BMI Calculator
Calculate your Body Mass Index from weight and height in metric or US units, with the WHO category and the healthy weight range for your height.
Important Medical Disclaimer
BMI is a screening tool and doesn't directly measure body fat or health. It may not be accurate for athletes, elderly individuals, or those with high muscle mass. Always consult healthcare professionals for comprehensive health assessments and personalized medical advice.
Updated Formula covered by automated tests
Body Mass Index compares your weight with your height to give one number that clinicians use as a first screen for weight-related health risk. Enter weight and height in metric or US units and the calculator returns your BMI, the World Health Organization category it falls in, and the weight range that would put you between 18.5 and 24.9 at your height.
People usually check BMI before a doctor visit, when starting a weight-loss plan, or when an insurance or fitness form asks for it. Because it needs only a scale and a tape measure, it is the most widely reported body-size statistic in public-health data, which also makes it easy to compare your number with population averages.
The formula
- weight
- Body weight in kilograms, or in pounds for the US-unit version
- height
- Standing height in metres, or in inches for the US-unit version
- 703
- Conversion constant that makes the pound-inch result match the kilogram-metre result
The index was proposed by the Belgian statistician Adolphe Quetelet in the 1830s, who noticed that adult weight tends to rise with roughly the square of height rather than in direct proportion. Dividing by height squared removes most of that height effect, so a short and a tall person of similar build land on similar numbers.
The US-unit version is the same equation with the units converted inside the constant: one kilogram is 2.20462 pounds and one metre is 39.3701 inches, and 39.3701² ÷ 2.20462 rounds to 703. The calculator converts your inputs to metric first, so both unit systems give an identical result.
Worked example
Maya is 1.68 m tall and weighs 72 kg. She wants to know her BMI and how much weight would bring her into the 18.5 to 24.9 range.
- 1 Square the height: 1.68 × 1.68 = 2.8224 m².
- 2 Divide weight by that value: 72 ÷ 2.8224 = 25.51.
- 3 Look up the category: 25.51 is between 25.0 and 29.9, so the WHO class is overweight.
- 4 Find the top of the healthy range at her height: 24.9 × 2.8224 = 70.3 kg.
- 5 Find the bottom of the range: 18.5 × 2.8224 = 52.2 kg.
Result: Maya’s BMI is 25.5, just inside the overweight band. Losing about 1.7 kg (to 70.3 kg) would put her at the top of the 18.5 to 24.9 range.
Before you rely on the result
- BMI is a screening number, not a diagnosis. It does not measure body fat, fat distribution, fitness or blood markers.
- It overestimates fatness in muscular people and can underestimate it in older adults who have lost muscle mass.
- Do not use it during pregnancy or for anyone under 18; ask a clinician for the appropriate measure instead.
Frequently asked questions
Why does a muscular athlete get an “overweight” BMI?
BMI cannot tell muscle from fat; it only sees total weight. A rugby forward at 1.85 m and 98 kg scores 28.6 even with low body fat. For people with a lot of muscle, a waist measurement or a body-fat estimate is a better guide.
Are the BMI cut-offs the same for everyone?
The WHO adult cut-offs (18.5, 25 and 30) apply to adults of any sex. The WHO and several national health bodies suggest lower action points for people of South Asian, Chinese and some other Asian backgrounds, often 23 for increased risk and 27.5 for high risk, because type 2 diabetes risk rises at lower BMI in these groups.
Can I use this calculator for my child?
No. Children and teenagers are assessed with BMI-for-age percentiles, which compare a child with others of the same age and sex. A BMI of 20 can be healthy at 16 and high at 8, so the adult categories here do not apply under 18.
Is a BMI of 24.9 healthier than 18.6?
Both sit in the same WHO category and the band was set to cover the range where large studies found the lowest risk. Within it, differences in health depend far more on activity, diet, blood pressure and where fat is stored than on the exact BMI value.