Updated February 2026 • 15 min read

BMI Calculator Guide: How to Calculate and Understand Your Body Mass Index

Body Mass Index (BMI) is one of the most widely used health screening tools in the world. This comprehensive guide explains exactly how BMI is calculated, what the results mean, and how to use our free BMI calculator to check your own score in seconds.

What Is BMI?

Body Mass Index (BMI) is a numerical value calculated from your weight and height. It provides a quick, inexpensive way to screen for weight categories that may lead to health problems. BMI was developed by Belgian mathematician Adolphe Quetelet in the 1830s and has been used by healthcare professionals worldwide since the 1970s as a population-level indicator of weight status. To learn more about the origins and evolution of this measurement, see our BMI history guide.

BMI does not measure body fat directly. Instead, it uses your total body weight relative to your height to place you into one of several categories: underweight, normal weight, overweight, or obese. According to the CDC, research has shown that BMI correlates reasonably well with more direct measures of body fat, such as underwater weighing and dual-energy X-ray absorptiometry (DEXA scans), making it a useful first-line screening tool. For a deeper comparison, read our body fat vs. BMI analysis.

While BMI is not perfect, it remains the most commonly used metric in clinical settings because it requires only two measurements that are easy to obtain. It helps doctors identify patients who may benefit from further evaluation, lifestyle changes, or medical intervention related to their weight.

How to Calculate BMI: The Formula

BMI is calculated using one of two formulas, depending on whether you use metric or imperial units. Both formulas yield the same result.

Metric Formula

Example: A person who weighs 70 kg and is 1.75 m tall would calculate their BMI as follows:

BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9

Imperial Formula

Example: A person who weighs 154 lbs and is 5 feet 9 inches (69 inches) tall:

BMI = (154 ÷ (69 × 69)) × 703 = (154 ÷ 4,761) × 703 = 22.7

Both results fall in the “Normal weight” category (18.5–24.9). The slight difference is due to rounding in unit conversion.

Step-by-Step Calculation

If you prefer to calculate manually, follow these steps:

1. Measure your height in meters (or convert feet and inches to total inches)
2. Square your height — multiply it by itself
3. Measure your weight in kilograms (or pounds)
4. Divide your weight by your squared height
5. If using imperial units, multiply the result by 703

Or simply use our free BMI calculator to get your result instantly without any math.

BMI Categories: What Your Number Means

The World Health Organization (WHO) defines the following BMI categories for adults aged 20 and older. These categories apply to both men and women of all body types and ethnic backgrounds, though individual interpretation may vary.

For a deeper exploration of each category, read our BMI Categories Explained guide.

Underweight (BMI Below 18.5)

A BMI below 18.5 indicates that a person may be underweight. According to the NHS, this can result from insufficient caloric intake, malabsorption disorders, eating disorders, or underlying medical conditions. Being underweight is associated with a weakened immune system, nutrient deficiencies, osteoporosis, fertility problems, and increased surgical risk. If your BMI falls in this range, consult your healthcare provider to rule out underlying conditions and develop a plan to reach a healthier weight. Our healthy weight tips guide offers practical advice.

Normal Weight (BMI 18.5 – 24.9)

A BMI between 18.5 and 24.9 is considered normal or healthy weight. People in this range generally have the lowest risk for weight-related chronic diseases such as type 2 diabetes, cardiovascular disease, and certain cancers. However, a normal BMI does not guarantee good health. Other factors such as diet quality, physical activity level, smoking status, and family history all play important roles in overall health.

Overweight (BMI 25.0 – 29.9)

A BMI of 25.0 to 29.9 is classified as overweight. According to the NIH National Heart, Lung, and Blood Institute, people in this category have a moderately increased risk for developing hypertension, type 2 diabetes, dyslipidemia, coronary heart disease, stroke, gallbladder disease, sleep apnea, and certain cancers. Losing as little as 5–10% of body weight can significantly reduce these health risks. A combination of dietary changes, increased physical activity, and behavioral strategies is typically recommended. See our how to lower BMI guide for actionable strategies.

Obese (BMI 30.0 and Above)

A BMI of 30 or higher indicates obesity, which is further subdivided into three classes. The Cleveland Clinic notes that obesity substantially increases the risk for nearly all chronic diseases and reduces life expectancy. It is also associated with higher rates of depression, joint problems, and reduced quality of life. Treatment may include lifestyle changes, behavioral therapy, medication, and in severe cases, bariatric surgery. Read more about risks in our BMI and Health Risks guide.

How to Use Our Free BMI Calculator

Our online BMI calculator offers seven specialized tools in one interface. Here is how to use each one:

1. Standard BMI Calculator

The default tab calculates your BMI using the standard formula. Enter your height and weight in either imperial (feet/inches, pounds) or metric (centimeters, kilograms) units. Click “Calculate BMI” and instantly see your BMI value, category, a visual color-coded scale bar showing where you fall, and your healthy weight range based on your height.

2. BMI for Women

The Women tab uses the same BMI formula but presents results with female-specific context. Research suggests that a slightly different healthy range of 19–24 may be more appropriate for many women due to differences in body composition. This calculator highlights those female-specific considerations. See our BMI for women guide and BMI Chart for Women for detailed reference tables.

3. BMI for Men

The Men tab provides male-specific context, where a healthy BMI range of 20–25 is often referenced. Men tend to carry more muscle mass than women at the same BMI, which is why interpretation can differ between sexes. Check our BMI for men guide and BMI Chart for Men for comprehensive tables.

4. BMI by Age

The Age tab accounts for the fact that healthy BMI ranges may shift with age. For adults aged 45–65, some research suggests a BMI of 22–27 may be associated with the lowest mortality risk. For those over 65, a range of 23–28 may be optimal. This calculator adjusts its recommendations based on the age you enter. Learn more in our BMI Calculator by Age guide.

5. Pediatric BMI (Kids)

For children and teens aged 2–19, BMI is calculated the same way but interpreted differently using our kids BMI calculator. Instead of fixed cutoffs, pediatric BMI uses age- and sex-specific percentiles. A child at the 50th percentile has a BMI equal to or greater than 50% of children of the same age and sex. The healthy range is typically the 5th to 84th percentile. Read our BMI for children guide and Pediatric BMI Calculator guide for details.

6. Ideal Weight Calculator

The ideal weight tab estimates your ideal body weight using four clinically referenced formulas: Devine, Robinson, Miller, and Hamwi. Each formula gives a slightly different result based on your height and sex, and the calculator displays the average along with a BMI-based healthy weight range. Body frame size (small, medium, or large) is factored in. See our Ideal Weight Calculator guide for formula details.

7. Lean Body Mass Calculator

The Lean Body Mass tab estimates how much of your total body weight is lean tissue (muscle, bone, organs, water) versus fat mass. It uses the Boer formula to estimate lean body mass and calculates your approximate body fat percentage. This is especially useful for athletes and people who want to track body composition beyond simple BMI. Read our Lean Body Mass Calculator guide.

BMI for Different Groups

BMI interpretation is not one-size-fits-all. Different populations may require adjusted interpretation:

Women vs. Men

Women naturally carry a higher percentage of body fat than men (approximately 20–25% for healthy women versus 15–20% for healthy men). This means that two people with identical BMI values may have very different body compositions. A woman and a man who both have a BMI of 24 likely have different amounts of body fat. For this reason, some health organizations suggest sex-specific BMI ranges, and our calculator provides separate tabs for men and women with tailored guidance.

Children and Teenagers

BMI in children changes substantially as they grow. A 7-year-old boy with a BMI of 18 is at a very different percentile than a 15-year-old boy with the same number. This is why the CDC and WHO use growth charts that compare a child’s BMI to others of the same age and sex. Weight categories for children are defined as follows:

Athletes and Muscular Individuals

Athletes, bodybuilders, and people with above-average muscle mass often have BMIs in the overweight or even obese range despite having low body fat. As Harvard Health explains, muscle weighs more per unit volume than fat, so a 6-foot-tall muscular person weighing 220 lbs would have a BMI of 29.8 (overweight) even with a body fat percentage of just 12%. For these individuals, body fat percentage, waist circumference, or the lean body mass calculator provides a more meaningful assessment than BMI alone. Our BMI for athletes guide covers this topic in depth.

Older Adults

Adults over age 65 may benefit from carrying slightly more weight than younger adults. Multiple studies have found that a BMI in the 23–28 range is associated with the lowest mortality risk in older populations. This may be because modest fat reserves provide protection during illness or recovery from surgery. However, this does not mean that obesity is healthy at any age; the risks of very high BMI remain significant in older adults.

Ethnic Variations

BMI cutoffs were originally developed using data primarily from European populations. Research has shown that health risks associated with excess weight can vary by ethnicity. For example, people of South Asian descent may face higher metabolic risk at lower BMI values (23–25 rather than 25–30), while some Pacific Islander populations may have lower risk at higher BMIs. The WHO has suggested lower BMI cutoff points for Asian populations to better reflect these differences.

Limitations of BMI

While BMI is a useful screening tool, it has several well-documented limitations that are important to understand:

• Does not distinguish fat from muscle: BMI uses total body weight, so it cannot differentiate between fat mass and lean mass. A muscular person and an overweight person of the same height could have identical BMIs.
• Does not indicate fat distribution: BMI tells you nothing about where fat is stored. Abdominal (visceral) fat is far more dangerous than fat stored in the hips and thighs, but BMI cannot distinguish between these. Waist-to-height ratio addresses this limitation.
• Not accurate for all populations: As noted above, BMI cutoffs may not apply equally to all ethnic groups, age groups, and body types.
• Does not account for bone density: People with denser bones or larger frames will naturally weigh more, potentially pushing their BMI into the overweight range even when their body fat is normal.
• Cannot detect “normal weight obesity”: Some people have a normal BMI but a high percentage of body fat and low muscle mass. This condition, sometimes called “skinny fat,” carries metabolic risks that BMI alone will not identify.
• Not appropriate for pregnant women: Weight gain during pregnancy is expected and necessary. BMI should not be used to assess weight status during pregnancy.

For a thorough discussion, read our guide on BMI Limitations.

BMI and Health Risks

Despite its limitations, BMI remains a useful predictor of health risk at the population level. Research consistently shows associations between BMI and various health conditions:

Conditions Associated with High BMI

• Type 2 diabetes: Risk increases dramatically as BMI rises above 25. A BMI over 30 carries a 7-fold increased risk compared to normal weight.
• Cardiovascular disease: Higher BMI is linked to hypertension, high cholesterol, atherosclerosis, heart attack, and stroke.
• Certain cancers: Obesity is associated with increased risk for at least 13 types of cancer, including breast, colon, kidney, and pancreatic cancer.
• Sleep apnea: Excess weight around the neck and throat can obstruct airways during sleep.
• Osteoarthritis: Extra weight puts additional stress on weight-bearing joints, particularly the knees and hips.
• Fatty liver disease: Non-alcoholic fatty liver disease (NAFLD) is strongly linked to BMI above 30.
• Mental health: Obesity is associated with higher rates of depression, anxiety, and reduced quality of life.

Conditions Associated with Low BMI

• Malnutrition and nutrient deficiencies
• Weakened immune system and susceptibility to infections
• Osteoporosis and increased fracture risk
• Fertility problems in both men and women
• Anemia and chronic fatigue

Alternatives and Complements to BMI

BMI should ideally be used alongside other measurements for a more complete picture of health. Here are the most common alternatives:

Our calculator includes a Lean Body Mass tab that estimates your body composition using the Boer formula, giving you additional insight beyond standard BMI.

Tips for Reaching and Maintaining a Healthy BMI

If your BMI falls outside the healthy range, the following evidence-based strategies can help:

BMI Calculator History and Development

The Body Mass Index has a history spanning nearly two centuries. Belgian mathematician and astronomer Adolphe Quetelet developed the concept in the 1830s as part of his work on “social physics” — an attempt to apply statistical methods to human characteristics. He called it the Quetelet Index.

The term “Body Mass Index” was coined in 1972 by American physiologist Ancel Keys in a landmark paper published in the Journal of Chronic Diseases. Keys studied several indices for measuring body fat and concluded that BMI, while imperfect, was the best population-level indicator available at the time.

In 1985, the National Institutes of Health (NIH) began using BMI to define obesity. The current WHO classification system (underweight below 18.5, normal 18.5–24.9, overweight 25–29.9, obese 30+) was established in 1995 and has remained largely unchanged since.

In 2000, the WHO published a technical report on obesity that established the BMI cutoffs still in use today. While there has been ongoing debate about the accuracy and appropriateness of these cutoffs, particularly for non-European populations, BMI remains the most widely used screening tool for weight status in both clinical practice and public health research.

Common BMI Calculation Mistakes

When calculating BMI manually, several common errors can lead to inaccurate results:

• Forgetting to square the height: The most common error. You must multiply height by itself before dividing.
• Mixing units: If using the metric formula, height must be in meters (not centimeters). If your height is 170 cm, convert to 1.70 m before calculating.
• Not converting feet to inches: For the imperial formula, convert your total height to inches first. A person who is 5′7″ is 67 inches tall, not 57.
• Forgetting the 703 multiplier: The imperial formula requires multiplying by 703. Without this factor, the result will be far too low.
• Using outdated weight: Your weight fluctuates by 2–5 lbs throughout the day. Weigh yourself at the same time, ideally in the morning before eating, for the most consistent results.

To avoid these mistakes entirely, use our free online BMI calculator which handles all unit conversions automatically.

Healthy Weight Ranges by Height

The following table shows the healthy weight range (BMI 18.5–24.9) for common heights:

Enter your exact height in our calculator for a precise healthy weight range tailored to you.

When to See a Doctor About Your BMI

While our calculator can help you identify your BMI category, certain situations warrant professional medical advice:

• Your BMI is below 18.5 or above 30
• You have unexplained weight loss or weight gain
• You have a family history of obesity-related conditions (diabetes, heart disease)
• You are starting a new diet or exercise program after being sedentary
• You are concerned about your child’s growth or weight
• You have symptoms such as excessive fatigue, shortness of breath, or joint pain related to weight

A healthcare provider can perform additional assessments including blood work, body composition analysis, and a review of your medical history to give you personalized guidance beyond what BMI alone can provide.

Frequently Asked Questions

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Medical Disclaimer: This article is for informational purposes only and does not constitute medical advice. BMI is a screening tool, not a diagnostic measure. Always consult a qualified healthcare provider for personalized health assessment and recommendations.