BMI (Body Mass Index) is a quick screening number that relates your weight to your height. It won't tell you everything about your health, but it's a useful first check that health services use worldwide. This guide shows the formula in both metric and imperial units, a worked example, and what the result actually means — and the free BMI calculator does the maths for you.
What BMI is
BMI estimates whether you're a healthy weight for your height by dividing your weight by the square of your height. It's the same formula for adults of any age or sex, which makes it a simple, consistent screening tool — but also one with real limits (see below).
The BMI formula
Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = 703 × weight (lb) ÷ height (in)²
The 703 factor just converts pounds-and-inches into the same scale as the metric version, so both give the same BMI number.
Worked example
Someone 1.75 m tall weighing 70 kg:
| Height squared | 1.75 × 1.75 = 3.0625 |
| BMI | 70 ÷ 3.0625 ≈ 22.9 |
| Category | Healthy weight (18.5–24.9) |
The same person in imperial (5 ft 9 in ≈ 69 in, 154 lb): 703 × 154 ÷ 69² = 703 × 154 ÷ 4761 ≈ 22.7 — the same answer, give or take rounding.
BMI categories (WHO, adults)
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
Some health bodies use lower thresholds for people of South Asian descent, where health risks rise at a lower BMI.
The limits of BMI
BMI only knows your height and weight — not your body composition. A muscular athlete can read as “overweight” despite low body fat, while someone at a “healthy” BMI may carry excess fat. It also doesn't account for age, sex, or where fat is stored. Treat BMI as a rough screen, not a diagnosis — pair it with waist measurement and a doctor's advice. To plan around energy needs, the calorie calculator estimates your daily maintenance calories.
FAQ
What is a healthy BMI? For most adults, 18.5 to 24.9. Lower cut-offs may apply for some populations.
Does BMI work for athletes? Not well — extra muscle raises BMI without extra fat, so it can misclassify very muscular people.
Is BMI the same for men and women? The formula and adult categories are the same, though body composition differs on average.
Is my data private? Yes — the calculator runs entirely in your browser and stores nothing.