BMI calculations might seem like some esoteric and complicated process that you have no hope of figuring out. That’s not true. BMI isn’t magic or some scientific secret withheld by those in the know. If you’ve ever used a BMI calculator online, then you know that you put in your height and weight, and it cranks out special number.

There’s always a chart handy, or the website interprets it for you, and tells you whether you’re underweight, normal weight, overweight or obese, all based on this single number. The website did the calculations for you, but you might be wondering just how it arrived at that number. BMI calculations are fairly simple once you understand the basic formula. But first, let’s look at what BMI is and what BMI calculations are used for in the first place.

BMI is the measure of body mass. By comparing your weight and height, specifically dividing your weight by your height, squared, the BMI calculations give us a single number. Depending on where that number falls on a special scale, it tells you if you’re underweight for your height, in the normal range, overweight, or obese.

BMI doesn’t measure body fat, however, so often the number is wrong when it comes to athletes or very muscular people. Muscle weighs more than fat, so for them, BMI isn’t an accurate measure at all. For most people, though, it’s a very good estimate of weight in relation to height. So what BMI calculations allow us to arrive at that magic BMI number?

BMI calculations may seem complicated, but they’re really not. At its most basic, the formula is kg/m2. Or your weight in kilograms divided by your height in meters, squared (your height in meters times itself). A person who’s five-and-a-half-feet tall stands about 1.7 meters. Square that number (1.7 times 1.7) and you come up with 2.89. A person who weighs 150 pounds weights about 68 kilograms. So the BMI calculations to determine this person’s BMI would be 68 divided by 2.89. The result of that is about 23.5.

A BMI of 18.5 or below indicates that an individual is underweight. The normal weight range is from 18.5 to 24.9. Overweight people will calculate a BMI of 25 to 29.9. A BMI of 30 or greater indicates obesity. So a person with a BMI of 23.5 is within his or her normal weight range, according to the BMI calculations.

If you don’t want to convert feet and pounds to meters and kilograms, then you can use one of the other BMI calculations to figure your number. This person’s new formula would be weight in pounds, 150, times 4.88, or 732, divided by height in feet squared (5.5 times 5.5) which is 30.25. Then divide weight by height squared (732 divided by 30.25) and the result is 24.1, very similar to the metric formula’s result. BMI calculations can also be done by taking weight in pounds times 703, and dividing by height in inches, squared.