How to Calculate BMI: Formula and Examples

BMI can be calculated from only two measurements: weight and height.

Quick answer: In metric units, BMI equals weight in kilograms divided by height in meters squared. In U.S. customary units, multiply weight in pounds by 703 and divide by height in inches squared.

BMI Calculator

See your body mass index from height and weight.

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BMI formula in metric units

The metric formula is:

BMI = weight (kg) ÷ height (m)²

Example

Imagine:

• Weight: 70 kg
• Height: 1.75 m

First square the height:

1.75 × 1.75 = 3.0625

Then divide weight by that number:

70 ÷ 3.0625 = 22.9

The BMI is approximately 22.9.

BMI formula using centimeters

If your height is measured in centimeters, CDC also gives this equivalent formula:

BMI = [weight (kg) ÷ height (cm) ÷ height (cm)] × 10,000

For 70 kg and 175 cm:

70 ÷ 175 ÷ 175 × 10,000 = 22.9

BMI formula in pounds and inches

The formula is:

BMI = weight (lb) ÷ height (in)² × 703

Example

Imagine:

• Weight: 170 lb
• Height: 68 in

170 ÷ 68² × 703 ≈ 25.8

How to interpret the result

For adults age 20 and older, CDC categories are:

BMI	Category
Below 18.5	Underweight
18.5 to <25	Healthy weight
25 to <30	Overweight
30 or greater	Obesity

Obesity is further divided into three classes.

The easiest method: use a BMI calculator

Manual calculation is useful for understanding the formula, but a calculator is faster and reduces arithmetic errors.

BMI Calculator

See your body mass index from height and weight.

Calculate →

Enter:

• height
• weight

and the calculator returns the BMI value and category.

Common BMI calculation mistakes

1. Using centimeters as meters

175 cm is 1.75 m, not 175 m.

2. Forgetting to square height

BMI divides weight by height squared, not height once.

3. Mixing kilograms and inches

Use one complete unit system.

4. Applying adult categories to children

Children and teens require age- and sex-specific BMI percentiles.

5. Treating the result as a diagnosis

BMI is a screening measure.

Why two people with the same BMI can be different

Imagine two adults with the same height and weight.

One may have:

• more muscle
• less body fat
• high activity levels

while the other may have a different body composition.

BMI cannot distinguish those differences.

Should you track BMI over time?

Tracking can be useful when it provides context, but small short-term changes in body weight can reflect:

• hydration

• food intake

• glycogen

• menstrual-cycle changes

• normal day-to-day variation

Look at trends rather than individual measurements.

Explore related EatBetter tools

• BMI Calculator
• Calorie Calculator

Sources

1. CDC — BMI Frequently Asked Questions: https://www.cdc.gov/bmi/faq/
2. CDC — Adult BMI Categories: https://www.cdc.gov/bmi/adult-calculator/bmi-categories.html
3. CDC — Adult BMI Calculator: https://www.cdc.gov/bmi/adult-calculator/index.html

Note: BMI is a screening measure and is not a substitute for individualized medical advice.