BMI calculation
BMI Formula
BMI, or Body Mass Index, compares weight with height. It is commonly used as an adult screening calculation because it is fast, repeatable, and easy to explain. The result is still only a broad reference and does not measure body fat directly.
Metric BMI formula
BMI = weight (kg) / height (m)^2
- Measure weight in kilograms.
- Convert height from centimeters to meters by dividing by 100.
- Square the height in meters.
- Divide weight by the squared height.
Example: 70 kg and 175 cm becomes 70 / 1.75^2 = 22.9.
US BMI formula
BMI = 703 x weight (lb) / height (in)^2
- Measure weight in pounds.
- Convert height to total inches.
- Square the height in inches.
- Multiply weight by 703, then divide by squared height.
Example: 165 lb and 5 ft 9 in is 165 lb and 69 in, so BMI = 703 x 165 / 69^2 = 24.4.
Example BMI calculations
| Measurements | Calculation | BMI |
|---|---|---|
| 70 kg, 175 cm | 70 / 1.75^2 | 22.9 |
| 82 kg, 180 cm | 82 / 1.80^2 | 25.3 |
| 165 lb, 69 in | 703 x 165 / 69^2 | 24.4 |
| 210 lb, 70 in | 703 x 210 / 70^2 | 30.1 |
Common BMI formula mistakes
- Using height in centimeters directly instead of converting to meters.
- Using feet and inches separately instead of total inches.
- Forgetting to square height before dividing.
- Mixing kilograms with inches, or pounds with meters.
- Rounding height or weight before the calculation is finished.
- Treating the result as a complete health assessment.
Why height is squared
Taller people naturally weigh more than shorter people. Squaring height is a simple statistical adjustment that makes weight more comparable across adult heights. It is not perfect, which is one reason BMI can misclassify some individuals.
The formula works best as a population screening tool. For individual decisions, it is better to consider waist circumference, body composition, symptoms, history, and advice from a qualified healthcare professional.
How to interpret the result
After calculating BMI, compare the result with adult BMI categories: below 18.5 is underweight, 18.5 to 24.9 is healthy weight, 25 to 29.9 is overweight, and 30 or above falls in the obesity range. These are broad screening ranges, not medical diagnoses.
The calculator on BMI Checks applies the formula automatically, reduces unit errors, and shows the adult category plus a healthy weight estimate for your height.
BMI formula FAQ
Why is height squared in the BMI formula?
BMI adjusts weight for height by dividing by height squared. This makes the number more comparable across adults of different heights than weight alone.
Do I use centimeters directly?
No. In the metric formula, height must be in meters. Convert centimeters to meters first by dividing by 100.
Why does the US formula use 703?
The 703 factor converts pounds and inches into the same scale as kilograms and meters.
Should I round my height or weight first?
Avoid rounding early. Calculate with the most accurate measurements you have, then round the final BMI to one decimal place.
Is BMI the same for men and women?
The formula is the same, but interpretation can differ because body composition and fat distribution vary between individuals.
Can BMI diagnose health?
No. BMI is a screening calculation, not a diagnosis or a direct measure of body fat.
References
Medical disclaimer
This page is for general informational use only and does not replace professional medical advice, diagnosis, or treatment.