BMI is reasonably simple to calculate but before we look at that, let's take a look at BMI and what it actually is. BMI is short for Body Mass Index and this is a tool that was created in the 1800s to determine a persons weight range. The formula will give a result that will indicate whether a person is in a normal weight range for their height or whether they are over or under weight. Even though this formula was created 200 years ago, it is still widely used today.
There are some situations where BMI results won't be accurate but generally for most people it will give an accurate result. The problem with BMI is that it doesn't distinguish between fat and muscle so those people, like body builders, who have a lot of muscle, their BMI results would not be accurate. For most of us though, BMI will give us a pretty good idea of whether we are in a healthy weight range, underweight, overweight or obese.
How to Calculate BMI
There is a basic formula that is used to calculate BMI and that is weight divided by height squared – kg/m2. You take your height (in meters) and square it, so you times it by itself. Then you divide the answer into your weight (in kilograms).
Let's do an example so you can see exactly how it works. If a person is 1.7 meters tall we would times 1.7 by 1.7 which gives us an answer of 2.89. Now if that person weights 90 kilograms we then divide the weight by the height squared, so that is 90 divided by 2.89. This gives us a BMI of 31. A BMI of 31 would indicate that this person is obese.
The normal weight range is a BMI between 18.5 and 24.9. If you fall below 18.5 then you would be underweight. If you fall in the range of 25 to 29.9 then you would be considered overweight. If your BMI results was 30 or over then you are considered obese. So now you can see how the answer we got above of 31 would put that person in the obese category.
Let's do another example using the same height but a different weight. So the person is 1.7 meters and 1.7 squared is 2.89. This time let's say that they weight 68 kilograms so we divide 68 by 2.89 which gives us a BMI result of 23.5. This person would be within the normal weight range. You can use BMI calculations to determine how much weight you need to lose to be in a healthy range. With these two examples, the person in the first example would need to lose around 20 kilograms to get down to a healthy weight range.
If you don't like to work in meters and kilograms there is another formula that you can use for pounds and feet. With this formula you take your weight in pounds and times it by 4.88. Then you divide that number by your height in feet, squared. Let's work that out using a person that is 200 pounds and 6 feet tall. You would times 200 by 4.88 which is 976. Then you would take the height in feet and times it by itself, so 6 times 6 equals 36. Now take the weight answer of 976 and divide it by 36 which equals 27.1 – this is your BMI. A result of 27.1 would put a person in the overweight range.
When calculating your BMI just remember to factor in whether you have a lot of muscle or not as this can make the results inaccurate.