Body mass index shows up on every doctor's intake form, but most people have never seen the actual formula behind it or understood what the number does and does not tell them. BMI is a screening tool, not a diagnosis, and knowing where it comes from helps you read your own result the right way. The BMI Calculator turns your height and weight into that same standardized number in seconds.

This article walks through the formula, the CDC and WHO category cutoffs, four worked examples covering different body types and unit systems, and the specific situations where BMI is known to give a misleading picture. Every number here traces back to CDC and WHO reference data, not a rounded approximation.

What Is a BMI Calculator and Why Does It Matter

A BMI calculator converts a person's height and weight into a single number, body mass index, used to screen whether someone's weight falls into a category associated with higher or lower health risk. It solves a practical problem: doctors, researchers, and public health agencies needed a quick, low-cost way to flag weight-related risk across large populations without individual body composition testing. In plain terms, the method divides weight in kilograms by height in meters squared. Relying on BMI as a final answer rather than a screening signal can mean missing that a muscular person is misclassified as overweight, or that an older adult with low muscle mass looks healthier on paper than they actually are.

What You Need Before You Start

Gather these two measurements before calculating, since accuracy depends on them.

- Your current weight, measured as precisely as your scale allows, in pounds or kilograms.

- Your height without shoes, in feet and inches or in centimeters.

- Your age, since BMI interpretation for adults over 65 and for children and teens differs from the standard adult ranges.

- Whether you are asking on behalf of a child or teen between ages 2 and 19, since their BMI is calculated against age- and sex-specific percentiles rather than fixed adult cutoffs.

- Any relevant context about muscle mass or athletic training, since this affects how much weight you should place on the result.

- Whether you or your family are of Asian descent, since some health organizations recommend lower BMI cutoffs for this population due to different risk patterns at the same BMI level.

How to Use This BMI Calculator Step by Step

Calculating BMI takes only your height and weight.

1. Enter your weight in pounds or kilograms, whichever your scale displays.

2. Enter your height in feet and inches or in centimeters.

3. Confirm your age, since the calculator applies adult standards only to those 20 and older.

4. Review your calculated BMI number.

5. Check which CDC category your number falls into: underweight, healthy weight, overweight, or obesity.

6. Read the plain-English note on what that category does and does not mean on its own.

7. Bring your result to your healthcare provider if you have questions about what it means for you specifically.

The BMI Calculator Formula Explained Simply

BMI compares your weight to the square of your height, producing a single number that lets researchers and clinicians compare body size across people of different heights on the same scale. This formula is the World Health Organization and CDC standard, originally developed in the 1830s by Belgian mathematician Adolphe Quetelet and later adopted for modern clinical and public health use.

BMI = Weight (kg) divided by Height (m) squared

For pounds and inches: BMI = (Weight in pounds divided by Height in inches squared) multiplied by 703

- Weight: your body weight in kilograms for the metric formula, or pounds for the US customary version, measured without heavy clothing for best accuracy.

- Height: your height in meters for the metric formula, or inches for the US customary version, measured without shoes.

- 703: the conversion constant that adjusts the pounds-and-inches formula to produce the same result as the metric kilograms-and-meters formula.

- BMI: the resulting number, typically falling between about 15 and 45 for most adults, expressed in kilograms per square meter.

The intuition behind squaring height is that body weight tends to scale roughly with the square of height across a population, not directly with height itself, so dividing by height squared produces a number that stays relatively stable across different heights for people with similar body proportions. This is why BMI works reasonably well as a population screening tool even though it was never designed to measure body fat directly in any one individual. The formula and the standard adult category cutoffs come from the CDC and the World Health Organization.

BMI Calculator Worked Examples

Example1:Beginner — Standard Adult Calculation in US Customary Units

Rachel is 34 years old, weighs 160 pounds, and is 5 feet 6 inches tall, and wants to calculate her BMI using the US customary formula.

1. Convert height to total inches: 5 feet 6 inches equals 66 inches.

2. Square the height: 66 multiplied by 66 equals 4,356 square inches.

3. Divide weight by squared height: 160 divided by 4,356 equals 0.03673.

4. Multiply by the conversion constant: 0.03673 multiplied by 703 equals 25.82.

Rachel's BMI of 25.82 falls into the CDC's overweight category, which spans 25.0 to 29.9. This is a screening result, not a diagnosis, and Rachel can discuss what it means for her specifically with her doctor, who can weigh in other factors like muscle mass, waist circumference, and overall health history.

Example2:Professional — Metric Calculation for a Clinical Setting

A clinic nurse is recording a patient's BMI using metric measurements: the patient weighs 82 kilograms and is 1.78 meters tall.

1. Square the height: 1.78 multiplied by 1.78 equals 3.1684 square meters.

2. Divide weight by squared height: 82 divided by 3.1684 equals 25.88.

The patient's BMI of 25.88 falls just inside the overweight category by the standard WHO and CDC cutoffs. In a clinical setting, this number would typically be recorded alongside blood pressure, waist circumference, and other vitals rather than used alone, consistent with CDC guidance that BMI should be considered alongside other health indicators.

Example3:Edge Case — An Athletic Individual With High Muscle Mass

Marcus is a competitive powerlifter who weighs 220 pounds and is 5 feet 10 inches tall, with a body fat percentage his trainer estimates at around 14 percent based on caliper measurements.

1. Convert height to inches: 5 feet 10 inches equals 70 inches.

2. Square the height: 70 multiplied by 70 equals 4,900 square inches.

3. Divide weight by squared height: 220 divided by 4,900 equals 0.04490.

4. Multiply by the conversion constant: 0.04490 multiplied by 703 equals 31.56.

Marcus's calculated BMI of 31.56 falls into the CDC's obesity category, yet his estimated 14 percent body fat is well within a lean, healthy range for an athletic man. This is the textbook case where BMI is known to overestimate body fat in muscular individuals, since the formula cannot distinguish muscle mass from fat mass. Marcus is a clear example of why BMI should never be the sole measure used to assess an individual's health.

Example4:Regional Variant — Applying the Lower Asian BMI Cutoffs

Wei Lin weighs 65 kilograms and is 1.65 meters tall, and wants to know how her BMI reads under both the standard WHO cutoffs and the lower cutoffs some health organizations recommend for people of Asian descent.

1. Square the height: 1.65 multiplied by 1.65 equals 2.7225 square meters.

2. Divide weight by squared height: 65 divided by 2.7225 equals 23.88.

3. Compared to standard WHO cutoffs: 23.88 falls within the standard healthy weight range of 18.5 to 24.9.

4. Compared to the lower Asian-specific overweight cutoff some researchers recommend, at 23 and above: 23.88 falls into the overweight range under that alternative threshold.

Wei Lin's result illustrates why the same BMI number can fall into different categories depending on which population-specific cutoffs are applied, since some research indicates health risk begins at a lower BMI for people of Asian descent. This is a case where discussing the result with a healthcare provider familiar with population-specific risk research adds real value beyond the raw number.

BMI Calculator Results What the Numbers Mean

The CDC defines four standard adult categories: below 18.5 is underweight, 18.5 to 24.9 is healthy weight, 25.0 to 29.9 is overweight, and 30.0 and above is obesity, which is further divided into Class 1 (30.0 to 34.9), Class 2 (35.0 to 39.9), and Class 3 (40.0 and above). A result in the healthy weight range is generally reassuring as a population-level indicator, but it does not rule out other health concerns, and someone with low muscle mass can fall into this range while still carrying higher body fat than the number suggests. A result in the overweight or obesity range is a signal worth discussing with a healthcare provider, not an automatic health verdict, since the CDC itself states that BMI is a screening measure not intended to diagnose disease or illness.

The most common misunderstanding is treating BMI as a direct measurement of body fat rather than an indirect estimate based on weight and height alone. Two people with an identical BMI can have very different body compositions, health markers, and risk profiles depending on muscle mass, fat distribution, age, and overall fitness. The practical move, per CDC guidance, is to view your BMI category as one data point among several, alongside waist circumference, blood pressure, and other factors your doctor considers, rather than as a standalone verdict on your health.

How Your Inputs Change the Result

Weight has a direct, linear effect on BMI: for a fixed height, every additional pound or kilogram raises BMI by the same proportional amount. A 10-pound weight change for someone at 5 feet 6 inches shifts BMI by roughly 1.6 points, enough to cross a full category boundary in some cases.

Height has a much larger effect on BMI than weight does, because the formula squares it. Two people of the same weight but a few inches different in height can land in entirely different BMI categories, which is exactly the imbalance the formula was designed to correct for across a population, though it can feel counterintuitive for any one individual comparing themselves to someone of a different height.

Age changes how BMI should be interpreted even though it does not change the calculation itself for adults 20 and older. Research reviewed by the CDC suggests that for adults over 65, a slightly higher BMI range, often cited around 23 to 27, may be associated with lower mortality risk than the standard 18.5 to 24.9 range, since some muscle loss with age is common and a bit of extra reserve can be protective.

Population-specific cutoffs shift where the category lines fall without changing your calculated number at all. The table below shows how the same BMI value can land in different categories depending on which reference standard is applied.

BMI value | Standard WHO/CDC category | Lower Asian-specific category (where used)

22.5 | Healthy weight | Healthy weight

23.5 | Healthy weight | Overweight

26.0 | Overweight | Obese

29.0 | Overweight | Obese

BMI Calculator Common Mistakes to Avoid

- Treating BMI as a direct measure of body fat percentage. BMI estimates weight relative to height only; it cannot distinguish fat mass from muscle mass, bone density, or fluid.

- Applying standard adult cutoffs to a child or teenager. The CDC uses age- and sex-specific percentiles for people ages 2 through 19, not the fixed adult ranges of 18.5, 25, and 30.

- Ignoring muscle mass in athletic individuals. Competitive athletes and people who strength train regularly commonly show a BMI in the overweight or obese range despite low body fat, as shown in the worked example above.

- Using a single BMI category to make a medical decision without professional input. The CDC explicitly states that BMI is a screening measure, not intended to diagnose disease, and should be discussed with a healthcare provider.

- Overlooking that healthy BMI ranges may differ by population. Some research supports lower overweight and obesity cutoffs for people of Asian descent due to different risk patterns at the same BMI.

- Forgetting the standard ranges do not adjust for older adults. Some research suggests a slightly higher BMI may be associated with better outcomes for adults over 65 compared to the standard adult range.

- Mixing up the pounds-inches and kilograms-meters formulas. Using the metric formula with pounds and inches, or vice versa, produces a wildly incorrect result rather than a small rounding error.

- Using BMI alone during pregnancy. Standard adult BMI categories are not designed for pregnant individuals, whose weight changes reflect normal physiological changes rather than body fat shifts.

Key Terms Explained

Body mass index (BMI)

Body mass index is a calculated screening measure of weight relative to height, expressed in kilograms per square meter, used to categorize adults into weight-related risk groups.

Underweight

Underweight is the CDC category for adult BMI below 18.5, associated with risks such as nutrient deficiency and reduced bone density.

Obesity class

Obesity class refers to the CDC's three-tier subdivision of the obesity category: Class 1 (30.0-34.9), Class 2 (35.0-39.9), and Class 3 (40.0 and above).

BMI percentile

BMI percentile is the method used to interpret BMI in children and teens ages 2 through 19, comparing a child's BMI to others of the same age and sex rather than using fixed adult cutoffs.

Waist circumference

Waist circumference is a separate body measurement, often used alongside BMI, since excess abdominal fat carries independent health risk information that BMI alone does not capture.

Body composition

Body composition refers to the proportion of a person's body made up of fat mass versus lean mass such as muscle and bone, information that BMI does not directly measure.

Screening measure

A screening measure is a tool used to flag individuals who may benefit from further evaluation, as distinct from a diagnostic test that confirms a specific condition.

How This Calculator Works Methodology

This calculator uses the standard WHO and CDC BMI formula, weight in kilograms divided by height in meters squared, applied to adults 20 years and older. Category cutoffs follow the CDC's published adult ranges: underweight below 18.5, healthy weight 18.5 to 24.9, overweight 25.0 to 29.9, and obesity 30.0 and above, subdivided into three classes. The calculator does not apply the child and teen percentile system used for ages 2 through 19, and does not adjust automatically for age, athletic build, or population-specific cutoffs, since the CDC treats these as factors for a healthcare provider to weigh individually rather than fixed formula adjustments. Results are rounded to two decimal places. This calculator excludes children under 2, since standard BMI methodology is not applied to infants and toddlers.

Frequently Asked Questions About the BMI Calculator

1. What is a healthy BMI for adults?

The CDC defines a healthy BMI for adults as 18.5 to 24.9. This range applies to adults 20 and older and is based on population-level health risk data, not an individual diagnosis.

2. How accurate is BMI as a health measure?

BMI is a reasonably reliable screening tool at the population level but has known limitations for individuals, particularly athletes, older adults, and people with unusual muscle-to-fat ratios. The CDC states it should be considered alongside other health factors, not used alone.

3. What is the BMI formula in simple terms?

BMI equals weight in kilograms divided by height in meters squared, or for pounds and inches, weight divided by height squared, multiplied by 703. Both versions produce the same result, just using different units.

4. Is the BMI scale the same for everyone?

The standard WHO and CDC cutoffs apply broadly, but some health organizations recommend lower overweight and obesity thresholds for people of Asian descent due to different risk patterns at the same BMI. Children and teens use a completely different, age- and sex-based percentile system.

5. When should I not rely on my BMI result?

BMI is less reliable for competitive athletes and heavily muscled individuals, who often show an inflated BMI despite low body fat, and for older adults, who may show a lower BMI despite reduced muscle mass. It also is not designed for use during pregnancy or for children outside the 2-to-19 percentile system.

6. How can I get a more complete picture of my health than BMI alone provides?

Ask your healthcare provider about combining BMI with waist circumference, blood pressure, and other lab measures, since the CDC recommends considering these together rather than relying on BMI in isolation. Some providers also use body fat percentage tools to add more context to a BMI reading.

7. What is a typical BMI result for US adults?

National survey data reviewed by the CDC's National Center for Health Statistics shows a substantial share of US adults fall in the overweight range, with roughly 3 in 10 adults classified as overweight in recent national data, and additional adults falling into the obesity categories. These are population statistics and do not predict any individual's result.

8. Should I talk to a doctor about my BMI result?

Yes, especially if your result falls outside the healthy weight range or if you have questions about what your number means given your muscle mass, age, or family health history. The CDC's own BMI tool states plainly that it is not a substitute for professional medical advice.

Related Calculators Worth Bookmarking

- Body fat percentage calculator: provides a body composition estimate that BMI cannot, useful alongside BMI for a fuller picture.

- Waist-to-hip ratio calculator: measures fat distribution, which the CDC recommends considering alongside BMI for a more complete risk picture.

- Calorie needs calculator: estimates daily energy needs based on activity level, height, weight, and age, often used alongside a BMI result.

- Ideal weight range calculator: shows the weight range corresponding to a healthy BMI for a given height.

- Child and teen BMI percentile calculator: the correct tool for anyone ages 2 through 19, using the CDC's age- and sex-specific percentile system instead of adult cutoffs.

- Basal metabolic rate calculator: estimates resting energy expenditure, a common companion calculation to BMI in fitness and nutrition planning.

- Pregnancy weight gain calculator: appropriate for tracking healthy weight gain during pregnancy, since standard adult BMI categories do not apply.

- Blood pressure category calculator: another common screening measure often reviewed alongside BMI during a routine health checkup.

References and Further Reading

1. Centers for Disease Control and Prevention. Adult BMI Calculator. U.S. Department of Health and Human Services, 2024.

2. Centers for Disease Control and Prevention. About Body Mass Index (BMI). U.S. Department of Health and Human Services.

3. Centers for Disease Control and Prevention, National Center for Health Statistics. Overweight and Obesity Data Query System. U.S. Department of Health and Human Services, 2026.

4. World Health Organization. Body Mass Index (BMI) Classification. World Health Organization.

5. World Health Organization Expert Consultation. Appropriate Body-Mass Index for Asian Populations and Its Implications for Policy and Intervention Strategies. The Lancet, 2004.

Important Disclaimer

This calculator is for informational and educational purposes only and provides a general screening estimate, not a medical diagnosis. This tool does not constitute medical advice. Always consult a qualified healthcare provider to interpret your BMI result and discuss any concerns about your weight or overall health.