Body mass index, or BMI, is useful for assessing and adjusting body weight. It's not the most accurate way to find out what your body fat is, but it's the easiest and cheapest way to measure it. There are several different ways to measure BMI depending on the type of measurement chosen. Before you start, make sure you know what your current height and weight are, then start counting.
See the section When Should You Try? to know when you should measure your BMI.
Step
Method 1 of 4: Using Metric Measurement
Step 1. Measure your height in meters and square the number
You must multiply your height by the same number. For example, if you are 1.75 meters tall, you would multiply 1.75 by 1.75 and get around 3.06.
Step 2. Divide your weight in kilograms by meters squared
Next, you must divide your weight in kilograms by your height in meters squared. For example, if you weigh 75 kilograms and your height in meters squared is 3.06, you would divide 75 by 3.06 to get 24.5 as your BMI.
Step 3. Use the longer equation if your height is in centimeters
You can still calculate your BMI if your height is in centimeters, but you'll need to use a slightly different equation. The equation is weight in kilograms divided by height in centimeters, then divided by height in centimeters, then multiplied by 10,000.
For example, if your weight in kilograms is 60 and your height in centimeters is 152, you would divide 60 by 152, by 152 (60/152/152) to get 0.002596. Multiply this number by 10,000 and you get 25, 96 or roughly 30. So, this person's BMI is 30
Method 2 of 4: Using Imperial Measurement
Step 1. Square your height in inches
To square your height, multiply your height by the same number. For example, if you are 70 inches (177 cm) tall, multiply 70 by 70. The answer in this example is 4,900.
Step 2. Divide your weight by your height
Next, you need to divide your weight by your height squared. For example, if your weight in pounds is 180, divide 180 by 4,900. You will get the number 0.03673 as the answer.
Step 3. Multiply that answer by 703
To get your BMI, you must multiply the last answer by 703. For example, 0.03673 times 703 equals 25.83, so your estimated BMI in this example is 25. 8.
Method 3 of 4: Using Metric Conversion Factors
Step 1. Multiply your height in inches by 0.025
0.025 is the metric conversion factor needed to convert inches to meters. For example, if you are 60 inches (152 cm) tall, you must multiply 60 by 0.025 to get an answer of 1.5 meters.
Step 2. Square the last result
Next, you must multiply that last number by the same number. For example, if the last number is 1.5, multiply 1.5 by 1, 5. In this situation, the answer is 2.25.
Step 3. Multiply your weight in pounds by 0.45
0.45 is the metric conversion factor needed to convert pounds to kilograms. This will convert the weight to an equivalent metric. For example, if you weigh 150 pounds, the answer is 67.5.
Step 4. Divide the large number by the small number
Take the number you got for the weight and divide it by the number you got for the height squared. For example, 67.5 should be divided by 2.25. The answer is your BMI, and in this example it means 30.
Method 4 of 4: When Should You Try?
Step 1. Calculate your BMI to determine if you are at a healthy weight
The IMP is important because it can help determine whether you are underweight, normal weight, overweight, or obese.
- A BMI below 18.5 means underweight.
- BMI 18, 6 to 24, 9 means healthy.
- BMI 25 to 29.9 means overweight.
- A BMI of 30 and above indicates obesity.
Step 2. Use your BMI to find out if you are a candidate for bariatric surgery
In some situations, your BMI must be above a certain number if you want to have bariatric surgery. For example, to qualify for bariatric surgery in the UK, you must have a BMI of at least 35 if you don't have diabetes, and a BMI of at least 30 if you have diabetes.
Step 3. Record changes in BMI over time
You can also use BMI to help track changes in weight. For example, if you want to graph a weight loss chart, calculating your BMI on a regular basis will help. Or, if you want to track the growth of yourself or your child, one way is to calculate and record your BMI.
Step 4. Calculate your BMI before considering more expensive and invasive options
If you can determine that your weight is still within the normal range with your BMI, this is the best option. However, if you are an athlete or a sports fan and think that your BMI gives an inaccurate picture of your body fat content, then you should consider other options.
Skinfold testing, underwater weight measurement, dual-energy x-ray absorptiometry (DXA) and bioelectrical impedance are some of the options available to determine body fat content. It's just that you need to remember that these methods are more expensive and invasive than calculating BMI
Tips
- Maintaining a healthy weight is probably the single most important step you can take for optimal health and longevity. BMI is just a rough indicator of your general condition and physical health.
- Another way to determine whether your weight is healthy or not is to calculate your waist-to-hip ratio.
