BMI Calculator
Calculate Body Mass Index with the WHO classification, healthy weight range and BMI Prime, in metric or imperial units.
Inputs
Formula
- BMI = mass (kg) / height (m)²
- Imperial: BMI = 703 × mass (lb) / height (in)²
- BMI Prime = BMI / 25
Body Mass Index normalises body mass by the square of stature, giving a single number that correlates with body fat at population level. The WHO cut-offs are 18.5, 25 and 30 kg/m². BMI Prime expresses the same value as a ratio to the upper healthy limit, so 1.0 is exactly the top of the healthy range.
Step-by-step calculation
Mass in kilograms
given
70 kg
Height in metres
cm / 100
1.72 m
Height squared
h²
2.9584 m²
BMI
mass / h²
23.66 kg/m²
Worked example
- 70 kg at 172 cm → h = 1.72 m, h² = 2.9584 m².
- BMI = 70 / 2.9584 = 23.7 kg/m² → Normal weight.
- Healthy weight range at this height = 54.7 kg to 73.9 kg.
Assumptions
- WHO adult cut-offs are used; Asian populations often apply lower thresholds (23 overweight, 27.5 obese).
- BMI does not distinguish muscle from fat and is not valid for children, pregnancy or athletes.
Tips
- Waist-to-height ratio below 0.5 is a better single indicator of metabolic risk than BMI alone.
- Measure height without shoes and weight before breakfast for consistency.
Warnings
- BMI is a screening indicator, not a diagnosis. Discuss results with a healthcare professional.
Frequently asked questions
What is a healthy BMI?
The WHO healthy range for adults is 18.5 to 24.9 kg/m². Below 18.5 is underweight, 25.0–29.9 is overweight and 30 or above is obese.
Is BMI accurate for athletes?
No. Muscle is denser than fat, so muscular people often score as overweight despite low body fat. Use body-fat percentage instead.
Does BMI differ for men and women?
The formula and cut-offs are the same, although women typically carry more body fat at the same BMI.
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