What this calculates
Body mass index compares your weight to your height, giving a single number that places you in one of four bands: underweight, healthy, overweight or obese. Enter your measurements in metric or imperial and the result updates as you type.
Alongside the number, this shows where you sit on the scale and what weight range would put you in the healthy band at your height — usually the more actionable of the two.
The formula
BMI is weight in kilograms divided by height in metres squared. Everything else is unit conversion.
Body mass index
BMI = kg ÷ m²From centimetres
m = cm ÷ 100From feet, inches and pounds
m = (ft × 12 + in) × 2.54 ÷ 100 · kg = lb × 0.45359237The squared term is what makes BMI awkward. Because height is squared and weight is not, the index drifts upward for tall people and downward for short ones, even at identical body composition.
Worked example
Now read it the other way round. At 1.78 m, the healthy band of 18.5 to 25 works out to roughly 58.6 kg to 79.2 kg. So this person is about 3 kg above the top of the healthy range — a much more useful piece of information than the label "overweight".
When to use it, and the mistakes to avoid
BMI is worth checking when you want a quick screening figure, when tracking a deliberate change in weight over months, or when a form asks for it. It is a starting point for a conversation, not a verdict.
Where people go wrong:
- Treating it as a diagnosis. BMI cannot see body composition. Muscular people are routinely classed as overweight, and it says nothing about fitness.
- Reading a single reading too closely. Weight swings by a kilo or two across a day with food and hydration. The trend over months is the signal.
- Using adult bands on children. Under 18s are assessed on percentile charts for their age and sex, not these thresholds.
- Ignoring ancestry. Several guidelines use lower cut-offs for people of South Asian and Chinese descent, because risk rises at a lower BMI.
- Applying it in pregnancy. BMI is not valid while pregnant or breastfeeding.
- Confusing the units. Pounds with metres, or kilograms with inches, produces a number that looks plausible and is nonsense. Pick a system and stay in it.
FAQ
What counts as a healthy BMI?
The World Health Organization puts the healthy band between 18.5 and 25. Below 18.5 is classed as underweight, 25 to 30 as overweight, and 30 or above as obese. Those cut-offs are for adults; children and teenagers are assessed against age and sex percentiles instead.
Why is BMI criticised so much?
Because it only knows two things about you: your height and your weight. It cannot tell muscle from fat, it says nothing about where fat sits, and it ignores age, sex and frame. A lean athlete and a sedentary person of the same build get the same number, which is why BMI screens populations well and individuals poorly.
Should the thresholds be different for South Asian people?
Many health bodies think so. Research shows that people of South Asian, Chinese and some other Asian ancestries develop diabetes and heart disease at lower BMIs, and several national guidelines lower the overweight threshold to around 23 and the obesity threshold to around 27.5 for these groups. This calculator shows the standard WHO bands; if that applies to you, read your result against the lower cut-offs and talk to a doctor.
Is waist measurement a better guide?
For health risk, often yes. Waist circumference and waist-to-height ratio capture abdominal fat, which is the fat most strongly linked to metabolic disease. A useful rule of thumb is to keep your waist under half your height. Use it alongside BMI rather than instead of it.
How much weight would move me into the healthy band?
The calculator shows the healthy weight range for your exact height — the weights that would put your BMI between 18.5 and 25. The distance from your current weight to the nearest end of that range is the answer, though a gradual change of half a kilo a week is the pace most guidance recommends.
Does BMI work during pregnancy?
No. BMI is not valid during pregnancy or while breastfeeding, and it is not designed for people with significant muscle wastage or oedema either. Weight gain in pregnancy is tracked against separate guidance based on your pre-pregnancy BMI.