How To Calculate BMI (Body Mass Index)? A Complete Guide

Step 1. Measure your Weight

You can weigh yourself by using a weighing scale. Most people don't know the perfect time to measure their body weight. Well, just after you come out of the bathroom in the morning is an excellent time.

It is also very important to keep the scale on a flat surface and, if your scale is digital, in a place whose temperature will remain relatively constant throughout the day. The general formula for BMI considers weight in Kg.

Likewise, you can also weigh yourself if the scale shows weight in pounds. There is no need to worry about the measurement style.

Step 2. Measure your Height

You can measure your height using a regular measuring tape. It is even more accurate when done with the assistance of another person.

Simply stand upright on your barefoot, keeping your feet together and heels touching the wall. Take something that has a flat surface, put it on your head, and perpendicular to the wall. If needed, ask someone to hold it for you.

Mark the point on the wall where the tape meets your head and the lower edge of your leg. Then, take a measuring tape and determine the distance from the ground to that mark.

Step 3. Compute your BMI

You can calculate your BMI using simple math skills, either with a calculator or by hand. It can be done with the metric system (kilograms and meters) or the imperial system (pounds and inches).

The calculations are described in the following formulas:

BMI FORMULA

In Metric Units,

BMI = Weight (kg) / [Height(m)]^2

In Imperial Units,

BMI = 703* Weight (lbs) / [Height(in)]^2

Example 1: Calculating BMI With Kilograms

If a person weighs 75 kilograms and is 1.75 meters tall, you would calculate the BMI as follows:

• Start by calculating the meters squared. Using this example, it would be 1.75 x 1.75 = 3.06 sq. m.
• Next, divide the weight in kilograms by the meters squared result. Using this example, it would be 75 ÷ 3.06 = 24.5 BMI.
• The BMI for this person is 24.5.

Example 2: Calculating BMI With Pounds 

If a person weighs 180 pounds and is 70 inches tall, you would calculate the BMI as follows:

• Start by calculating the inches squared. Using this example, it would be 70 x 70 = 4,900 sq. in.
• Next, divide the weight in pounds by the inches squared result. For this example, it would be 180 ÷ 4,900 = 0.03673.
• Finally, multiply that result by 703. The final result would be 0.03673 x 703 = 2.58 BMI.                                                                  
• The BMI for this person is 25.8.

The optimum range for a healthy BMI is between 18.5 and 24.9. Anything above and below these values is technically considered underweight and overweight, respectively.

Body mass index scores are broken down into the following categories:

• Underweight - BMI less than 18.5
• Normal weight - BMI of 18.5 to 24.9
• Overweight - BMI of 25 to 29.9
• Obesity - BMI of 30 or higher

Obesity is sometimes broken down into additional categories:

• Class 1 (mild obesity) - BMI 30 to 34.9
• Class 2 (moderate obesity) -BMI 35 to 39.9
• Class 3 (severe obesity) - BMI of 40 or higher

BMI helps assess whether you are underweight, normal weight, overweight, or obese, which can indicate potential health risks. A high or low BMI may be linked to various health issues, including:

1. Heart disease
2. High blood pressure
3. Liver diseases
4. High cholesterol
5. Type 2 diabetes
6. Gallstones
7. Sleep apnea
8. Osteoarthritis 
9. Cancer 

While excess body fat has been linked to the risk of several health conditions, it is essential to emphasize, however, that BMI calculations alone cannot diagnose health problems - they only point out potential red flags.

A simple index of body fat, BMI is convenient and inexpensive to use, and it can be measured anywhere. A person's BMI indicates whether they have a health risk, and due to that fact, it is universally used by health professionals globally.

Yet it also involves significant limitations: as a tool, a BMI score has no real diagnostic value but is rather used to help calculate your risk for certain diseases such as with a cardiac risk estimator.

The diagnosis of metabolic syndrome conditions has absolutely nothing to do with BMI, though it may be used to help set weight loss goals for an individual who could be diagnosed with such conditions. Also, the factors that BMI does not account for make it a false alarm for many people.

Here are some cons of BMI:

1. Muscle mass

BMI cannot distinguish between fat and muscles so it can overestimate body fat in athletes or individuals with lots of muscle mass. It can also underestimate body fat in people with very little muscle mass.

2. Age 

Because structure and composition change with age, BMI sometimes gives misleading results among different age groups. Younger individuals will have more muscle mass, while with aging, muscle is lost and fat is gained, which cannot be reflected by the calculation of BMI.

3. Body Shape

BMI can be inaccurate because it overlooks body shape and how weight is distributed. Even with the same BMI, individuals may have varying health risks depending on whether their weight comes from muscle or fat and the areas where fat accumulates.

4. Gender

BMI can be less reliable because it does not differentiate between sexes, which can affect body composition. 

Men and women often have different fat and muscle distributions, so two people with the same BMI might have different health implications based on their sex and how fat is distributed in their bodies.

1. To Determine if you are at a Healthy Weight

Determining BMI can also inform whether a person is within a healthy weight range for his or her height – a pertinent benchmark for overarching health.

While BMI is not a whole-picture measure, it can do the job of indicating a health risk through excess weight, and prompt follow-up discussion between individuals and their health professionals about lifestyle and wellness.

2. To track changes in your BMI over time

Tracking your BMI over time is essential for monitoring how your weight changes in relation to your height. This practice can offer valuable insights into your health and fitness journey.

Consistently checking will enable you to identify patterns of weight fluctuations, empowering you to make well-informed decisions regarding adjustments to your diet, routine, and other health-related habits.

3. To avoid more costly and invasive options

Understanding your BMI can provide a valuable preliminary assessment before exploring more expensive and invasive treatments. This initial evaluation helps to identify whether simpler, less costly options might be effective in managing weight-related health issues.

4. To see if you are a Candidate for Bariatric Surgery

BMI might be a critical component in deciding whether you are a candidate for bariatric surgery. Health professionals will use the BMI to determine whether your weight-related health problems qualify for surgery and will support decisions about appropriate treatment options.

For instance, to be eligible for bariatric surgery in the UK, you would require a minimum BMI of 35 should you be devoid of diabetes. Likewise, you would need a minimum of 30 in case you happen to be suffering from diabetes.

Here are some alternative methods for measuring body fat and assessing health risks that can be used in place of BMI:

1. Waist Circumference

Waist circumference is a good substitute indicator for estimating body fat distribution. Measuring waist circumference gives some idea about intra-abdominal fat, which is associated with health risks of critical diseases such as heart disease and diabetes.

This is a very easy, accessible measurement that can be done at home, as well as clinically. It gives a more direct look at the abdominal accumulation of fat that is not provided with BMI and perhaps is a better indication of some health risks.

2. Waist-to-hip Ratio

Another alternative to BMI for understanding body fat distribution is the waist-to-hip ratio. This is the ratio of the circumference of your waist compared to that of your hips to estimate the proportion of abdominal fat to total body fat.

This is a priceless measurement, as it gives quite valuable insight into your risk regarding specific conditions such as heart disease and stroke. It can give a more differential evaluation of fat distribution than the more generalized BMI that does not take into account the actual body fat or weight.

3. Smart scale 

Using a smart scale goes all the way into showing a person's full-body composition. These devices measure not only weight but also body fat, muscle mass, and water content using highly developed technology such as bioelectrical impedance analysis.

Smart scales can keep track of these over time to paint a fuller picture of health and fitness progress. This, in turn, allows you to make more informed adjustments based on a broader tract of data.

4. Skin-fold Measurements 

Skin-fold measurements offer an alternative to BMI by estimating body fat percentage through the thickness of skin folds at various body sites. This method involves using calipers to measure the folds of skin and underlying fat at specific locations, such as the triceps or abdomen.

These measurements can aid in tracking the changes in body fat levels over time, so they offer a comprehensive understanding of fat distribution and body composition. This method can be a more precise evaluation of body fat as opposed to BMI, which fails to account between fat and muscle mass.

5. Underwater Weighing

Underwater weighing is used to assess body fat percentage by comparing the body density. When you weigh yourself on land and then again in water, you can use the ratio of the two weights along with body volume to calculate your fat content. This way of measuring body fat is more accurate than methods such as BMI.

It offers enhanced precision, providing a detailed assessment of body composition changes. It stands out as more precise for evaluating alterations in both lean muscle mass and fat mass.

6. Dual-energy X-ray Absorptiometry (DEXA)

Dual-energy X-ray absorptiometry (DEXA) utilizes edge imaging technology to offer a thorough examination of composition. Through the use of low-level X, it assesses bone, fat distribution, and muscle mass, offering a holistic insight into the role of elements in one's general well-being.

This method is especially useful for understanding fat distribution and monitoring bone health, offering precise insights that go beyond what BMI can reveal. DEXA's detailed analysis helps in making informed decisions about health and fitness.

7. Air Displacement Plethysmography (ADP)

This method offers a precise way to assess body composition by measuring body volume and density. Using a specialized chamber to calculate these factors through air displacement provides detailed information about fat and lean body mass.

The accuracy of ADP makes it an excellent tool for tracking changes in body composition. It offers a more nuanced understanding of health compared to BMI, aiding in making well-informed decisions about fitness and wellness.