BMI Formula Calculator

Metric: BMI = weight (kg) / height (m)²

Imperial: BMI = (weight (lbs) × 703) / height (inches)²

Body Mass Index (BMI) is a simple screening tool that estimates body fat based on your weight relative to your height.
It was developed by Belgian mathematician Adolphe Quetelet in the 1830s, and for over a century it was simply called the Quetelet index.
The name body mass index came from the American physiologist Ancel Keys in 1972, who found it tracked body fat better than the other weight-for-height formulas of the day.

How the formula works:

In the metric version, divide your weight in kilograms by your height in meters squared.
For example, a person weighing 70 kg who is 1.75 m tall: BMI = 70 / (1.75 × 1.75) = 70 / 3.0625 = 22.9.

In the imperial version, multiply your weight in pounds by 703, then divide by your height in inches squared.
For example, a person weighing 154 lbs who is 5'9" (69 inches): BMI = (154 × 703) / (69 × 69) = 108,262 / 4,761 = 22.7.

BMI categories (WHO standard):

You will usually see that middle row written as “18.5 to 24.9”, which makes a BMI of 24.95 look like it belongs nowhere.
The bands are really half-open: 18.5 up to but not including 25.
This calculator uses 25 as the cutoff, the same value the category ladder uses, so the table and the answer can never disagree.

Where the 703 comes from

It is not a magic number, and you can derive it in one line.
One pound is 0.45359 kg and one inch is 0.0254 m, so lb/in² converts to kg/m² by multiplying by 0.45359 / 0.0254², which is 0.45359 / 0.00064516 = 703.07.
Rounding that to 703 costs you about one part in ten thousand, far below the precision of your bathroom scale.

Important context:

BMI is a useful population-level screening tool, but it has limitations for individuals.
It does not distinguish between muscle mass and fat mass. A very muscular athlete might have a high BMI while having low body fat.
It also does not account for age, bone density, or body fat distribution.

Tips:

• Switch the unit selector to work in kilograms and centimetres or in pounds and inches. Both paths give the same answer, which is the whole point of the 703.
• For a more complete health picture, combine BMI with waist circumference and body fat percentage measurements.
• Children and teenagers have different BMI categories based on age-specific growth charts. Use the teen BMI calculator instead if you are under 20.
• For adults of Asian heritage the risk starts lower. A WHO expert consultation in 2004 flagged 23 and 27.5 as action points, and in the UK NICE treats 23 as overweight and 27.5 as obese for adults with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background.