How to Calculate Your BMI Correctly (Formula, Chart & Examples)

Body Mass Index (BMI) is the quickest way to check whether your weight sits in a healthy range for your height. It's not perfect — but for most adults it takes 10 seconds and gives you a useful starting point.

What is BMI?

BMI is a single number that compares your weight to the square of your height. It was designed in the 1830s by Belgian statistician Adolphe Quetelet as a population-level measure of average body size, and was renamed Body Mass Index in 1972 by physiologist Ancel Keys. Today the World Health Organization uses it as the standard screening tool for underweight, healthy weight, overweight and obesity in adults.

The BMI Formula

Metric

BMI = weight (kg) ÷ height (m)²

Example: a person weighing 70 kg with a height of 1.75 m has a BMI of 70 ÷ (1.75 × 1.75) = 22.86.

Imperial

BMI = (weight in lb ÷ height in inches²) × 703

Example: 154 lb at 69 inches → (154 ÷ 4761) × 703 ≈ 22.74. The 703 factor exists to make the imperial formula match the metric result.

BMI Categories (WHO)

• Underweight — below 18.5
• Healthy weight — 18.5 to 24.9
• Overweight — 25.0 to 29.9
• Obesity class I — 30.0 to 34.9
• Obesity class II — 35.0 to 39.9
• Obesity class III — 40.0 and above

Worked Examples

Example 1. 60 kg, 1.65 m → 60 ÷ 2.7225 = 22.0 → healthy weight.

Example 2. 95 kg, 1.80 m → 95 ÷ 3.24 = 29.3 → overweight.

Example 3. 180 lb, 70 in → (180 ÷ 4900) × 703 = 25.8 → overweight.

When BMI Is Misleading

BMI ignores body composition. Muscle is roughly 18% denser than fat, so a lean athlete can land in the "overweight" range while carrying very little fat. The same number can also underestimate risk for older adults who have lost muscle but kept fat. For a more complete picture, pair BMI with one of:

• Waist circumference — under 94 cm (men) or 80 cm (women) is generally healthy.
• Waist-to-height ratio — aim to keep your waist under half your height.
• Body-fat percentage — measured with calipers, DEXA or a smart scale.

BMI for Children

Adult categories don't apply to under-18s. Children use age- and sex-specific BMI percentiles from CDC or WHO growth charts. A pediatrician (or a dedicated child BMI calculator) is the right tool here.

Key Takeaways

• BMI is a screening number, not a diagnosis.
• Healthy adult range is 18.5–24.9.
• Combine BMI with waist measurements for a clearer health picture.
• Children and very muscular adults need different tools.

Frequently Asked Questions