About the BMI Calculator
Body Mass Index (BMI) is one of the most widely used screening tools for weight-related health, used by doctors, insurers, and public health organizations worldwide. Our BMI Calculator gives you an instant result in either imperial or metric units, along with the weight category it falls into.
How It Works
BMI is calculated by dividing your weight in kilograms by the square of your height in meters. It's a screening measure, not a diagnosis - it doesn't distinguish between muscle mass and fat mass, which is why an athlete and a sedentary person of the same height and weight can have identical BMI but very different body compositions.
Formula & Methodology
Belgian mathematician Adolphe Quetelet devised this formula in the 1830s while studying population averages, not individual health - a detail often left out of modern discussions of BMI. Dividing by height squared (rather than height alone) was an empirical choice: across a population, body weight tends to scale roughly with the square of height, which made this ratio useful for comparing groups. It was never designed to assess a single person's individual body composition.
For imperial units, the formula becomes BMI = 703 × weight (lb) ÷ height (in)². The constant 703 is simply the unit-conversion factor that makes the imperial formula produce the same result as the metric one (it converts pounds to kilograms and inches to meters within a single multiplication).
Step-by-Step: Calculating It By Hand
- 1Convert your height to meters (divide inches by 39.37, or centimeters by 100).
- 2Square your height in meters (multiply it by itself).
- 3Convert your weight to kilograms if needed (divide pounds by 2.205).
- 4Divide your weight in kilograms by your squared height in meters.
- 5Compare the result to the standard categories: under 18.5 is underweight, 18.5–24.9 is normal weight, 25–29.9 is overweight, and 30+ is obese.
Examples
Example calculation
A person who is 5'8" (1.73m) and weighs 160 lb (72.6 kg) has a BMI of about 24.3 - within the 'normal weight' range of 18.5–24.9.
Metric example
A person who is 175 cm and 80 kg has a BMI of about 26.1, which falls in the 'overweight' range of 25–29.9.
Advantages
- Quick, free screening tool used consistently by medical professionals
- Works in both imperial and metric units
- Useful for tracking trends in your own weight over time
- No account or personal data storage required
Common Mistakes
- Treating BMI as a precise diagnosis rather than a general screening number
- Not accounting for muscle mass, which can inflate BMI in athletes
- Using outdated height or weight measurements
- Comparing BMI across very different body types without context
Edge Cases to Watch For
- Athletes and heavily muscled individuals often score 'overweight' or 'obese' by BMI despite low body fat, since the formula can't distinguish muscle from fat.
- Older adults tend to carry more fat at the same BMI as younger adults due to natural muscle loss with age, so the same number can mean different things across age groups.
- BMI categories were developed primarily from studies of adult populations and don't apply the same way to children or teenagers, who use age- and sex-specific percentile charts instead.
- Different ethnic groups have been shown to carry different health risks at the same BMI - some public health bodies use adjusted thresholds for South and East Asian populations, for example.
- Pregnancy substantially changes weight in ways unrelated to fat gain, making standard BMI categories inapplicable during that time.
Common Use Cases
- General health screening and tracking over time
- Understanding standard weight categories used by doctors
- Setting a baseline before starting a fitness or nutrition plan
- Quick reference before a medical appointment