BMI Calculation

BMI (Body Mass Index) calculation is a simple indicator to understand if your body size is healthy. This number, calculated from your height and weight, helps objectively evaluate your physique, such as obesity or being underweight, and serves as a first step in understanding the risk of various diseases.
This article comprehensively explains BMI calculation, from the accurate calculation method and the meaning of the number to its association with health risks.
Through this article, get to know your current body size and use it for your future health management.

What is BMI calculation?

BMI calculation, or Body Mass Index calculation, is a widely used method to assess a person’s body weight in relation to their height. It provides a numerical value that serves as a screening tool for weight categories that may lead to health problems. Essentially, BMI helps classify individuals as underweight, normal weight, overweight, or obese.

While not a direct measure of body fat, BMI is a cost-effective and easy-to-perform method for categorizing weight status in the general population. It is recognized globally by health organizations like the World Health Organization (WHO) and used by national health bodies, such as the Japan Society for the Study of Obesity (JASSO), to set standards and guide public health initiatives related to weight management.
Calculating your BMI is often the first step in understanding if your weight might be impacting your health.

How to do BMI calculation

Calculating your BMI requires only two pieces of information: your height and your weight. The formula is simple, but it’s important to use the correct units of measurement for an accurate result.

BMI calculation formula

The basic formula for calculating BMI is the same worldwide, but the units of measurement used for height and weight differ between metric and imperial systems.

BMI calculation with kg and meters

In the metric system, which is commonly used in Japan and most parts of the world, the BMI formula is:

$$ \text{BMI} = \frac{\text{Weight (kg)}}{\text{Height (m)}^2} $$

This means you divide your weight in kilograms by the square of your height in meters.

For example, if a person weighs 60 kilograms and is 1.70 meters tall:
$$ \text{BMI} = \frac{60 \text{ kg}}{(1.70 \text{ m})^2} = \frac{60}{2.89} \approx 20.76 $$\nSo, their BMI would be approximately 20.8.

BMI calculation with pounds and inches

In countries that use the imperial system, such as the United States, the formula involves pounds for weight and inches for height. To align with the metric formula’s outcome, a conversion factor is included:

$$ \text{BMI} = \frac{\text{Weight (lb)}}{\text{Height (in)}^2} \times 703 $$

Here, you divide your weight in pounds by the square of your height in inches, and then multiply the result by a conversion factor of 703.

For example, if a person weighs 150 pounds and is 67 inches tall (5 feet 7 inches):
$$ \text{BMI} = \frac{150 \text{ lb}}{(67 \text{ in})^2} \times 703 = \frac{150}{4489} \times 703 \approx 0.0334 \times 703 \approx 23.5 $$\nTheir BMI would be approximately 23.5.

Step-by-step BMI calculation

Calculating your BMI manually is straightforward if you follow these steps:

1. Measure your current weight: Step onto a reliable scale and record your weight. For the metric system, use kilograms (kg). For the imperial system, use pounds (lb). Weigh yourself at the same time of day, ideally in the morning before eating or drinking, for consistency.
2. Measure your height: Stand straight against a wall without shoes. Use a measuring tape or stadiometer to find the top of your head. For the metric system, measure in meters (m). If you measure in centimeters (cm), divide the value by 100 to convert it to meters (e.g., 170 cm becomes 1.70 m). For the imperial system, measure in inches (in). Convert feet and inches to total inches if necessary (e.g., 5 feet 7 inches is $5 \times 12 + 7 = 67$ inches).
3. Square your height: Multiply your height value by itself.
   Metric: Height (m) $\times$ Height (m)
   Imperial: Height (in) $\times$ Height (in)
4. Apply the formula:
   Metric: Divide your weight (kg) by the result from step 3.
   Imperial: Divide your weight (lb) by the result from step 3, then multiply the result by 703.
5. Round the result: BMI is typically rounded to one decimal place (e.g., 20.76 becomes 20.8).

Once you have calculated your BMI, the next step is to interpret what that number means for your health.

BMI categories and interpretation

After calculating your BMI, the resulting number falls into a specific category that indicates your weight status. These categories are linked to potential health risks, although it’s crucial to remember that BMI is a screening tool and not a definitive diagnosis.

Normal BMI range

Maintaining a BMI within the “normal” or “healthy weight” range is generally associated with the lowest risk of various health problems. Different health organizations have slightly varying cut-off points for these categories.

In Japan, the Japan Society for the Study of Obesity (JASSO) defines the standard BMI range:

• Low weight (Underweight): Less than 18.5
• Normal weight (Healthy weight): 18.5 to less than 25
• Obesity Class 1: 25 to less than 30
• Obesity Class 2: 30 to less than 35
• Obesity Class 3: 35 to less than 40
• Obesity Class 4: 40 or higher

The World Health Organization (WHO) uses slightly different categories for obesity:

• Underweight: < 18.5
• Normal weight: 18.5 – 24.9
• Overweight: 25.0 – 29.9
• Obesity Class I: 30.0 – 34.9
• Obesity Class II: 35.0 – 39.9
• Obesity Class III: $\ge$ 40.0

While the WHO and JASSO classifications for underweight and normal weight are the same, JASSO uses stricter cut-offs for obesity categories (starting at 25) based on studies indicating increased health risks for Japanese populations at lower BMI values compared to some Western populations.

BMI chart

A BMI chart, also known as a BMI table, provides a visual reference to quickly find your BMI category based on your height and weight. Here is a simplified table based on the Japan Society for the Study of Obesity (JASSO) standards:

BMI Range	Weight Category	Potential Health Risk (General)
Less than 18.5	Low weight (Underweight)	Increased
18.5 to < 25	Normal weight	Lowest
25 to < 30	Obesity Class 1	Increased
30 to < 35	Obesity Class 2	High
35 to < 40	Obesity Class 3	Very High
40 or higher	Obesity Class 4	Extremely High

Note: This table is a general guide for adults. Specific health risks vary greatly depending on individual factors.

BMI calculation for adults

For most adults (aged 20 and over), BMI is a reliable indicator of weight status. The categories and associated risks described above generally apply to this age group. However, as we will discuss in the limitations section, individual circumstances can influence the interpretation of BMI. It’s often used in clinical settings alongside other assessments to evaluate overall health. For example, a doctor might calculate your BMI during a routine check-up as part of a broader health assessment.

BMI calculation for children and teens

Calculating BMI for children and teenagers (aged 2 to 19 years) uses the same formula as for adults: weight (kg) / height (m)^2. However, interpreting the result is different. Children’s BMI changes significantly with age and also varies between sexes as they grow. Therefore, a child’s BMI is plotted on a growth chart (specifically, a BMI-for-age percentile chart) that compares them to other children of the same age and sex.

Instead of fixed categories like adults, weight status for children and teens is determined by the percentile their BMI falls into:

• Underweight: Less than the 5th percentile
• Healthy weight: 5th percentile to less than the 85th percentile
• Overweight: 85th percentile to less than the 95th percentile
• Obese: 95th percentile or higher

It is crucial not to use adult BMI categories for children. If you are concerned about a child’s weight, it is best to consult with a pediatrician or healthcare provider who can interpret their BMI in the context of their individual growth and development.

BMI calculation limitations

While BMI is a useful screening tool, it has significant limitations and should not be the sole basis for assessing an individual’s health. It doesn’t differentiate between muscle mass, bone density, and fat mass.

• Muscle Mass: Athletes or individuals with high muscle mass may have a high BMI even if they have low body fat. Muscle is denser than fat, so a very muscular person might fall into the “overweight” or “obese” BMI category, yet be very healthy.
• Age: As people age, muscle mass tends to decrease and body fat may increase, often shifting towards the abdomen. BMI might not fully capture these changes, and the relationship between BMI and health risk can differ in older adults. Some studies suggest a slightly higher BMI might be protective in very old age.
• Sex: Men generally have more muscle mass than women, which can affect their BMI relative to body fat percentage.
• Ethnicity: The relationship between BMI, body fat percentage, and health risks can vary among different ethnic groups.
• Body Composition: BMI provides no information about the distribution of fat. Abdominal obesity (carrying excess fat around the waist) is associated with higher health risks than fat carried on the hips and thighs, even at the same BMI.
• Health Conditions: BMI doesn’t account for underlying health conditions, genetics, lifestyle factors (diet, exercise, smoking), or medical history, all of which significantly impact overall health and risk.

Therefore, BMI should be used in conjunction with other health assessments, such as waist circumference measurement, body fat percentage analysis, blood pressure, cholesterol levels, and discussions about lifestyle and family history, to get a more complete picture of health status.

Using a BMI calculation tool

Manually calculating your BMI is simple, but using a BMI calculation tool can be faster and more convenient, especially if you want to quickly check your status or calculate for others. Many online tools and health apps are available.

BMI calculation kg input

Most online BMI calculators designed for users outside the US will primarily use the metric system, requiring input for weight in kilograms (kg) and height in meters (m) or centimeters (cm). You simply enter your measurements into the designated fields, and the tool will instantly provide your BMI result. If your height is in cm, ensure the tool accepts cm or convert it to meters (e.g., 175 cm = 1.75 m) before entering.

BMI calculation with age and gender (female, men)

Some advanced BMI calculators or health assessment tools may ask for your age and gender. While age and gender do not change the fundamental BMI formula itself (weight/height^2), providing this information can be important for interpreting the result, particularly for children and teenagers as discussed earlier. For adults, age and gender might be used by the tool to provide more context about typical body composition changes or to link BMI to age/gender-specific health risk data. However, for a standard adult BMI category assessment, age and gender are not strictly necessary for the calculation itself. For children, inputting age and gender is essential for using the correct percentile charts for interpretation.

BMI calculation FAQs (People Also Ask)

Here are answers to some frequently asked questions about BMI calculation.

How to calculate the BMI?

To calculate BMI, divide your weight in kilograms by the square of your height in meters (Weight (kg) / (Height (m))^2). Alternatively, you can use an online BMI calculator by entering your height and weight in the specified units.

What is the formula for counting BMI?

The formula for calculating BMI using the metric system is: BMI = Weight (kg) / Height (m)^2. Using the imperial system (pounds and inches): BMI = [Weight (lb) / Height (in)^2] * 703.

How do you calculate BMI levels?

BMI levels, or categories, are determined by comparing your calculated BMI value to standard ranges set by health organizations. For adults in Japan, a BMI below 18.5 is underweight, 18.5 to under 25 is normal weight, 25 to under 30 is Obesity Class 1, and so on. For children and teens, BMI is plotted on age- and sex-specific percentile charts to determine their category (underweight, healthy weight, overweight, obese).

How do you calculate weight per BMI?

You can reverse the BMI formula to calculate a target weight for a desired BMI.
Using the metric system: Target Weight (kg) = Desired BMI $\times$ Height (m)^2.
For example, if you are 1.70 m tall and want to achieve a BMI of 22:
Target Weight = 22 $\times$ (1.70 m)^2 = 22 $\times$ 2.89 $\approx$ 63.58 kg.
This means a weight of about 63.6 kg would give you a BMI of 22.

What is the healthy weight range for my height?

You can use the formula above with the “normal weight” BMI range (18.5 to < 25 in Japan) to find your healthy weight range.
For a person 1.70 m tall:
Minimum healthy weight: 18.5 $\times$ (1.70 m)^2 = 18.5 $\times$ 2.89 $\approx$ 53.47 kg
Maximum healthy weight: 25 $\times$ (1.70 m)^2 = 25 $\times$ 2.89 = 72.25 kg
So, the healthy weight range for a person 1.70 m tall is approximately 53.5 kg to 72.3 kg.

Is BMI a good indicator of health?

BMI is a useful screening tool, but not a perfect indicator of health. It correlates well with body fat in many people but doesn’t measure body fat directly or indicate its distribution. It has limitations for muscular individuals, the elderly, pregnant women, and certain ethnic groups. It should be used alongside other health assessments.

BMI calculation and health risks

Your BMI category is associated with different levels of risk for various health problems.

High BMI (Overweight and Obesity) Risks:

Being overweight (BMI $\ge$ 25 in Japan) or obese (BMI $\ge$ 30 in Japan) significantly increases the risk of developing numerous chronic diseases. These include:

• Metabolic Syndrome: A cluster of conditions (high blood pressure, high blood sugar, unhealthy cholesterol levels, excess abdominal fat) that increase risk of heart disease, stroke, and diabetes.
• Type 2 Diabetes: Excess body weight, particularly around the waist, can lead to insulin resistance.
• Cardiovascular Diseases: High BMI is a major risk factor for high blood pressure, high cholesterol, coronary artery disease, heart attack, and stroke.
• Certain Cancers: Including cancers of the colon, breast (postmenopausal), uterus, kidney, esophagus, and pancreas.
• Sleep Apnea: A sleep disorder where breathing repeatedly stops and starts.
• Osteoarthritis: The increased weight puts extra stress on joints, particularly knees and hips.
• Gallbladder Disease:
• Fatty Liver Disease:

The higher the BMI category (Obesity Class 2, 3, 4), the greater the risk generally becomes.

Low BMI (Underweight) Risks:

While the focus is often on the risks of high BMI, being underweight (BMI < 18.5) also poses health risks, such as:

• Malnutrition and Nutrient Deficiencies: Not getting enough calories or essential vitamins and minerals.
• Weakened Immune System: Making you more susceptible to infections.
• Osteoporosis: Especially in women, low body weight can contribute to decreased bone density and increased fracture risk.
• Anemia: Due to insufficient iron, B12, or folate.
• Muscle Wasting (Sarcopenia): Loss of muscle mass, leading to weakness and fatigue.
• Hormonal Imbalances: Can affect reproductive health (e.g., irregular periods in women) and overall well-being.
• Increased Risk of Complications from Surgery:
• Higher Mortality Risk in some populations, particularly older adults or those with certain chronic illnesses.

Maintaining a BMI within the normal range (18.5 to < 25) is generally associated with the lowest risk of these weight-related health issues. However, it is possible to be within the normal BMI range and still have health risks due to poor diet, lack of exercise, high body fat percentage, or other factors. This highlights the importance of looking beyond just the BMI number.

Other related metrics (Calorie calculator)

As mentioned, BMI is just one piece of the puzzle when assessing health. Other metrics provide valuable insights into body composition and metabolic health.

• Waist Circumference: Measures the distance around the natural waist. It is a strong indicator of abdominal fat, which is particularly associated with increased risk of type 2 diabetes, heart disease, and stroke, even in individuals with a normal BMI. A waist circumference of over 85 cm for men and over 90 cm for women (according to Japanese criteria for metabolic syndrome) is considered high risk.
• Body Fat Percentage: Measures the proportion of total body weight that is fat. This can be measured using various methods, such as skinfold calipers, bioelectrical impedance analysis (BIA) devices (like smart scales), or more advanced techniques like DEXA scans. While ideal percentages vary by age and sex, generally, higher body fat percentages (especially without significant muscle mass) are linked to increased health risks.
• Waist-to-Hip Ratio (WHR): The ratio of your waist circumference to your hip circumference. It indicates how fat is distributed. A higher WHR suggests more fat in the abdominal area (“apple” shape), which is riskier than fat stored around the hips and thighs (“pear” shape).

Managing your weight and body composition often involves balancing calorie intake and expenditure. Understanding how many calories you consume and burn is fundamental to weight management. A calorie calculator can help you estimate your daily calorie needs based on your age, sex, weight, height, and activity level. This can be a useful tool for planning your diet, whether you are trying to maintain, lose, or gain weight. While BMI gives you a snapshot of your current weight status, calorie calculation and tracking are tools for managing the factors that influence your BMI.

Conclusion: Make BMI Calculation Your First Step in Health Management

BMI calculation is a simple and effective way to understand your body size and how it relates to your health status. You can calculate it manually using your height and weight, or easily use one of the many online tools available. However, remember that BMI is only one indicator and does not account for muscle mass, body fat percentage, or fat distribution. Special care is needed when interpreting BMI for certain groups, such as athletes, the elderly, and growing children.

Both high and low BMI can increase the risk of different health problems. If your BMI falls outside the normal range, it’s important to review your lifestyle and consult a doctor or specialist (such as a registered dietitian) if necessary.

Calculating your BMI is the first step on your journey towards a healthy physique. Let this number be an opportunity to look at other indicators like waist circumference and body fat percentage, incorporate a balanced diet and regular exercise, and proactively manage your overall health.

Disclaimer: The information in this article is for general knowledge purposes only and is not intended as a substitute for medical diagnosis or treatment advice. For individual health conditions or specific symptoms, always consult with a doctor or qualified healthcare professional. We assume no responsibility for the outcomes of actions taken based on the information provided in this article.