BMI Calculator Formula: How to Calculate BMI by Hand

BMI calculator formula in kg and cm

1. Change height from cm to metres: 165 cm ÷ 100 = 1.65 m.
2. Square it: 1.65 × 1.65 = 2.7225.
3. Divide weight by that number: 68 ÷ 2.7225 = 25.0.

Round to one decimal place. Most charts list bands to one decimal, for example 22.9 and 23.0.

How to calculate BMI from feet and inches

1. Turn feet into inches: 5 feet = 60 inches; add the extra inches, so 5 feet 4 inches = 64 inches.
2. Turn inches into cm: 64 × 2.54 = 162.56 cm.
3. Carry on with the metric steps: 1.6256 × 1.6256 = 2.6426.
4. Divide: 58 kg ÷ 2.6426 = 21.9.

The kg and feet calculator accepts feet directly and skips the conversion.

The 703 formula for pounds

703 is a conversion factor that turns pounds per square inch into kg per square metre. Both formulas give the same BMI.

BMI full form and where the formula came from

BMI stands for Body Mass Index. The idea dates back to the nineteenth century, when a Belgian scientist studying average body sizes noticed that adult weight rises roughly with the square of height. Much later, researchers showed that this simple ratio tracks body fat reasonably well across large groups, and the WHO adopted it for adult weight bands.

The formula was never meant to diagnose one person. It is a quick screen that tells you whether to look closer.

Worked examples you can check

Person A sits at 21.6, healthy. B at 25.0 is obese by Indian cut-offs and overweight by WHO. C at 24.9 is overweight in India. D at 27.8 is obese in India and overweight by WHO. The BMI chart lists every band.

Variants of the formula

• BMI Prime: your BMI divided by the upper healthy limit. With an upper limit of 22.9, a BMI of 25 gives 1.09, about 9 percent above the line.
• New BMI: a proposed version that divides by height to the power 2.5 and multiplies by 1.3, so tall and short people are treated more evenly. It is not used in clinics.
• Child BMI: the same formula, but read on age and sex charts; see the kids page.

Using the formula backwards

To find the weight for a target BMI, multiply that BMI by height in metres squared. At 1.65 m, a BMI of 22.9 means 22.9 × 2.7225 = 62.3 kg. That is how the ideal weight calculator builds a healthy range.

The same trick answers the question people ask most after seeing their result: how many kilos do I need to lose or gain? Work out the weight at the edge of the healthy band, then subtract it from your current weight. If you weigh 70 kg at 1.65 m, the gap to 62.3 kg is 7.7 kg, which at half a kilo a week is about four months of steady effort.

Common mistakes

• Forgetting to square the height, which gives a number around 40.
• Using cm instead of metres, which gives a tiny number such as 0.002.
• Mixing kg with inches.
• Rounding height too early. Keep three decimals until the end.
• Measuring height with shoes on, which can add 2 to 3 cm and lower your BMI by about half a point.