It's a formula using someone's height and weight to determine if they are a healthy weight.
It's what most health care professionals use, but now a mathematician at Oxford University says the formula is flawed.
Professor Nick Trefethen says the BMI underestimates the bulk of short people while tall people aren't as overweight as they are being told.
He has come up with his own version of the BMI.
Traditionally, a BMI between 18.5 and 24.9 is considered normal. Less than 18.5 is considered underweight, while 25 to 29.9 is considered overnight. A BMI of 30 or more is considered obese.
Under Trefethen's new calculations, someone who is 5' tall would be overweight at 122 pounds, instead of 129 pounds, on the current chart.
But a person who is 6' tall wouldn't be overweight until they hit 192 pounds. That's 7 pounds above the current level.
If you want to check out your 'new' BMI, Trefethen has put a calculator on his website.
But regardless of this controversy, it is important to know your BMI, at least the current standard.
Many times people underestimate their weight and you could be at risk for heart disease or diabetes and not know it.
So we have an easy way to calculate your BMI and what it means: http://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/english_bmi_calculator/bmi_calculator.html