BMI Calculator

The formula

What Is BMI?

BMI, or Body Mass Index, is a numerical value calculated from a person's weight and height. It is a widely used tool to categorize individuals into different weight status groups, such as underweight, normal weight, overweight, or obese. The formula for calculating BMI is simple: divide a person's weight in kilograms by the square of their height in meters (BMI = kg/m²).

Here are some key points about BMI:

• It provides a quick snapshot of whether a person's weight is in a healthy range for their height.
• BMI does not directly measure body fat but correlates with more direct measures of body fat.
• It is used by healthcare professionals to screen for potential weight-related health issues.

While BMI is a useful screening tool, it has limitations:

• It does not distinguish between muscle and fat, so athletes with high muscle mass may have a high BMI without being overweight.
• It may not accurately reflect health risks for certain populations, such as older adults or those with a sedentary lifestyle.

To calculate your BMI, follow these steps:

1. Measure your weight in kilograms.
2. Measure your height in meters.
3. Square your height (multiply it by itself).
4. Divide your weight by the squared height.

For example, if you weigh 70 kg and are 1.75 meters tall, your BMI would be calculated as follows: 70 / (1.75 × 1.75) = 22.86, which falls into the normal weight category.

How to calculate bMI

Body mass index is weight scaled against height squared, giving a single number used as a quick population-level screen for whether someone is underweight, a healthy weight, overweight or obese. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet for population studies, not as an individual diagnostic tool, which is part of why its limits are so often debated.

The formula divides weight in kilograms by height in metres squared, so a 70kg person at 1.75m comes out to 70 ÷ 1.75² = 22.9. The WHO bands are under 18.5 underweight, 18.5-24.9 healthy weight, 25-29.9 overweight and 30+ obese, applied regardless of age, sex or build.

What to enter:

• Weight (kg)
• Height (cm)

Results appear immediately — there is nothing to submit. Changing a field rewrites the link, so you can share the exact scenario you are looking at.

Consistency in how the underlying measurements are taken matters more than precision to the last decimal — measuring at the same time of day, in the same state of rest or hydration, makes readings taken weeks apart genuinely comparable in a way that a single very precise reading on its own is not.

Why Is BMI Important?

Understanding your Body Mass Index (BMI) is a crucial step in assessing your overall health. BMI is a simple calculation that uses your height and weight to determine whether you fall into a healthy weight range. While it doesn't measure body fat directly, it provides a reliable indicator for most people to gauge potential health risks associated with being underweight, overweight, or obese.

Here are some key reasons why BMI is important:

• Health Risk Assessment: A high BMI can indicate an increased risk for chronic conditions like heart disease, diabetes, and hypertension.
• Early Intervention: Tracking your BMI over time helps identify trends, allowing for early lifestyle changes or medical advice.
• Population Health Studies: BMI is widely used in research to analyze trends in obesity and malnutrition across populations.

While BMI is a useful tool, it has limitations. For example, it doesn't account for muscle mass, bone density, or fat distribution. Athletes with high muscle mass may have a high BMI without excess fat, while older adults may have a normal BMI but higher body fat.

To calculate your BMI, use the formula: BMI = weight (kg) / (height (m))². For imperial units, the formula is BMI = (weight (lbs) / (height (in))²) × 703.

Regularly monitoring your BMI can help you stay informed about your health and take proactive steps toward a balanced lifestyle. Remember, BMI is just one piece of the puzzle—consult a healthcare professional for a comprehensive evaluation.

Why bMI matters

This kind of calculation gets used both out of general interest and ahead of a specific conversation — with a trainer, a midwife or a GP — where having an actual number ready, rather than a description, makes that conversation more concrete and specific.

This tends to get checked before or after a change in routine — a new training block, a change in diet, a different stage of a pregnancy — as a way of putting a figure on something that would otherwise only be felt rather than measured.

A single number like this is most useful placed in context rather than read in isolation. Reference ranges exist because bodies vary considerably and still count as healthy across a wide spread of values, so a result sitting outside a commonly quoted range is a prompt to look more closely rather than a verdict on its own. The same figure, tracked consistently over time, tends to say more than any single reading ever will.

A calculator like this one is frequently used to prepare a specific number for an upcoming appointment, so that a description like "I think it has changed" can be replaced with an actual figure and a date it was measured on. Clinicians and trainers generally find a concrete number, however approximate, more useful to work from than a general impression.

How to Calculate Your BMI

Calculating your BMI (Body Mass Index) is a straightforward way to assess whether your weight is in a healthy range for your height. Here’s how you can do it:

1. Measure your weight in kilograms (kg). If you only have your weight in pounds (lbs), divide it by 2.205 to convert it to kilograms.
2. Measure your height in meters (m). If your height is in inches, multiply by 0.0254 to convert it to meters.
3. Use the formula: BMI = weight (kg) / (height (m) × height (m)).

For example, if you weigh 70 kg and are 1.75 m tall, your BMI would be calculated as follows:
70 / (1.75 × 1.75) = 22.86.

Here’s a quick reference table for BMI categories:

Keep in mind that BMI is a general guideline and doesn’t account for muscle mass, bone density, or other individual factors. For a more comprehensive health assessment, consult a healthcare professional.

Worked example

Work through the defaults on this page:

• Weight: 70 kg
• Height: 175 cm

That gives:

• Body Mass Index: 22.86

Those starting figures exist only to show the calculation working, not as a target or a reference value to compare yourself against — replace them with your own measurements in the calculator above and the same formula applies exactly as shown.

BMI Categories Explained

Understanding your BMI (Body Mass Index) is essential for assessing your weight status and overall health. BMI is calculated by dividing your weight in kilograms by the square of your height in meters. The resulting number falls into one of the following categories:

• Underweight: BMI less than 18.5. This may indicate insufficient body fat, which can lead to health issues like weakened immunity or nutrient deficiencies.
• Normal weight: BMI between 18.5 and 24.9. This range is generally associated with the lowest risk of chronic diseases.
• Overweight: BMI between 25 and 29.9. Being overweight increases the risk of conditions like heart disease and type 2 diabetes.
• Obesity (Class I): BMI between 30 and 34.9. This category signifies a higher risk of severe health problems, including hypertension and joint issues.
• Obesity (Class II): BMI between 35 and 39.9. The health risks here are even more pronounced, often requiring medical intervention.
• Extreme Obesity (Class III): BMI of 40 or higher. This is the highest risk category, linked to life-threatening conditions like stroke and certain cancers.

While BMI is a useful screening tool, it doesn’t account for muscle mass, bone density, or fat distribution. For a comprehensive health assessment, consult a healthcare professional.

Reading the result

The bands were derived from population-level mortality risk data, so a result in the healthy range is a reasonable population-level signal — but BMI can't distinguish muscle from fat. A well-muscled athlete can register as "overweight" despite low body fat, while someone with low muscle and high visceral fat can register as "healthy weight."

Where this goes wrong. Using BMI as the sole measure ignores fat distribution, which matters clinically — waist-to-height ratio and waist-to-hip ratio are generally better predictors of cardiovascular risk because they capture where fat is carried, not just total mass. This is a screening estimate, not a diagnosis; any concern about weight and health is worth raising with a GP.

As with any general-purpose formula, the result here is an estimate derived from population averages, not a measurement specific to you — a reasonable starting point for a conversation with a qualified professional, not a replacement for one. Nothing on this page is medical advice.

Limitations of BMI

While BMI (Body Mass Index) is a widely used tool to assess whether a person has a healthy weight for their height, it has several limitations that can affect its accuracy and usefulness. Here are some key limitations:

• Does not distinguish between fat and muscle: BMI calculates weight relative to height but does not account for muscle mass. Athletes or individuals with high muscle density may have a high BMI despite having low body fat.
• Ignores fat distribution: BMI does not consider where fat is stored in the body. Visceral fat (around organs) poses higher health risks than subcutaneous fat (under the skin), but BMI cannot differentiate between the two.
• Age and gender differences: BMI does not adjust for age or gender, even though body composition changes over time and varies between men and women.
• Ethnicity variations: BMI thresholds may not be equally applicable to all ethnic groups. For example, some populations may have higher health risks at lower BMI levels.
• Limited for certain groups: BMI may not be suitable for children, pregnant women, or the elderly, as their body composition differs significantly from the general adult population.

Despite these limitations, BMI remains a quick and easy screening tool. However, it should not be the sole measure of health. Combining BMI with other assessments, such as waist circumference or body fat percentage, provides a more comprehensive picture of overall health.

Related calculators

A few other calculators in this section cover similar ground: Anion gap, BMI (NHS) and BMI.

Healthy BMI Ranges for Adults

Understanding your Body Mass Index (BMI) is a crucial step in assessing your overall health. For adults, a healthy BMI typically falls within the range of 18.5 to 24.9. This range is associated with the lowest risk of chronic diseases such as heart disease, diabetes, and hypertension.

Here’s a breakdown of BMI categories for adults:

• Underweight: BMI less than 18.5
• Normal weight: BMI 18.5–24.9
• Overweight: BMI 25–29.9
• Obesity: BMI 30 or higher

Maintaining a BMI within the healthy range can significantly improve your quality of life. Here are some key benefits:

• Reduced risk of chronic illnesses
• Better energy levels and physical performance
• Improved mental health and self-esteem

It’s important to note that BMI is a general guideline and doesn’t account for factors like muscle mass or body composition. For a more accurate assessment, consider consulting a healthcare professional.

To calculate your BMI, use the formula:

BMI = weight (kg) / (height (m))²

Or, for pounds and inches:

BMI = (weight (lbs) / (height (in))²) x 703

Regularly monitoring your BMI can help you stay on track with your health goals. Small lifestyle changes, such as a balanced diet and regular exercise, can make a big difference in achieving and maintaining a healthy BMI.

BMI for Children and Teens

BMI (Body Mass Index) is a useful tool to assess whether a child or teen is at a healthy weight for their age, height, and sex. Unlike adults, children and teens are still growing, so their BMI is interpreted differently. It is calculated using the same formula as for adults—weight in kilograms divided by height in meters squared—but the results are compared to growth charts specific to their age and gender.

Here’s why BMI is important for children and teens:

• Early detection: Identifying weight issues early can help prevent health problems like diabetes or heart disease later in life.
• Growth tracking: BMI-for-age percentiles show how a child’s weight compares to others of the same age and sex.
• Healthy habits: Encouraging balanced nutrition and physical activity can be guided by BMI results.

BMI categories for children and teens are as follows:

Parents and caregivers should consult a healthcare provider to interpret BMI results accurately. A high or low BMI percentile may require further evaluation, including dietary or lifestyle changes. Remember, BMI is just one indicator of health—other factors like muscle mass and overall fitness also play a role.

How to Improve Your BMI

Improving your BMI (Body Mass Index) is a key step toward better health. Whether your goal is to lower or raise your BMI, the process involves a combination of diet, exercise, and lifestyle changes. Here’s how you can achieve a healthier BMI:

• Eat a Balanced Diet: Focus on nutrient-dense foods like fruits, vegetables, lean proteins, and whole grains. Avoid processed foods and sugary drinks, which can contribute to weight gain.
• Exercise Regularly: Aim for at least 150 minutes of moderate aerobic activity or 75 minutes of vigorous activity each week. Strength training twice a week can also help build muscle and improve metabolism.
• Monitor Portion Sizes: Overeating, even healthy foods, can lead to weight gain. Use smaller plates and pay attention to hunger cues.
• Stay Hydrated: Drinking water before meals can help control appetite and prevent overeating.
• Get Enough Sleep: Poor sleep is linked to weight gain and higher BMI. Aim for 7-9 hours of quality sleep each night.
• Manage Stress: Chronic stress can lead to emotional eating. Practice relaxation techniques like meditation or yoga.

If your BMI is below the healthy range, focus on gaining weight in a healthy way by increasing calorie intake with nutritious foods and strength training to build muscle.

Remember, BMI is just one indicator of health. Consult a healthcare professional before making significant changes to your diet or exercise routine.

BMI Calculator for Adults

Calculating your BMI (Body Mass Index) is a straightforward way to assess whether your weight is in a healthy range for your height. The formula for BMI is simple: divide your weight in kilograms by the square of your height in meters. Here’s the formula:

For those using pounds and inches, the formula adjusts slightly:

Understanding your BMI can help you gauge potential health risks associated with being underweight, overweight, or obese. Here’s a quick reference for BMI categories:

• Underweight: BMI less than 18.5
• Normal weight: BMI 18.5–24.9
• Overweight: BMI 25–29.9
• Obese: BMI 30 or higher

While BMI is a useful screening tool, it doesn’t account for muscle mass, bone density, or overall body composition. For a more comprehensive assessment, consider consulting a healthcare professional.

Here’s an example of BMI calculations for different heights and weights:

BMI Calculator for Children

Calculating BMI for children is slightly different from adults, as it takes into account age and gender to provide a more accurate assessment of growth and development. The formula remains the same: BMI = weight (kg) / (height (m))^2, but the results are interpreted using growth charts specific to children.

Here’s why BMI for children is unique:

• It compares the child’s BMI to others of the same age and gender.
• It helps identify potential health risks early.
• It accounts for growth spurts and developmental changes.

To calculate your child’s BMI, follow these steps:

1. Measure their weight in kilograms.
2. Measure their height in meters.
3. Plug the numbers into the formula: BMI = weight / (height * height).
4. Compare the result to a CDC or WHO growth chart for their age and gender.

Below is an example of how BMI percentiles might look for children:

BMI Chart for Quick Reference

Understanding your BMI (Body Mass Index) is a quick way to assess whether your weight falls within a healthy range. A BMI chart provides a visual reference to categorize your results into underweight, normal weight, overweight, or obese. Here’s how to use it:

First, calculate your BMI using the formula:

Or, if using pounds and inches:

Once you have your BMI value, refer to the chart below to interpret your results:

• Underweight: BMI less than 18.5
• Normal weight: BMI 18.5–24.9
• Overweight: BMI 25–29.9
• Obese: BMI 30 or higher

For a more detailed breakdown, here’s a reference table:

Conclusion: Understanding Your BMI for Better Health

Understanding your BMI is a crucial step toward better health. By calculating your Body Mass Index, you gain insight into whether your weight falls within a healthy range for your height. This simple metric can serve as a starting point for making informed decisions about your lifestyle, diet, and exercise habits.

Here are some key takeaways about BMI:

• BMI Categories: A BMI below 18.5 is underweight, 18.5–24.9 is healthy, 25–29.9 is overweight, and 30 or above is obese.
• Limitations: BMI doesn’t account for muscle mass or fat distribution, so it’s not a complete measure of health.
• Next Steps: If your BMI is outside the healthy range, consult a healthcare professional for personalized advice.

While BMI is a useful tool, it’s not the only indicator of health. Pair it with other metrics like waist circumference, blood pressure, and cholesterol levels for a fuller picture. Small changes, such as increasing physical activity or adjusting your diet, can significantly impact your BMI and overall well-being.

Remember, the goal isn’t just to achieve a "perfect" BMI but to foster habits that support long-term health. Regular check-ups and a balanced lifestyle are the foundation of a healthier you.