About the BMI Calculator
This BMI calculator works out your Body Mass Index from height and weight in either metric or imperial units and places the result against the World Health Organization categories. It also does something most calculators skip: it converts the healthy 18.5 to 24.9 band back into an actual weight range for your height, so instead of an abstract number you get a target in kilograms or pounds. BMI is a screening measure with real limitations, discussed below, and it should inform a conversation with a clinician rather than replace one.
How to use the BMI Calculator
- 1
Choose Units — metric for centimetres and kilograms, or imperial for feet, inches and pounds.
- 2
In metric, enter Height (cm) and Weight (kg). In imperial, fill Height (feet), Height (inches) and Weight (lb).
- 3
Read your BMI, the category it falls into, and the healthy weight range calculated for your height.
- 4
Check the WHO category table, where a tick marks the band you are currently in.
What people use it for
Setting a realistic weight target
Someone 170 cm tall has a healthy band of roughly 53.5 to 72 kg. Seeing the range rather than a single ideal weight makes it clear how wide the healthy zone actually is.
Insurance and medical forms
Many application forms and pre-operative assessments ask for BMI directly. Calculating it beforehand avoids the guesswork of converting stones and pounds under pressure.
Tracking a change over time
Recalculating monthly at the same time of day gives a smoother signal than daily weight, which swings by a kilogram or more with hydration and food.
The formula and where it came from
BMI is weight in kilograms divided by height in metres squared: BMI = kg ÷ m². Someone 1.70 m tall weighing 70 kg has a BMI of 70 ÷ 2.89 = 24.2. In imperial units the equivalent is (pounds × 703) ÷ inches², where 703 is the conversion constant that reconciles the two systems; this tool instead converts pounds to kilograms at exactly 0.45359237 and inches to metres at exactly 0.0254, which is marginally more precise. The index was devised in the 1830s by the Belgian statistician Adolphe Quetelet, who was describing populations rather than diagnosing individuals, and it was popularised as BMI only in 1972. The WHO thresholds used here are: below 18.5 underweight, 18.5 to 24.9 healthy, 25.0 to 29.9 overweight, and 30.0 and above obese. Reversing the formula gives the healthy range: multiply 18.5 and 24.9 by your height in metres squared.
Why BMI misclassifies people, and the Asian thresholds
The squared-height denominator is the core problem. Because bodies scale in three dimensions but the formula divides by two, BMI systematically overstates the adiposity of tall people and understates it in short people. It also cannot tell muscle from fat, and muscle is denser. A well-trained rugby forward or a competitive rower routinely registers a BMI above 30 with body fat in the low teens, while a sedentary person of the same BMI may carry two or three times the fat mass. In the other direction, older adults lose muscle and bone density and can sit inside the healthy band while carrying substantial visceral fat, a pattern sometimes called normal-weight obesity. Ethnicity matters too: the WHO expert consultation of 2004 concluded that cardiometabolic risk rises at lower BMI in South Asian, Chinese and other Asian populations, and recommended public health action points around 23 and 27.5 rather than 25 and 30. BMI is also not valid for children, who need age and sex percentile charts, or for pregnancy. Waist circumference and waist-to-height ratio are better single indicators of health risk. Treat this result as a screening prompt, not a diagnosis, and discuss anything concerning with a healthcare professional.
Tips
- Waist-to-height ratio is a stronger risk indicator than BMI — keeping your waist under half your height is the usual guidance.
- Weigh yourself first thing in the morning after using the bathroom for the most consistent readings.
- If you are muscular, athletic, pregnant, over 65 or under 18, treat the BMI number with particular scepticism and use other measures.