NumberTally How to Calculate BMI
By NumberTally Tools · Updated October 2026 · 7 min read
Body Mass Index (BMI) is a simple screening number calculated from your height and weight. Doctors and health organizations use it as a quick first check for weight categories — though it does not measure body fat directly.
Here is the exact formula, in both imperial and metric units, plus a worked example.
The BMI formula
Imperial (pounds and inches):
Metric (kilograms and meters):
Step-by-step example
A person weighs 180 lb and is 5'10" (70 inches) tall:
- Square the height: 70 × 70 = 4,900
- Divide weight by that: 180 ÷ 4,900 = 0.03673
- Multiply by 703: 0.03673 × 703 = 25.8
Worked example
BMI of 25.8 — just inside the "overweight" category (see chart below).
BMI categories (WHO)
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25 – 29.9 | Overweight |
| 30 and above | Obesity |
BMI is a screening tool, not a diagnosis. Athletes with high muscle mass can score "overweight" while being very fit — body fat percentage gives a fuller picture.
Calculate yours in seconds:
Frequently asked questions
What is the formula for BMI?
BMI = 703 × weight in pounds ÷ (height in inches)². In metric: BMI = weight in kg ÷ (height in meters)².
What is a healthy BMI?
The World Health Organization defines a healthy BMI as 18.5 to 24.9.
Is BMI accurate?
BMI is a useful screening tool but it does not distinguish muscle from fat. Very muscular people may score high while being healthy. Use it alongside waist measurements or body fat estimates.