How to Calculate Your Body Mass Index (BMI)

Knowing your body mass index, or BMI, can be useful for assessing and adjusting your weight. It is not the most accurate measure of how much body fat you have, but it is the easiest and least expensive way to measure it.^{[1]
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Centers for Disease Control and Prevention
Main public health institute for the US, run by the Dept. of Health and Human Services

Go to source} There are different ways of calculating your BMI depending on the type of measurements you have taken. Make sure that you know your current height and weight before you get started and then try calculating your BMI.

See When Should You Try This? to learn more about when calculating your BMI might be a good course of action.

How do I calculate my BMI?

If you use the imperial system, the BMI equation is: weight (lb) / [height (in)]² x 703. If you use the metric system, the BMI equation is: weight (kg) / height (m)².

Method 1

Method 1 of 3:

Using Metric Measurements

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You will need to multiply your height in meters by itself first. For example, if you are 1.75 meters tall, then you would multiply 1.75 by 1.75 and get a result of approximately 3.06.^{[2]
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2
Divide your weight in kilograms by meters squared. Next, you will need to divide your weight in kilograms by your height in meters squared. For example, if your weight is 75 kilograms and your height in meters squared is 3.06, then you would divide 75 by 3.06 for an answer of 24.5 as your BMI.^{[3]
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- The full equation is kg/m² in which kg is equal to your weight in kilograms and m is equal to your height in meters.
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3
Use an extended equation if your height is in centimeters. You can still calculate your BMI if your height is in centimeters, but you will need to use a slightly different equation to do so. This equation is your weight in kilograms divided by your height in centimeters, then divided again by your height in centimeters, and then multiplied by 10,000.^{[4]
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- For example, if your weight in kilograms is 60 and your height in centimeters is 152, then you would divide 60 by 152, by 152 (60 / 152 / 152) for an answer of 0.002596. Multiply this number by 10,000 and you get 25.96 or about 26. The approximate BMI for this person would be 26.
- Another option is to simply change your height in centimeters to meters by moving the decimal two places to the left. For example, 152 centimeters equals 1.52 meters. Then, find your BMI by squaring your height in meters and then dividing your weight by your height in meters squared. For example, 1.52 multiplied by 1.52 equals 2.31. If you weigh 80 kilograms, then you would divide 80 by 2.31 and your result would be a BMI of 34.6.

Method 2

Method 2 of 3:

Using Imperial Measurements

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To square your height, multiply your height in inches by itself. For example, if you are 70 inches tall, then multiply 70 by 70. Your answer for this example would be 4,900.^{[5]
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2
Divide weight by height. Next, you will need to divide your weight by your squared height. For example, if your weight in pounds is 180, then divide 180 by 4,900. You would get an answer of 0.03673.^{[6]
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- The equation is weight/height².
To get your BMI, you will then need to multiply your last answer by 703. For example, 0.03673 multiplied by 703 equals 25.82, so your approximate BMI in this example would be 25.8.^{[7]
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Method 3

Method 3 of 3:

When Should You Try This?

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1
Calculate your BMI to determine if you are at a healthy weight. Your BMI is important because it can help you to determine if you are underweight, normal weight, overweight, or obese.^{[8]
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- A BMI below 18.5 means that you are underweight.
- A BMI of 18.6 to 24.9 is healthy.
- A BMI of 25 to 29.9 means that you are overweight.
- A BMI of 30 or greater indicates obesity.
In some situations, your BMI may need to be above a certain number if you want to have bariatric surgery. For example, to qualify for bariatric surgery in the UK, you would need to have a BMI of at least 35 if you do not have diabetes and a BMI of at least 30 if you do have diabetes.^{[9]
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You can also use your BMI to help you track changes in your weight over time. For example, if you want to chart your weight loss, then calculating your BMI on a regular basis might be helpful. Or, if you want to track growth in yourself or in a child, then calculating and tracking BMI is another way to do that.
4
Calculate BMI before considering more expensive and invasive options. A person with a BMI under 25 is considered to have a healthy body weight. However, if you have a higher than normal muscle percentage, your BMI might be higher. In that case, a BMI higher than 25 may not necessarily indicate that you are overweight. If you are muscled, consider getting skin fold testing to determine whether you have too much fat.
- Along with skin fold tests, underwater weighing, dual-energy x-ray absorptiometry (DXA) and bioelectrical impedance are some of the other options available for determining your body's fat content. Just keep in mind that these methods are more expensive and invasive than calculating BMI.^{[10]
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Question

What is the formula to check BMI?

Michele Dolan
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Michele Dolan is a BCRPA certified Personal Trainer in British Columbia. She has been a personal trainer and fitness instructor since 2002.

Michele Dolan

Certified Fitness Trainer

Expert Answer

Body Mass Index (BMI) = (body weight in Kg) divided by your height (in metres) squared

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My height 60 inches and weight is 40 kg. What is my BMI?

Michele Dolan
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Michele Dolan is a BCRPA certified Personal Trainer in British Columbia. She has been a personal trainer and fitness instructor since 2002.

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First you must have measurements all in metric or all in Imperial. 60 inches equals 1.5 meters (60 x 0.025). Then square that number for your height, so 1.5 x 1.5 = 2.25. Then your weight, divided by your height would be 40 kg/2.25 m² equals 17.7. That is your BMI.

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What BMI is considered morbidly obese?

Michele Dolan
Certified Fitness Trainer
Michele Dolan is a BCRPA certified Personal Trainer in British Columbia. She has been a personal trainer and fitness instructor since 2002.

Michele Dolan

Certified Fitness Trainer

Expert Answer

A BMI of 40 or being 100 pounds overweight with health problems constitutes being morbidly obese. (Based on urmc.rochester.edu/highland/bariatric-surgery-center/questions/morbid-obesity.aspx)

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There are also online calculators available if you are having trouble calculating your BMI manually.

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Another easy way to determine if you are at a healthy weight is to calculate your waist to hip ratio, which indicates the proportion of fat your body holds around your waist, or how much fat is visceral. Too much fat around the internal organs, or 'viscera', is a serious health risk.

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Maintaining a healthy weight is perhaps the single most important step you can take toward optimal health and long life. Knowing your BMI can help you determine if weight loss would be recommended for you. Remember a BMI over 25 indicates you are over weight and a BMI of 30 indicates obesity, which is a serious health risk.

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Warnings

Keep in mind that BMI doesn't take into account muscle mass.^{[11]
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Dean Theriot
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Expert Interview. 22 January 2021.} It also doesn't take into account overall body type.

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