How to Calculate BMI: Formula, Chart & Examples

What Is BMI?

Body mass index, commonly called BMI, is a numerical measure calculated from a person’s weight and height. It is widely used as a screening measure to classify weight status in adults (CDC, About BMI).

BMI does not directly measure body fat. Two people with the same BMI can have different amounts and distributions of body fat and muscle. For that reason, BMI is best understood as one piece of health information rather than a complete assessment of an individual’s health.

How to Calculate BMI

To calculate BMI manually:

1. Measure your weight in kilograms.
2. Measure your height in metres.
3. Square your height.
4. Divide your weight by the squared height.
5. Round the result to one decimal place for easy reading.

What Is the BMI Formula?

The same BMI value can be calculated using metric or U.S. customary measurements. Both versions below are consistent with CDC guidance.

Metric vs Imperial BMI Formula

BMI Calculation Examples

These examples demonstrate the calculation only. Category interpretation depends on the applicable reference system and on the person’s age and circumstances.

How to Calculate BMI From kg and cm

If your weight is in kilograms and your height is in centimetres, use BMI = weight ÷ height² × 10,000.

How to Calculate BMI From Pounds and Inches

The BMI unit is expressed as kg/m² even when the calculation uses the imperial conversion formula.

Adult BMI Chart

For adults aged 20 and older, the standard categories used by the CDC (Adult BMI Categories) are:

What Does Your BMI Mean?

Below 18.5

This falls in the underweight range under standard adult BMI categories. A low BMI can have many possible causes. Unintentional weight loss or concerns about nutrition should be discussed with a healthcare professional.

18.5 to less than 25

This falls in the standard healthy-weight category for adults. A BMI in this range does not guarantee good health, because BMI does not capture every factor that affects health.

25 to less than 30

This falls in the standard overweight category and should be interpreted alongside other factors rather than used as a diagnosis.

30 or higher

This falls in the standard adult obesity category, which is further divided into three classes. A healthcare professional can help interpret BMI in the context of medical history, physical findings, lifestyle, laboratory results and other risk factors.

BMI for South Asian and Indian Adults

The standard international adult BMI categories remain widely used. However, research and clinical guidance recognise that people from South Asian and some other Asian populations may develop cardiometabolic risk at lower BMI values. A WHO expert consultation identified additional BMI action points for Asian populations, including 23.0 and 27.5 kg/m², while retaining the international classification system.

These thresholds are guideline-specific (NICE CG189) and should not be presented as a universal replacement for every country’s BMI classification. For an Indian reader the practical takeaway is: do not interpret BMI using the number alone. Ethnicity, waist circumference, blood pressure, blood glucose, cholesterol and family history can also matter.

Does BMI Change With Age?

For standard adult classification, the CDC uses the same basic BMI categories for adults aged 20 and older. BMI may still need additional clinical context in older adults — NICE recommends interpreting BMI with caution in people aged 65 and over, considering comorbidities and functional capacity.

Is Adult BMI Used for Children?

No. Children and teenagers should not use the adult BMI category table. BMI is still calculated from height and weight, but it is interpreted using age- and sex-specific growth references or percentiles. The CDC provides BMI-for-age interpretation for children and teens aged 2–19. Use an age- and sex-specific child/teen BMI tool instead of the adult chart.

Is BMI Different for Men and Women?

The adult formula is the same for men and women, and the standard numerical ranges are also the same. BMI does not account for differences in body composition, fat distribution or muscle mass, so the same BMI can represent different body compositions in different people.

How Accurate Is BMI?

BMI is useful as a population-level and individual screening measure, but it is not a direct measurement of body fat. It can be less informative for:

• people with very high muscle mass
• some older adults
• pregnancy
• situations where body composition differs substantially from average
• children, if adult categories are incorrectly applied

For a fuller assessment, healthcare professionals may also consider waist circumference, blood pressure, blood glucose, cholesterol, medical and family history, physical activity, body composition and examination findings.

BMI vs Body Fat: What’s the Difference?

BMI and body-fat percentage are not interchangeable. A muscular person may have a higher BMI without excess body fat, while someone with a BMI in the standard range may still have risks BMI does not capture. Our body fat calculator uses the US Navy tape method as a complementary estimate.

Why Waist Measurement Can Add Context

BMI does not show where body fat is distributed. Waist circumference or waist-to-height ratio can add information about central adiposity. Ask a healthcare professional which waist threshold is appropriate for your age, sex, ethnicity and clinical context.

How to Measure Height and Weight Before Calculating BMI

Height

• Remove shoes and stand upright on a flat surface.
• Keep your head in a neutral position.
• Record height in centimetres or metres.

Weight

• Use a reliable scale on a hard, level surface.
• Remove heavy outer clothing or shoes when practical.
• Record the measurement in kilograms or pounds.

Small measurement errors can change the calculated BMI, especially near a category threshold.

Common BMI Calculation Mistakes

What Are the Limitations of BMI?

BMI is simple, inexpensive and easy to calculate, but it does not directly measure:

• body-fat percentage
• muscle mass
• bone density
• fat distribution
• metabolic health
• blood pressure
• blood glucose
• cholesterol
• fitness level

A BMI result is therefore a screening signal, not a complete health assessment.

When Should BMI Be Interpreted With Extra Caution?

• you are pregnant
• you are under 20
• you have very high muscle mass
• you are 65 or older
• you have had unexplained weight loss or gain
• you have a condition affecting weight or body composition
• you are concerned about disordered eating
• you are using BMI to make a medical treatment decision

In these situations, use professional medical guidance rather than relying on a BMI number alone.

Is BMI the Same as Ideal Weight?

No. BMI does not calculate an “ideal weight”; it provides a weight-to-height ratio used to classify adults into broad screening categories. If you want a height-based weight range, the ideal weight calculator shows the healthy-BMI range alongside the Devine, Hamwi and Robinson formulas — none of which is a personalised clinical target.

How ToolKit Pro Calculates BMI

The calculator on this page uses the standard equations, with no rounding until display:

• Calculations run at full floating-point precision; only the displayed BMI is rounded to one decimal place.
• The calculated BMI is displayed separately from the category label, so the number and the interpretation stay distinct.
• The category labels follow the standard adult CDC/WHO ranges shown in the chart above.

Sources and Editorial Review

Key claims on this page link to the source that supports them. Primary references:

• CDC — Adult BMI Categories
• CDC — About Body Mass Index
• CDC — Child & Teen BMI (BMI-for-age)
• WHO — Obesity and Overweight fact sheet
• WHO Expert Consultation — Appropriate BMI for Asian populations
• NICE CG189 — Overweight and obesity: identification and assessment

Our health content standards

• Health pages name the author and the review date.
• Formulas and thresholds are cited to CDC, WHO or NICE rather than to other blogs.
• Screening information is clearly separated from diagnosis.
• Limitations are disclosed instead of hidden.
• No treatment claims, guaranteed outcomes or personalised medical advice.

Corrections and updates

Medical guidance evolves. If you find an outdated, unclear or incorrect statement on this page, contact the editorial team and it will be reviewed, corrected and re-dated.

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