Food labels in the United States list grams. Gym weights are in pounds. Road signs are in miles and your doctor's chart is in kilograms per meter squared. Mixing unit systems is not a character flaw, but it is the most common reason people get a BMI that looks wrong, so it is worth getting straight.
The formula in both systems
Metric is the natural form. Weight in kilograms divided by height in meters, squared:
BMI = kg / m²
The US version adds a constant to make pounds and inches give the same answer:
BMI = 703 × lb / in²
The 703 is not arbitrary. It is the conversion factor that reconciles pounds per square inch with kilograms per square meter, rounded to the nearest whole number. If you forget it, your answer will be about 703 times too large, which is the kind of mistake that produces a BMI of 19,000.
Feet and inches, done right
Height in the imperial formula is in total inches, not feet. Someone who is 5 ft 9 in is 69 inches tall, not 5.9 feet or 5 feet 9 inches typed as 5.9. This is the single most common error. Entering 5.9 as a height turns a person of 5 ft 9 in into someone closer to 5 ft 10.8 in, which lowers the calculated BMI by roughly three percent.
| Height | Total inches | Centimeters |
|---|---|---|
| 5 ft 4 in | 64 | 162.6 |
| 5 ft 6 in | 66 | 167.6 |
| 5 ft 8 in | 68 | 172.7 |
| 5 ft 10 in | 70 | 177.8 |
| 6 ft 0 in | 72 | 182.9 |
The conversions worth memorizing
- 1 inch = 2.54 centimeters exactly.
- 1 pound = 0.45359237 kilograms exactly.
- 1 stone = 14 pounds, useful if you read British sources.
- 1 foot = 12 inches, and always convert to inches before using any formula.
Anyone who has tried to do this in their head knows it is tedious. The BMI calculator on this site takes feet, inches and pounds directly and does the conversion internally, which removes the whole category of mistake.
Three checks before you trust a number
First, sanity-check the result. For an adult, BMI should land somewhere between about 14 and 60. Anything outside that range is a unit error. Second, confirm that your height is in inches and your weight is in pounds, not stone or kilograms. Third, if the answer changed dramatically without your body changing, check what you typed rather than what you did at the gym.
One more thing worth knowing: BMI is unit-invariant by design. A person at 175 cm and 72.6 kg has exactly the same BMI as a person at 5 ft 9 in and 160 lb, because those are the same body measured two ways. If two inputs that describe the same person produce different BMIs, the conversion went wrong somewhere, not the formula.