Body Mass Index (BMI) is calculated using your weight and height. For adults, the formula is your weight in kilograms divided by your height in metres squared. The result helps you understand your weight category in relation to your height.
You can calculate BMI manually using a simple formula or use an online BMI Calculator to get your result quickly.
What Is the BMI Formula?
The standard BMI formula is:
BMI = Weight (kg) ÷ Height (m)²
For example, if your weight is 65 kg and your height is 1.65 metres:
BMI = 65 ÷ (1.65 × 1.65)
BMI = 65 ÷ 2.7225
BMI = 23.87
Your calculated BMI is approximately 23.9.
How to Calculate BMI Step by Step
Follow these three steps to calculate your BMI manually.
Step 1: Measure Your Weight
Measure your weight in kilograms. If your weight is recorded in pounds, convert it to kilograms by dividing the weight in pounds by 2.20462.
For example, 154 pounds is approximately 69.85 kg.
Step 2: Convert Your Height Into Metres
If your height is recorded in centimetres, divide it by 100 to convert it into metres.
For example, 170 cm = 1.70 metres.
Step 3: Apply the BMI Formula
Divide your weight in kilograms by your height in metres multiplied by itself.
For a person weighing 70 kg with a height of 170 cm:
BMI = 70 ÷ (1.70 × 1.70) = 24.22
The result is approximately 24.2.
How to Calculate BMI in kg and cm
You can also calculate BMI directly when your weight is in kilograms and your height is in centimetres.
BMI = Weight (kg) ÷ [Height (cm) ÷ 100]²
For example:
- Weight: 75 kg
- Height: 175 cm
- Height in metres: 1.75 m
- BMI: 75 ÷ (1.75 × 1.75)
- Result: 24.49
This is the same calculation used when centimetres are first converted into metres.
How to Calculate BMI in Feet and Inches
If your height is recorded in feet and inches and your weight is in pounds, use the imperial BMI formula:
BMI = 703 × Weight (lb) ÷ Height (in)²
First, convert your total height into inches. For example, a height of 5 feet 7 inches equals 67 inches.
For someone weighing 150 pounds:
BMI = (703 × 150) ÷ (67 × 67)
BMI = approximately 23.49.
How to Interpret Your BMI Result
For adults aged 20 and older, the commonly used BMI categories are:
| BMI range | Category |
| Below 18.5 | Underweight |
| 18.5 to below 25 | Healthy weight |
| 25 to below 30 | Overweight |
| 30 or above | Obesity |
These are standard adult categories. Appropriate interpretation can vary by population and clinical context.
Why Use an Online BMI Calculator?
Manual calculations are useful for understanding the formula, but an online Body Mass Index Calculator can save time and reduce arithmetic errors.
Enter your weight and height in the appropriate units, calculate your BMI, and review the resulting category. Always check that the correct units have been selected.
Frequently Asked Questions
Can I calculate BMI without a calculator?
Yes. Convert your height into metres, square it, and divide your weight in kilograms by that number.
Does BMI use age and gender?
For standard adult BMI classification, the same categories are generally used for adults aged 20 and older, regardless of age or sex. Children and teenagers require age- and sex-specific BMI interpretation.
Is BMI the same as body fat percentage?
No. BMI is based on height and weight. Body fat percentage estimates the proportion of body weight that is fat.
How accurate is an online BMI calculator?
An online calculator can accurately perform the BMI formula when the measurements and units are entered correctly. However, the resulting BMI is a screening measure and does not directly measure body fat or diagnose a health condition.