Your Body Mass In­dex (BMI) is a way for doc­tors and per­son­al train­ers alike to tell if you have too much fat on your body. This is a val­ue that is de­rived from the height and mass/weight of the in­di­vid­ual. The re­sult of this test can give an “ac­cu­rate” (or not-so-ac­cu­rate) mea­sure­ment of whether a per­son’s weight match­es their height.

How is it cal­cu­lat­ed?

You take a per­son’s weight in pounds and mul­ti­ply it by 703, and then you di­vide the sum by their height in inch­es, squared. To make life eas­i­er, you can al­ways go on­line and search for a “BMI cal­cu­la­tor”—it does the cal­cu­la­tion for you, so you won’t have to do the math.

What do the re­sults mean?

An­oth­er key part of cal­cu­lat­ing your BMI is un­der­stand­ing what the re­sults re­al­ly mean. If the re­sults are be­low 18.5, that means the in­di­vid­ual is un­der­weight; if the re­sults are be­tween 18.4 and 24.9, the in­di­vid­ual is con­sid­ered a healthy weight; if the re­sults are 25.0 to 29.9, this is con­sid­ered over­weight; and if the re­sults are 30.0 and above, the in­di­vid­ual is clas­si­fied as obese.

How and why doc­tors use your BMI.

BMI is not ac­cu­rate enough for doc­tors to use it as a means for di­ag­no­sis, but it can be used as a flag for un­der­ly­ing weight-re­lat­ed health is­sues. If an in­di­vid­ual has a very high BMI, the doc­tor may then con­sid­er us­ing oth­er types of tests to de­ter­mine how to move for­ward. The doc­tor can then make in­formed de­ci­sions on what changes the in­di­vid­ual may need to make to their di­et, ex­er­cise and gen­er­al lifestyle to avoid fur­ther health com­pli­ca­tions.

Hav­ing ex­tra weight on your body is nev­er a good thing. Ex­cess weight can lead to a num­ber of health is­sues and lifestyle dis­eases. Here are a few neg­a­tive im­pacts of ex­tra weight:

In­creased blood pres­sure and cho­les­terol

High­er risk of lifestyle dis­eases

High­er risk of di­a­betes

Coro­nary heart dis­ease

Stroke and more.

Is BMI an ac­cu­rate mea­sure­ment of body fat?

Ac­cord­ing to Dr Hope Mecot­ti of the Har­vard Med­ical School, “BMI (your weight in kilo­grams di­vid­ed by the square of your height in me­ters) is a pret­ty re­li­able way to de­ter­mine whether you have too much body fat. That said, BMI is not a per­fect mea­sure be­cause it does not di­rect­ly as­sess body fat. Mus­cle and bone are denser than fat, so BMI can over­es­ti­mate body fat in ath­letes with high bone den­si­ty and mus­cle mass, or un­der­es­ti­mate it in old­er peo­ple who have low bone den­si­ty and mus­cle mass.”

BMI can be a use­ful tool for iden­ti­fy­ing cer­tain health risks but it should be used with cau­tion as it does not take in­to con­sid­er­a­tion body com­po­si­tion.