BMI Calculator
Calculate BMI in metric (kg and cm) or imperial (lb and inches).
Returns WHO classification, healthy weight range for your height, and a visual scale.
BMI stands for Body Mass Index — a screening number derived from your height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and is widely used in medicine as a quick population-level health indicator. It’s a screening tool, not a diagnosis.
The Formula
Metric: BMI = weight (kg) / height (m)²
Imperial: BMI = (weight (lbs) × 703) / height (inches)²
The 703 is just a unit conversion factor — both formulas give the same number for the same person.
Worked Example
A person weighs 75 kg and is 1.72 m tall. BMI = 75 / (1.72)² = 75 / 2.9584 = 25.35
A 5'8" (68 inches), 165 lb person: BMI = (165 × 703) / 68² = 116,000 / 4,624 = 25.09
Both fall in the Overweight band (25.0 to 29.9).
WHO BMI Classification (Adults)
| BMI Range | Category |
|---|---|
| Under 18.5 | Underweight |
| 18.5 to 24.9 | Normal / Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 to 34.9 | Obese Class I |
| 35.0 to 39.9 | Obese Class II |
| 40.0 and above | Obese Class III (severe) |
Healthy Weight Range for Your Height
The “healthy” BMI band (18.5 to 24.9) translates to a weight range:
Lower limit (kg) = 18.5 × height² (m) Upper limit (kg) = 24.9 × height² (m)
For someone 1.72 m tall:
- Lower healthy weight: 18.5 × 2.9584 = 54.7 kg (120 lb)
- Upper healthy weight: 24.9 × 2.9584 = 73.7 kg (162 lb)
What BMI Doesn’t Tell You
BMI does not distinguish between fat and muscle. Athletes with high muscle mass often score in the Overweight range despite being very lean. It also doesn’t account for where fat is distributed — abdominal fat carries more health risk than fat on hips and thighs.
For a fuller picture, pair BMI with:
- Waist circumference — over 40 inches (men) or 35 inches (women) indicates higher risk regardless of BMI
- Waist-to-height ratio — should be under 0.5; this is increasingly considered a better single indicator than BMI
- Body fat percentage — measured by skinfold calipers, DEXA scan, or smart scales
When BMI Is Less Reliable
- Elderly adults lose muscle and bone mass, so a slightly higher BMI (24 to 27) often correlates with better outcomes
- Children and teens use age-and-sex-specific percentile charts, not adult cutoffs
- Pregnant women should use pre-pregnancy BMI, not current
- Athletes and bodybuilders routinely score “overweight” while having very low body fat
- Asian populations may face elevated health risk at lower BMI thresholds (some health bodies use 23 as the overweight cutoff for Asian adults)
What to Do Next
If your BMI is in the healthy range, the priority is maintenance — regular activity, balanced eating, sleep. If it’s outside the range, the most-evidence-based first step is a small calorie surplus or deficit (300 to 500 per day) combined with strength training, not extreme diets. Big swings tend to reverse; small consistent changes stick.