How to Calculate BMI
Body Mass Index (BMI) is a simple screening measure that relates your weight to your height. This calculator supports both metric (centimeters and kilograms) and imperial (feet, inches, and pounds) units. The formula was popularized by the World Health Organization as a quick, low-cost way to flag potential weight-related health risk at a population level.
Worked example (metric): Height 170 cm (1.70 m), weight 65 kg → BMI = 65 ÷ (1.70 × 1.70) = 65 ÷ 2.89 = 22.5 — Normal weight.
Worked example (imperial): Height 5 ft 7 in (67 in total), weight 145 lb → BMI = (145 ÷ 67²) × 703 = (145 ÷ 4489) × 703 = 22.7 — Normal weight.
BMI Categories (WHO Standard)
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
Limitations of BMI
BMI is a useful population-level screening tool, but it doesn't distinguish between muscle and fat mass. Athletes with high muscle mass can show a high BMI despite having low body fat, and BMI ranges can also vary in relevance across different ages, sexes, and ethnic groups. It's a starting point for a conversation about health, not a diagnosis — the CDC and NHS both recommend using BMI alongside other measures like waist circumference and body composition when assessing health risk.
Formula Used
Metric: weight (kg) ÷ height (m)². Imperial: weight (lb) ÷ height (in)² × 703. Both formulas produce the same standardized BMI value.
Frequently Asked Questions
Is BMI accurate for everyone?
BMI is a general screening tool and doesn't account for muscle mass, bone density, or body composition, so it may be less accurate for athletes or older adults.
What is a healthy BMI range?
18.5 to 24.9 is generally considered a normal weight range for most adults.
Should I make health decisions based on BMI alone?
No, BMI is just one indicator. Consult a healthcare professional for a full picture of your health.
Does this tool store my data?
No, all calculations happen locally in your browser and nothing is saved.
Does BMI apply the same way to children?
No, BMI-for-age percentile charts are used for children and teens instead of the fixed adult categories shown here.