If you have ever tried an online BMI calculator, then you're aware that all you do is enter your weight and height, and then it gives you your results. There's usually a chart handy, or the website tells you if you are underweight, normal, overweight or obese. Even though the online calculator figured it out for you, you may still wonder how it got that number.
The good news is that while online BMI calculators may make it a bit easier, figuring them out on your own is quite simple once you know the formula. Before we get to the formula, let's take a look at what BMI is and what it's used for.
Basically, BMI attempts to give people an objective measure of their body mass. This is attained by comparing your height and weight, relative to each other (more on that in a bit), and then giving you a single BMI number. This number is then compared to the standard BMI scale to see where you lie between underweight and obesity.
One thing that BMI doesn't measure is body fat. That means the number tends to get more skewed as the overall amount of muscle goes up. That's because it weighs more than fat. Therefore athletes do not usually view it as an accurate measurement of their fitness. For most people, however, the BMI works as a pretty good calculation of their weight to height ratio. So, what is the formula for calculating your BMI number? Let's take a look now.
This may sound complicated at first, but it's really quite simple. Those in the US will have to take the additional step of converting their weight and height to kilograms and meters, respectively. Here's the formula: kg/m^2 , which is your weight in kilograms divided by height in meters squared (your height times itself). So, somebody that's 1.7 meters (about 5' 6") and weighs 70 kilograms (about 154 pounds) would do the following:
1.7 X 1.7 = 2.89 (1.7 meters, squared)
70/2.89 = 24.2 (70 kilograms divided by 1.7 meters, squared)
The number 24.2 is the BMI. The final step is to see where it falls on the scale. Anything below 18.5 is underweight. 18.5 to 24.9 is normal. 25.0 to 29.9 is overweight, and anything 30.0 and above is considered obese. Our example of 24.2 falls within the range for those with normal weight.
So that you know how to do it yourself, you won't have to be near a computer to figure out your BMI. If you still want to use an online BMI calculator, that's fine. At least now you will understand how the number is calculated - and, as you've seen, it's no mystery.
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