BMI Calculator
Calculate your Body Mass Index (BMI) using metric or imperial units. Get your WHO category, a visual scale, and a step-by-step breakdown.
Your BMI
Formula
BMI formula.
Metric
BMI = weight (kg) ÷ height² (m)
Imperial
BMI = 703 × weight (lb) ÷ height² (in)
The constant 703 converts the imperial result to the metric scale. Height must be in total inches (feet × 12 + remaining inches).
Step by step
How to calculate BMI.
- 01
Measure your height
Stand straight without shoes. Measure from floor to crown of head. Convert to metres for metric (e.g. 170 cm = 1.70 m) or total inches for imperial (e.g. 5 ft 7 in = 67 in).
- 02
Measure your weight
Weigh yourself in the morning before eating, without shoes or heavy clothing. Use kilograms for metric or pounds for imperial.
- 03
Square your height
Multiply your height value by itself. For 1.70 m: 1.70 × 1.70 = 2.89 m². For 67 in: 67 × 67 = 4,489 in².
- 04
Divide weight by height²
Metric: weight (kg) ÷ height² (m²). Imperial: 703 × weight (lb) ÷ height² (in²).
- 05
Interpret the result
Look up your BMI in the WHO categories. A result of 18.5–24.9 is considered normal weight for most adults aged 18–65.
Example
Worked example.
Metric
- Height
- 175 cm = 1.75 m
- Weight
- 70 kg
- Height²
- 3.0625 m²
→ Normal weight
Imperial
- Height
- 5 ft 9 in = 69 in
- Weight
- 160 lbs
- Height²
- 4,761 in²
→ Normal weight
Note
Limitations of BMI.
BMI is a population-level screening tool. It does not account for:
- —Muscle vs. fat mass — athletes may read "overweight" despite low body fat
- —Fat distribution — abdominal fat carries more health risk than BMI captures
- —Age — older adults naturally carry more fat at the same BMI
- —Sex — women carry more body fat than men at the same BMI value
- —Ethnicity — health risks begin at lower thresholds for South and East Asian populations
Consult a healthcare professional before making health decisions based on BMI alone.
FAQ
Frequently asked questions.
What is a healthy BMI for adults?
For adults aged 18–65, a BMI between 18.5 and 24.9 is considered healthy. Below 18.5 is underweight; 25–29.9 is overweight; 30 and above is obese. These thresholds are set by the World Health Organization (WHO).
Does BMI differ for men and women?
The BMI formula is the same for both sexes. However, women naturally carry more body fat than men at the same BMI. For a more sex-specific assessment, tools like body fat percentage are more informative.
Is BMI accurate for athletes and muscular people?
No. BMI cannot distinguish between fat mass and muscle mass. A heavily muscular person may have a BMI in the "overweight" range despite having low body fat. Athletes should use body fat percentage or DEXA scans for a more accurate assessment.
Does BMI apply to children?
BMI for children (ages 2–19) is interpreted differently. It uses age- and sex-specific percentile charts rather than fixed cutoffs. A pediatrician should always interpret BMI for children.
What should I do if my BMI is outside the normal range?
BMI is a screening tool, not a diagnosis. If your BMI is outside the 18.5–24.9 range, consult a healthcare provider. They can assess your overall health, body composition, diet, and activity level before recommending any action.
What are the limitations of BMI?
BMI does not account for muscle mass, bone density, fat distribution, age, sex, or ethnicity. For example, Asian populations face higher health risks at lower BMI thresholds. It is best used as a quick screening tool alongside other health metrics.
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