BMI Calculator
Calculate Body Mass Index (BMI) from weight and height in metric or imperial units. See your WHO category and a step-by-step breakdown of the formula.
What BMI measures
BMI (Body Mass Index) compares your weight to your height to give a quick screening number for body-weight ranges. It does not measure body fat directly — it is a fast, low-cost estimate used to flag categories worth a closer look.
The formula (two unit systems)
- Metric: BMI = weight (kg) ÷ [height (m)]²
- Imperial: BMI = 703 × weight (lb) ÷ [height (in)]²
Both formulas describe the same relationship; 703 is just the conversion constant that makes the pounds-and-inches version agree with the kilograms-and-meters version.
Worked examples
- 70 kg, 1.75 m → 70 ÷ 1.75² = 70 ÷ 3.0625 ≈ 22.9 → Normal weight.
- 154 lb, 69 in → 703 × 154 ÷ 69² ≈ 22.7 → Normal weight.
- 92 kg, 1.75 m → 92 ÷ 3.0625 ≈ 30.0 → Obese.
BMI categories (WHO, adults)
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
What BMI does not capture
BMI cannot tell the difference between muscle and fat, so a muscular athlete can score "Overweight" without carrying excess fat. It also does not account for age, sex, or where fat sits on the body.
Common mistakes
- Using centimeters instead of meters in the metric formula (must be squared meters).
- Forgetting the 703 constant in the imperial formula.
- Treating BMI as a diagnosis rather than a screening estimate.
BMI is a form of unit rate — weight per unit of height-squared. To work with rates and ratios directly, see the percentage calculator.
BMI is a screening index, not a medical diagnosis. It does not account for muscle mass, bone density, or fat distribution. Consult a healthcare professional for a full health assessment.
Frequently asked questions
- What is BMI?
- BMI (Body Mass Index) is a screening number that compares your weight to the square of your height. It is a quick way to flag possible underweight, normal, overweight, or obese ranges — not a diagnosis.
- What is the BMI formula?
- In metric units, BMI = weight (kg) divided by height (m) squared. In imperial units, BMI = 703 times weight (lb) divided by height (in) squared. Both formulas give the same result for the same person.
- Why does the imperial formula use 703?
- 703 is a unit-conversion constant that makes the pounds-and-inches formula land on the same BMI value as the kilograms-and-meters formula. Without it, the two unit systems would disagree.
- What do the BMI categories mean?
- Under 18.5 is Underweight, 18.5 up to 25 is Normal weight, 25 up to 30 is Overweight, and 30 or above is Obese. These are the standard WHO adult bands used worldwide.
- Is BMI accurate for everyone?
- No. BMI does not distinguish muscle from fat, so very muscular people can score high without being overfat. It also ignores age, sex, and where fat is distributed on the body.
- Is BMI a medical diagnosis?
- No. BMI is a screening index, not a diagnosis. Use it as a starting point and talk to a healthcare professional for a full assessment of your health.
- Can BMI be used for children?
- This calculator uses the adult WHO bands. Children and teens need age- and sex-specific BMI percentile charts instead, since growth patterns differ from adults.