Before we start calculating BMI, let’s discuss what BMI is, and what it isn’t. BMI, or the body mass index, is a tool that’s used to determine whether or not a person is at a normal weight for his or her height. It was invented between 1830 and 1850 by mathematician and scientist Adolphe Quetelet. Though it’s been almost 200 years since he created the BMI, it’s used today almost exactly as he outlined.

While BMI isn’t the final word in whether or not someone is at a normal weight, it’s one of the first tools often used to determine if someone’s overweight. Because the BMI doesn’t distinguish between weight from fat and weight from muscle (other tools must be used to determine the actual amount of fat) it doesn’t work well for very muscular people like athletes. But for most of us, BMI is a pretty accurate measure of whether we’re underweight, a normal weight, overweight or obese.

Calculating BMI isn’t difficult once you understand the formula. The basic formula is weight divided by height squared, or kg/m2. You simply take your height in meters and square it which means you take the number of meters times itself. Then divide that number into your weight in kilograms. So a person who’s five-and-a-half-feet tall stands about 1.7 meters. To get 1.7 meters squared, you simply multiply 1.7 times 1.7, which equals 2.89. Now, convert weight to kilograms.

So if someone weigh 200 pounds, that’s about 91 kilograms. To determine the BMI of someone about five-and-a-half-feet tall who weights 200 pounds, or who stands 1.7 meters tall who weights 91 kilograms, we divide the weight by the height squared, or 91 divided by 2.89. That gives us the result of about 31. This individual is classified as obese. How do I know that?

A BMI of 18.5 or below indicates an underweight person. The normal weight range is from 18.5 to 24.9. Overweight people will have a BMI of 25 to 29.9. A BMI of 30 or greater indicates obesity. So a person with a BMI of 31 is in the obese range, but not by much. By calculating BMI again with different weights, you can see that the person would only need to lose 5 kilograms, or about 11 pounds, to no longer be obese, but merely overweight. And a loss of 19 kilograms, or about 42 pounds, will put this person within a normal weight range. Calculating BMI to determine how much weight one should lose can be a big help for someone who’s not sure.

Calculating BMI can be made even more simple if you’re not sure how to convert to kilograms or meters. Simply take your weight in pounds times 4.88. Then divide that number by your height in feet, squared. So a 200 pound person who is 6 feet tall: 976 divided by 36 = 27.1. After calculating BMI, it’s clear this person is within a normal and healthy weight range.

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.