MINNEAPOLIS, Minn. (WCCO) - Most of us think we know obesity when we see it.

And we’ll be seeing a lot more of it, by 2030, according to researchers, when 42 percent of Americans will be obese.

But with so much discussion of the obesity epidemic, what makes someone obese?

We asked Allen Levine, director of the Minnesota Obesity Center at the University of Minnesota.

“We define with something called the body mass index,” said Levine.

Body Mass Index is calculated by taking your weight in kilograms divided by your height (in meters) squared.

A BMI greater than 25 means you’re considered overweight. A BMI greater than 30 has you classified as obese.

For someone 5 foot 2 inches tall, obesity would be defined as 164 pounds. For someone 5 foot 9 inches tall, obese is 203 pounds, about 30 pounds overweight.

“There are more health problems with obese people, more diabetes,” said Levine.

Today, nearly 36 percent of adults are obese. When you’re surrounded by it, it starts to look normal.

“The numbers have changed over the years,” said Levine, who said the definition of obesity used to be a BMI of 27.

“They’ve done it by taking a look at the populations and seeing where the risk is,” he said.

Levine said he feels the current BMI formula is useful and accurate.

“I think everyone should know where their BMI is,” said Levine. “It gives them some idea, some notion that they should do something about their weight.”

Doctors say if we can keep obesity flat, we’ll save $550 billion by 2030.

“Are we gonna have enough physicians to take care of kidney disease when all these people have diabetes?” wondered Levine.

If we can reduce the trend by 1 percent, that will result in 2.9 million fewer obese Americans by 2030.

Levine said the same trend is happening in first world, developed countries around the world.

Researchers aren’t sure exactly why there is so much more obesity, but it’s not as simple as blaming one thing.

Levine’s theory: “Food is ever-present, and it’s inexpensive.”