How to calculate BMI: metric and US formulas with examples

The metric BMI formula

Using kilograms and metres, the formula is BMI = weight in kg ÷ (height in m × height in m). The CDC also gives a centimetre version: weight in kg ÷ height in cm² × 10,000. ^[1]

Example: an adult who is 70 kg and 1.75 m tall has BMI = 70 ÷ (1.75 × 1.75). That is 70 ÷ 3.0625, or approximately 22.9 after rounding to one decimal place.

The pounds-and-inches BMI formula

Using US units, BMI = weight in lb ÷ (height in inches × height in inches) × 703. Convert the height first: total inches = feet × 12 + remaining inches. ^[1]

Example: 5 ft 9 in is 69 inches. At 154 lb, the calculation is 154 ÷ (69 × 69) × 703, which is approximately 22.7. Small differences from a metric example usually come from rounded conversions, not from a different BMI system.

Four common calculation mistakes

A correct formula can still produce a wrong result when the units are inconsistent. Check the following before interpreting the answer.

• Using centimetres in the metres formula without converting or multiplying by 10,000.

• Dividing by height once instead of height squared.

• Entering feet as a decimal—for example, treating 5 ft 9 in as 5.9 feet.

• Rounding height, converted weight, or intermediate results too early.

From a number to a category

After calculating, compare the result with the standard adult categories: below 18.5, 18.5 to less than 25, 25 to less than 30, and 30 or above. These categories apply to adults 20 and older. ^[2]

The category does not turn the result into a diagnosis. BMI is a screening measure and cannot directly measure body fat or distinguish it from muscle and bone. ^{[3] [1]}

Let the calculator handle the arithmetic

Massy accepts metric, US, and stones-and-pounds inputs in the iPhone app and performs the conversion locally, without requiring an account.

Whether you calculate by hand or use the tool, measure carefully, round only at the end, and keep the limitations next to the result.