BMI (Body Mass Index) is a quick way to see whether your weight is in a healthy range for your height. The math is simple — here's the formula in both metric and imperial, what the numbers mean, and where BMI falls short.
⚖️ Open the free BMI CalculatorYour BMI, weight category and healthy weight rangeBMI is your weight divided by the square of your height:
Metric: BMI = weight (kg) ÷ [ height (m) ]²
Imperial: BMI = 703 × weight (lb) ÷ [ height (in) ]²
Worked example (metric): someone who is 70 kg and 1.75 m tall. Height squared is 1.75 × 1.75 = 3.0625. So BMI = 70 ÷ 3.0625 ≈ 22.9 — within the healthy range.
Worked example (imperial): 154 lb and 69 in tall. That's 703 × 154 ÷ (69 × 69) = 108,262 ÷ 4,761 ≈ 22.7 — the same person, essentially the same BMI.
For most adults, the World Health Organization uses these standard ranges:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obesity |
You can turn the formula around to find the weight range that keeps your BMI between 18.5 and 24.9 for your height: multiply each end of that range by your height squared.
Example: at 1.75 m, height squared is 3.0625, so a healthy weight is about 18.5 × 3.0625 ≈ 57 kg up to 24.9 × 3.0625 ≈ 76 kg. The calculator works this out for you automatically.
BMI is a useful screening number, but it isn't a full picture of health. It doesn't distinguish muscle from fat, so very muscular people can read as "overweight" while carrying little fat. It also doesn't account for where fat is stored, age, or ethnicity. Treat BMI as a starting point, not a diagnosis — and talk to a doctor about your individual situation.
Is BMI the same for men and women?
The formula and adult categories are the same. Body composition differs on average, which is one reason BMI is a rough guide rather than a precise measure.
Does BMI work for children?
Not with these adult categories. Children and teens use age- and sex-specific BMI percentiles instead.
What's a "healthy" BMI?
For adults, 18.5 to 24.9 is the standard healthy range — but it's a guide, not a guarantee of health on its own.
Are my numbers sent anywhere?
No. The calculation runs entirely in your browser. Nothing you type is uploaded.