute for a clinical assessment

Questions people ask about this in United Kingdom