How to Calculate BMI: An Easy-to-Follow Guide

How to Calculate BMI
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Body Mass Index (BMI) is a widely used method for assessing an individual’s healthy weight, considering their height and weight. It helps in determining the likelihood of health problems arising from excessive body fat or being underweight. Calculating your BMI can be an important step toward understanding your overall health and taking action to maintain or improve it.

To calculate BMI, you’ll need to know your weight in kilograms (or pounds) and your height in meters (or feet). The process involves dividing your weight by the square of your height. Several online tools, such as the Adult BMI Calculator from the CDC or the BMI Calculator from, can help you easily determine your BMI and classify it, such as underweight, normal weight, overweight, or obese. Remember that while BMI is useful, it is not a diagnostic tool for determining an individual’s exact body fatness or health.

What Is BMI

Definition of Body Mass Index

BMI, or Body Mass Index, is a widely used screening tool that measures the ratio of your height to your weight to estimate the amount of body fat you have. It is a fairly simple and accessible method for healthcare providers and individuals. BMI determines a person’s weight classification, which may indicate potential health risks and the need for a more in-depth evaluation.

The basic formula for calculating BMI is:

BMI = weight in kilograms / (height in meters)^2

For individuals using pounds and inches, the formula can be converted to:

BMI = (weight in pounds / (height in inches)^2) * 703

BMI is a helpful tool for most people, as it often correlates with body fat levels; however, it is important to note that it has its limitations. For example, athletes and bodybuilders with high muscle mass may have a high BMI without excessive body fat. Similarly, older individuals or those with depleted muscle mass may have a lower BMI, even with unhealthy body fat percentages.

BMI is split into several categories, these are:

  • Underweight: BMI < 18.5
  • Normal weight: 18.5 <= BMI < 24.9
  • Overweight: 25 <= BMI < 29.9
  • Obesity, Class I (moderate): 30 <= BMI < 34.9
  • Obesity, Class II (severe): 35 <= BMI < 39.9
  • Obesity, Class III (very severe or morbidly obese): BMI >= 40

Although BMI is not a definitive diagnostic tool, it serves as a useful starting point for discussing weight and potential health concerns. It is crucial to remember that it does not take into account factors such as muscle mass, bone density, or overall health conditions. Therefore, it remains essential for individuals to consult their healthcare providers about their BMI and the potential implications for their overall well-being.

How to Calculate BMI

Using the BMI Formula

To calculate your Body Mass Index (BMI) manually, you will need to know your height and weight. You can use either the metric system (meters and kilograms) or the imperial system (inches and pounds).

  • Metric System: To calculate BMI using meters (m) and kilograms (kg), simply take your weight in kg and divide it by your height in meters squared. The formula is: BMI = weight (kg) / height (m)^2

  • Imperial System: For those who prefer to use inches (in) and pounds (lbs), the formula is slightly different: BMI = (weight (lbs) x 703) / height (in)^2

For a better understanding, let’s break down these calculations into smaller steps:

  1. Convert your height to meters (if using the metric system) or inches (if using the imperial system). To do this, you can multiply your height in centimeters by 0.01 to get meters, or divide your height in centimeters by 2.54 to get inches.
  2. Square your height in meters (m) or inches (in).
  3. Divide your weight in kilograms (kg) or pounds (lbs) by the squared height value.
  4. The result is your BMI.

Using a BMI Calculator

If you prefer a simpler method or want to avoid manual calculations, use a BMI calculator available online. These calculators are often found on websites of health organizations and institutions like the CDC or Mayo Clinic.

To use a BMI calculator, follow these steps:

  1. Locate a reliable BMI calculator online.
  2. Enter your height and weight in the appropriate fields. Most calculators will allow you to choose between metric or imperial measurements.
  3. Click the button to calculate your BMI, and the calculator will display the result.

Remember that BMI calculations only estimate body fat and may not accurately reflect the health status of everyone. Always consult with a healthcare professional for personalized assessment and advice.

BMI Categories


Underweight is a term used to describe people who have a BMI less than 18.5.

  • Individuals in this category may have a higher risk of health problems, such as malnutrition, weakened immune systems, and potential growth and development issues in young people.
  • Underweight individuals need to consult with a healthcare professional to develop a suitable plan for maintaining a healthy weight.

Normal Weight

Normal weight refers to individuals with a BMI ranging from 18.5-24.9. This is considered a healthy weight for adults due to its association with a lower risk of chronic health conditions such as heart disease and type 2 diabetes.

  • Maintaining a normal weight through a balanced diet and regular physical activity is essential in promoting overall health and well-being.


Overweight is a term that applies to adults with a BMI of 25-29.9. This indicates the presence of excess body weight, which can lead to a higher risk of various health problems, including:

  • High blood pressure
  • Type 2 diabetes
  • Heart disease

Individuals who are overweight must consult with their healthcare provider to develop a personalized plan to achieve a healthier weight.


Obesity is characterized by a BMI of 30 or more, which indicates a significantly high amount of body fat in relation to lean body tissue. Obesity is further divided into three subclasses:

  • Class 1: BMI of 30 to < 35
  • Class 2: BMI of 35 to < 40
  • Class 3 (severe obesity): BMI of 40 or higher

Individuals with obesity are at an increased risk of chronic health conditions, such as:

  • Heart disease
  • Stroke
  • Type 2 diabetes
  • Certain types of cancer

Obese individuals should work closely with their healthcare provider to develop a weight management plan and establish realistic goals for achieving a healthier weight.

BMI for Different Age Groups


Calculating BMI for children is slightly different from adults since their bodies are constantly growing, and their BMI changes accordingly. Instead of using a simple BMI chart, healthcare providers consider age, sex, and growth patterns to interpret children’s BMI. The CDC offers a BMI calculator designed for children and teenagers under 20.

Here’s a breakdown of BMI percentiles for children:

  • Underweight: Below the 5th percentile
  • Healthy weight: 5th percentile to below the 85th percentile
  • Overweight: 85th percentile to below the 95th percentile
  • Obese: Equal to or greater than the 95th percentile


BMI calculation for teens requires the same formula as for other age groups: weight (in kilograms) divided by height (in meters) squared. However, just like for children, BMI interpretation for teens relies on percentiles. Using the same CDC calculator, BMI percentiles are compared with other teens of the same age and sex.

Here are the BMI percentiles for teens:

  • Underweight: Below the 5th percentile
  • Healthy weight: 5th percentile to below the 85th percentile
  • Overweight: 85th percentile to below the 95th percentile
  • Obese: Equal to or greater than the 95th percentile


The basic BMI formula remains the same for adults, but the interpretation relies on health risk categories rather than percentiles. A healthy BMI for adults ranges from 18.5 to 24.9, with variations for both adult men and women. According to Livestrong, the average BMI for women aged 20 and older is 28.7.

Here are the BMI categories for adults:

  • Underweight: Below 18.5
  • Healthy weight: 18.5 – 24.9
  • Overweight: 25 – 29.9
  • Obese: 30 or greater

To keep track of the changes in BMI throughout their lives, individuals should consult a healthcare provider for personalized advice, accurate assessments, and appropriate interventions when needed.

Health Risks Associated with BMI

Body Mass Index (BMI) is a valuable tool that measures the ratio of an individual’s weight to their height, which helps estimate their body fat. It is important to be aware of the health risks associated with both low and high BMI values.

Low BMI Health Risks

A BMI of under 18.5 kg/m² is considered underweight and can be associated with several potential health problems. Some of these risks include:

  • Malnutrition: Inadequate intake of essential nutrients can lead to various health issues, including weakened immune system and slow wound healing.
  • Anemia: Low BMI can be linked to iron deficiency anemia, causing fatigue, weakness, and shortness of breath.
  • Osteoporosis: Low body weight is a risk factor for developing fragile bones and osteoporosis, which can lead to fractures from even minor falls.

High BMI Health Risks

A BMI of 25 kg/m² or higher indicates being overweight or obese, which can put individuals at an increased risk of developing various health issues. Some of these health problems include:

  • Diabetes: High body fat can contribute to insulin resistance and the development of type 2 diabetes.
  • High blood pressure: Carrying excess weight can cause higher blood pressure, which strains the heart and arteries.
  • Stroke: Being overweight or obese increases the risk of stroke due to the increased pressure on blood vessels and a higher likelihood of inflammation and blood clots.
  • Osteoarthritis: Excess weight can increase joint pressure, leading to osteoarthritis and joint pain.
  • Sleep apnea: An increased BMI can cause respiratory issues like sleep apnea, characterized by pauses in breathing during sleep, potentially affecting overall sleep quality.
  • Certain cancers: A higher BMI has been linked to an increased risk of developing various cancers, including breast, colon, and kidney.

It is essential to maintain a healthy BMI to avoid these potential health risks. Proper diet, regular exercise, and consulting with healthcare professionals can help individuals achieve and maintain their optimal weight for improved overall health.

Limitations of BMI

It’s important to recognize that while BMI is a useful tool for estimating body fat and assessing potential health risks, it has several limitations. One of the primary drawbacks of using BMI as a measure of body composition is that it doesn’t consider an individual’s muscle mass. This can lead to an inaccurate representation of a person’s health status, especially in those with a high muscle-to-fat ratio, such as bodybuilders and athletes. In fact, people with higher muscle mass might have a BMI that categorizes them as overweight or obese despite being healthy and fit.

Another limitation is that the BMI formula doesn’t consider differences in body fat distribution, which varies among individuals and can impact health risks. For example, carrying excess fat around the abdominal area increases the risk of heart disease and type 2 diabetes, while fat stored in the hips and thighs tends to be less harmful . Additionally, BMI doesn’t account for differences in body composition related to age, sex, or ethnicity. Women generally have a higher body fat percentage than men at the same BMI level, and older adults tend to have more body fat than younger ones 1.

It’s also worth noting that BMI does not directly measure an individual’s activity level. A sedentary person may have a BMI in the normal range, but their lack of physical activity could still put them at risk for weight-related health problems. Conversely, someone with a higher BMI might lead an active and healthy lifestyle but have more muscle mass or a larger bone structure 4.

In summary, while BMI is a convenient tool for health screening and assessing weight status, it’s essential to consider these limitations when interpreting BMI measurements. In many cases, additional factors such as muscle mass, activity level, and body fat distribution should be considered to better assess an individual’s overall health. It’s always best to consult with a healthcare professional for personalized medical advice when interpreting and using BMI.

BMI Tools and Resources

BMI Table

A BMI table is a useful resource that allows you to easily find your BMI value based on your height and weight. This table usually categorizes BMI values into underweight, normal weight, overweight, and obese categories. Keep in mind that BMI alone is not a diagnostic tool, but a starting point for understanding your overall health risk related to weight.

BMI Calculator Apps

There are numerous BMI calculator apps available for both iPhone and Android devices. These apps help you quickly calculate your BMI and track your progress. In addition to basic BMI calculation, some advanced apps may also offer additional features such as personalized recommendations, goal setting, and integration with other health and fitness apps.

Online BMI Calculators

Apart from mobile apps, there are several online BMI calculators that allow you to input your height and weight and receive your BMI result instantly. These calculators use the same BMI formula (weight in kilograms divided by the square of height in meters) to determine your BMI value. Using an online BMI calculator can be a practical and convenient alternative to using a BMI table or mobile app.

Some popular online BMI calculators include:

  • The National Heart, Lung, and Blood Institute (NHLBI) BMI calculator, which is part of their “Aim for a Healthy Weight” program that offers reliable health information related to weight and obesity.
  • Mayo Clinic BMI calculator that provides both BMI and waist circumference calculations to assess health risks.
  • Cleveland Clinic BMI calculator, which offers helpful tips and resources for maintaining a healthy BMI and overall well-being.

Make sure to choose a trustworthy and accurate calculator, as the quality of the information provided and user experience may vary. Remember, always consult your healthcare provider for advice and recommendations regarding your BMI and overall health.

Maintaining a Healthy Weight

Maintaining a healthy body weight is essential for overall health and well-being. To find the right balance, it’s crucial to assess your risk, control your weight, and consume nutritious meals.

Assessing Your Risk

Estimating your body mass index (BMI) is an efficient way to determine whether your current weight falls within a healthy range. You can use the BMI calculator provided by CDC to find your BMI based on your height and weight. This information will help you assess your risk for potential health issues related to your weight.

Controlling Your Weight

Once you have determined your BMI, you can take steps to control your weight. Here are some general guidelines:

  • Adjust your daily caloric intake according to your needs
  • Engage in regular physical activity
  • Make healthier food choices
  • Monitor your progress and adjust your plan as needed

To successfully maintain or achieve a healthy weight, consider the following tips:

  1. Set realistic goals: Aim for a gradual and consistent weight loss if needed, rather than attempting rapid or drastic changes.
  2. Create a plan: Develop a diet and exercise regimen that fits your lifestyle, preferences, and medical needs.
  3. Stay accountable: Track your food intake, exercise, and weight progress regularly to ensure that you are staying on track.
  4. Stay motivated: Surround yourself with a supportive network of friends and family, or consider joining a weight loss group for additional encouragement.

Healthy Recipes

Incorporating healthy recipes into your meal plans can make controlling your weight more enjoyable. Here are a few ideas:

  • Breakfast: Opt for protein-rich foods such as Greek yogurt with berries and chia seeds, or whole-grain toast topped with mashed avocado and a poached egg.
  • Lunch: Choose filling and nutritious salads that combine leafy greens, colorful vegetables, lean proteins, and healthy fats, such as a spinach salad with grilled chicken, cherry tomatoes, avocado, and a lemon-tahini dressing.
  • Dinner: Experiment with different protein sources such as grilled salmon, baked tofu, or roasted chickpeas, and accompany them with various vegetables and whole grains.
  • Snacks: Keep it simple with fresh fruits and vegetables, a handful of nuts, or a small portion of hummus and whole-grain crackers.

Remember to consult your healthcare provider before embarking on a weight management plan, as they can provide personalized guidance and support. By assessing your risk, controlling your weight, and focusing on nutritious recipes, you can work toward maintaining a healthy weight and enjoying its numerous benefits.

Written by James Kosur

James is a 20-year veteran of the digital media industry, an avid gym builder, and a dad to four kids, three dogs, and two cats. He's a DIYer who loves building stuff with his hands and a gamer who enjoys all facets of gaming.