How to Calculate BMI

The BMI formula

• Metric: BMI = weight (kg) ÷ height (m)²
• US units: BMI = 703 × weight (lb) ÷ height (in)²

The 703 converts pounds and inches into the same result the metric formula gives (the exact factor is 703.07).

Step by step in metric units

1. Convert your height to meters: 170 cm = 1.70 m.
2. Square it: 1.70 × 1.70 = 2.89.
3. Divide your weight by that number: 70 kg ÷ 2.89 = 24.2.

Step by step in US units

1. Convert your height to inches: 5 ft 7 in = 67 in.
2. Square it: 67 × 67 = 4,489.
3. Multiply your weight by 703: 150 lb × 703 = 105,450.
4. Divide: 105,450 ÷ 4,489 = 23.5.

With stones and pounds

One stone is 14 pounds. For 11 st 4 lb at 5 ft 9 in: 11 × 14 + 4 = 158 lb, 69 in squared is 4,761, and 703 × 158 ÷ 4,761 = 23.3.

Common mistakes

• Treating feet as decimals. 5.7 ft is 5 ft 8.4 in, not 5 ft 7 in.
• Forgetting to square the height. 70 ÷ 1.70 gives 41.2, which isn’t a BMI.
• Using centimeters instead of meters. 70 ÷ 170² gives 0.0024.
• Rounding too early. Keep full precision and round only the final result.

Rounding and categories

BMI is usually rounded to one decimal place, and the category comes from the rounded number, as on the CDC calculator. A result of 24.96 shows as 25.0, which is in the overweight range.

In a spreadsheet

With weight in column B and height in column C:

=B2/(C2/100)^2      weight in kg, height in cm
=703*B2/C2^2        weight in lb, height in inches

Related measures

• BMI Prime is BMI divided by 25, the top of the healthy range. A value of 1.00 or more means a BMI of 25 or more.
• “New BMI”, proposed by mathematician Nick Trefethen in 2013, is 1.3 × weight (kg) ÷ height (m)^2.5. It aims to treat very short and very tall people more evenly, but it isn’t used in clinical guidelines.
• Waist-to-height ratio adds where fat is stored. Calculate it here →

See the BMI chart by height and weight →

Sources

• CDC: Adult BMI Calculator
• NIH NHLBI: Calculate Your BMI