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BMI Calculator

BMI calculator turns two simple numbers, your height and your weight, into a single score called body mass index. This guide shows you how to calculate BMI by hand, which formula to use, how to read the result, and why that number is only a starting point. Whether you want a BMI calculator for men, a BMI calculator for women, or a quick BMI calculator in kg and cm, you will find the steps, worked examples and a category table below.

BMI calculator

Enter your details to see your body mass index and healthy weight range.

BMI = 0

BMI is a screening measure, not a diagnosis. It does not separate muscle from fat. Speak to a doctor about your health.

BMI categories for adults

BMICategory
Below 18.5Underweight
18.5 to 24.9Healthy weight
25 to 29.9Overweight
30 and aboveObese

How BMI is calculated

BMI is weight in kilograms divided by height in metres squared. A person who is 1.70 m tall and weighs 65 kg has a BMI of 65 ÷ (1.70 × 1.70) = 22.5.

☰ Table of Contents

    What is a BMI Calculator?

    BMI stands for body mass index. It is a ratio of weight to height. A BMI calculator is any tool, whether an online widget, a phone app or a pen-and-paper formula, that performs this calculation and usually shows which standard category your result falls into.

    In everyday terms, BMI asks: “Given how tall you are, is your weight low, in a typical range, or high?” Because taller people naturally weigh more, the formula divides weight by height squared so that people of different heights can be compared on one scale.

    What BMI measures: the relationship between your total body weight and your height.

    What BMI cannot show:

    • How much of your weight is fat, muscle, bone or water
    • Where fat is stored on your body
    • Your blood pressure, cholesterol, blood sugar, fitness or diet quality
    • Your genetic or family risk for any condition

    Public health agencies such as the U.S. Centers for Disease Control and Prevention (CDC) describe BMI as a screening tool that is easy, inexpensive and useful for spotting possible weight-related health risks across large groups of people, while stressing that it does not diagnose body fatness or health in an individual.

    How to Calculate BMI ?

    You do not need special equipment. Follow these steps.

    1. Measure your height : Stand barefoot with your back against a wall, heels together, eyes looking straight ahead. Place a flat object such as a book on your head, mark where it touches the wall, and measure from the floor to the mark.
    2. Measure your weight : Use a scale on a hard, flat floor. For consistency, weigh yourself in the morning, after using the bathroom and before eating, wearing light clothing.
    3. Pick one unit system : Use kilograms and meters (or centimeters), or pounds and inches. Do not mix them.
    4. Square your height : Multiply your height by itself.
    5. Divide : In metric units, divide your weight by your squared height. In imperial units, multiply by 703 first (explained in the next section).
    6. Round the result : Round to one decimal place, for example 24.2.
    7. Compare it with the category table later in this article.

    Worked example

    Suppose an adult weighs 70 kg and is 170 cm tall.

    • Convert height to meters: 170 ÷ 100 = 1.70 m
    • Square the height: 1.70 × 1.70 = 2.89
    • Divide weight by squared height: 70 ÷ 2.89 = 24.22
    • Rounded BMI: 24.2

    A BMI of 24.2 falls in the “healthy weight” range for adults, just under the 25.0 boundary.

    BMI Calculation Formula

    There are two versions of the BMI calculation formula. Both give the same result when the measurements match.

    Metric formula

    BMI = weight (kg) ÷ height (m)²

    • Weight is in kilograms (kg).
    • Height is in meters (m). The result is in kg/m², though the unit is usually left out.

    Imperial formula

    BMI = 703 × weight (lb) ÷ height (in)²

    • Weight is in pounds (lb).
    • Height is in inches (in), so 5 feet 5 inches is 65 inches.
    • The number 703 converts pounds and inches into the same scale as kg/m².

    Imperial example: A person weighs 140 lb and is 65 inches tall.

    • Multiply weight by 703: 140 × 703 = 98,420
    • Square height: 65 × 65 = 4,225
    • Divide: 98,420 ÷ 4,225 = 23.30
    • Rounded BMI: 23.3

    Rounding tip: Round only at the end and always to one decimal place. Rounding height or weight early can shift a result across a category boundary.

    BMI Calculator in kg and cm

    Most people outside the United States know their weight in kilograms and their height in centimeters, so a BMI calculator in kg and cm is the most common format. The formula needs height in meters, so the only extra step is converting centimeters.

    Convert centimeters to meters by dividing by 100. For example, 178 cm ÷ 100 = 1.78 m.

    A shortcut that skips the conversion

    You can also use centimeters directly:

    BMI = weight (kg) ÷ height (cm)² × 10,000

    Practical example

    A person weighs 82 kg and is 178 cm tall.

    • Meters method: 178 ÷ 100 = 1.78; 1.78² = 3.1684; 82 ÷ 3.1684 = 25.88, which rounds to 25.9
    • Centimeters method: 178² = 31,684; 82 ÷ 31,684 = 0.002588; × 10,000 = 25.88, which also rounds to 25.9

    Both methods agree. If you use a spreadsheet, enter the formula =weight/(height_cm/100)^2.

    Useful extra: You can work backward to see the weight range that corresponds to the healthy BMI range (18.5 to 24.9) for your height. For a person 170 cm (1.70 m) tall, multiply 2.89 by 18.5 and by 24.9. That gives roughly 53.5 kg to 72.0 kg. Treat it as a reference range, not a target.

    Understanding Your BMI Result

    The table below shows the standard BMI categories for adults, as published by the CDC and consistent with World Health Organization (WHO) adult classifications.

    BMI (kg/m²)

    Category

    Below 18.5

    Underweight

    18.5 to 24.9

    Healthy weight

    25.0 to 29.9

    Overweight

    30.0 and above

    Obesity

    The CDC further divides obesity into Class 1 (30.0 to 34.9), Class 2 (35.0 to 39.9) and Class 3 (40.0 and above).

    Who the table applies to: The CDC uses these categories for adults aged 20 and older. WHO adult guidance generally applies from age 18. The table does not apply to children and teenagers (see below).

    What each category means

    • Underweight: Your weight is low for your height. This can be natural for some people, but it can also be linked to low nutrient intake, certain illnesses or bone health concerns. A healthcare professional can help work out the reason.
    • Healthy weight: Your weight-to-height ratio sits in the range most associated with lower risk of weight-related conditions across populations. It does not guarantee good health.
    • Overweight: Your weight is somewhat above the typical range for your height. Many people in this range are healthy, particularly if they are active and carry more muscle. Risk depends on other factors.
    • Obesity: Your weight is well above the typical range. Risk for conditions such as type 2 diabetes, high blood pressure and heart disease is statistically higher at the group level, though individuals vary widely.

    Context changes the meaning

    A BMI number is not read in isolation. Clinicians also consider age, medical history, waist size, blood tests, family history, activity level and ancestry. Some populations also use different cut-offs, which are discussed later in this article.

    BMI Calculator for Men

    If you search for a BMI calculator for male adults, the formula you find is the same one used for everyone. The standard adult BMI formula and category table do not differ by sex. A man and a woman with the same height and weight get the same BMI.

    What differs is what that number tends to represent.

    Muscle mass. On average, men carry more muscle than women. Muscle is denser than fat, so a muscular man can have a BMI in the “overweight” range with a low body fat percentage. This is common in strength athletes and people in physical jobs.

    Fat distribution. Men are more likely to store fat around the abdomen (sometimes called an “apple” shape). Abdominal fat is linked to higher metabolic risk, which is why waist circumference adds useful information. The U.S. National Heart, Lung, and Blood Institute notes that a waist measurement above 40 inches (102 cm) in men is associated with higher risk.

    Age. Men typically lose muscle gradually from midlife onward. A man whose weight stays the same can still have more body fat at 60 than at 30, so BMI may understate fatness in older men.

    Example: A man who is 185 cm tall and weighs 95 kg has a BMI of 95 ÷ 3.4225 = 27.8. That is in the overweight range, but if he trains regularly and has a lean build, his actual health risk may be low. A waist measurement and other checks give a clearer picture.

    How Accurate Is a BMI Calculator?

    A BMI calculator is mathematically exact but biologically approximate. The arithmetic is simple and precise. What is imperfect is how well one ratio represents the body.

    Strengths

    • Fast and free. It needs only a scale and a tape measure.
    • Consistent. The same method is used worldwide, so results can be compared over time and across studies.
    • Linked to risk at population level. Large studies find that very low and very high BMI values are associated with higher rates of certain health problems.
    • Useful for tracking. Changes in your BMI over months or years can prompt a helpful check-in.

    Limitations

    • It cannot separate fat from muscle. Athletes and people with high muscle mass may be classed as overweight or obese despite low body fat.
    • It can miss excess fat in people with low muscle. This is sometimes called “normal weight obesity” and may affect older adults or people who are not physically active.
    • It ignores fat distribution. Fat around the waist and organs carries different risks than fat on the hips or thighs.
    • It does not account for age-related changes. Older adults lose height and muscle, which can change what a given BMI means. Some research suggests that a slightly higher BMI may be protective in later life, though findings vary.
    • It does not account for ancestry. Risk at a given BMI differs between populations.
    • It says nothing about overall health. Someone with a “healthy” BMI can still have high blood pressure or diabetes, and someone with a higher BMI can have excellent fitness and normal test results.

    In 2025, an international expert commission published in The Lancet Diabetes & Endocrinology recommended that clinical obesity be diagnosed using BMI together with other measures such as waist size or direct body fat assessment, rather than BMI alone. This reflects a growing consensus that BMI works best as a first step.

    Special groups

    • Athletes and very muscular people: BMI often overstates body fat.
    • Older adults: BMI may understate body fat and does not reflect muscle loss.
    • Pregnant people: Standard BMI categories do not apply during pregnancy.
    • Children and teenagers (ages 2 to 19): Adult categories should not be used. The CDC uses BMI-for-age percentile charts that account for age and sex, and a pediatrician should interpret the result.
    • People of different ancestries: See the next section.

    Do BMI Categories Differ by Population?

    Yes, in some regions. WHO’s global categories are based largely on data from people of European descent. A WHO expert consultation published in 2004 found that people of Asian ancestry may face higher risk of type 2 diabetes and cardiovascular disease at lower BMI values. It suggested 23.0 and 27.5 as additional public health action points for Asian populations.

    Several countries use lower cut-offs. For example, Indian consensus guidelines commonly treat a BMI of 23.0 to 24.9 as overweight and 25.0 or above as obesity for adults. If you live in a region with its own guidance, or if your family background is South Asian, East Asian or from another group with different recommendations, ask your doctor which thresholds apply to you.

    Do BMI Categories Differ by Population?

    Yes, in some regions. WHO’s global categories are based largely on data from people of European descent. A WHO expert consultation published in 2004 found that people of Asian ancestry may face higher risk of type 2 diabetes and cardiovascular disease at lower BMI values. It suggested 23.0 and 27.5 as additional public health action points for Asian populations.

    Several countries use lower cut-offs. For example, Indian consensus guidelines commonly treat a BMI of 23.0 to 24.9 as overweight and 25.0 or above as obesity for adults. If you live in a region with its own guidance, or if your family background is South Asian, East Asian or from another group with different recommendations, ask your doctor which thresholds apply to you.

    BMI Calculator for Women

    A BMI calculator for female users also uses the identical formula and adult categories. Nothing in the standard calculation changes for women. What changes is the context.

    Body composition. Women naturally have a higher percentage of body fat than men at the same BMI, partly because of hormonal differences and the biology of reproduction. This is normal and does not mean the BMI categories are wrong for women, only that the same number can reflect a different body makeup.

    Waist circumference. A waist measurement above 35 inches (88 cm) in women is associated with higher risk, according to the NHLBI.

    Life stages. Weight and body composition shift during puberty, pregnancy, postpartum recovery and menopause. After menopause, fat tends to move toward the abdomen even when weight stays stable.

    Pregnancy. BMI is not a tool for judging weight during pregnancy, because the weight you gain includes the baby, placenta, amniotic fluid and increased blood volume. Your BMI before pregnancy is often used to guide how much weight gain is recommended, so ask your doctor or midwife for advice specific to you. Do not use a standard BMI calculator to decide whether you are “too heavy” or “too light” while pregnant.

    Example: A woman who is 160 cm tall and weighs 58 kg has a BMI of 58 ÷ 2.56 = 22.7, which falls within the healthy weight range.

    How Accurate is a BMI Calculator?

    BMI calculator is mathematically exact but biologically approximate. The arithmetic is simple and precise. What is imperfect is how well one ratio represents the body.

    Strengths

    • Fast and free. It needs only a scale and a tape measure.
    • Consistent. The same method is used worldwide, so results can be compared over time and across studies.
    • Linked to risk at population level. Large studies find that very low and very high BMI values are associated with higher rates of certain health problems.
    • Useful for tracking. Changes in your BMI over months or years can prompt a helpful check-in.

    Limitations

    • It cannot separate fat from muscle. Athletes and people with high muscle mass may be classed as overweight or obese despite low body fat.
    • It can miss excess fat in people with low muscle. This is sometimes called “normal weight obesity” and may affect older adults or people who are not physically active.
    • It ignores fat distribution. Fat around the waist and organs carries different risks than fat on the hips or thighs.
    • It does not account for age-related changes. Older adults lose height and muscle, which can change what a given BMI means. Some research suggests that a slightly higher BMI may be protective in later life, though findings vary.
    • It does not account for ancestry. Risk at a given BMI differs between populations.
    • It says nothing about overall health. Someone with a “healthy” BMI can still have high blood pressure or diabetes, and someone with a higher BMI can have excellent fitness and normal test results.

    In 2025, an international expert commission published in The Lancet Diabetes & Endocrinology recommended that clinical obesity be diagnosed using BMI together with other measures such as waist size or direct body fat assessment, rather than BMI alone. This reflects a growing consensus that BMI works best as a first step.

    Special groups

    • Athletes and very muscular people: BMI often overstates body fat.
    • Older adults: BMI may understate body fat and does not reflect muscle loss.
    • Pregnant people: Standard BMI categories do not apply during pregnancy.
    • Children and teenagers (ages 2 to 19): Adult categories should not be used. The CDC uses BMI-for-age percentile charts that account for age and sex, and a pediatrician should interpret the result.
    • People of different ancestries: See the next section.

    Do BMI Categories Differ by Population?

    Yes, in some regions. WHO’s global categories are based largely on data from people of European descent. A WHO expert consultation published in 2004 found that people of Asian ancestry may face higher risk of type 2 diabetes and cardiovascular disease at lower BMI values. It suggested 23.0 and 27.5 as additional public health action points for Asian populations.

    Several countries use lower cut-offs. For example, Indian consensus guidelines commonly treat a BMI of 23.0 to 24.9 as overweight and 25.0 or above as obesity for adults. If you live in a region with its own guidance, or if your family background is South Asian, East Asian or from another group with different recommendations, ask your doctor which thresholds apply to you.

    Conclusion

    A BMI calculator is a quick, free way to compare your weight with your height. You can work it out by hand with weight in kilograms divided by height in meters squared, use the centimeter version for convenience, or apply the imperial formula with the 703 factor. The same formula and adult categories apply to men and women, but muscle mass, age, ancestry, pregnancy and fat distribution all change what the number means.

    Frequently Asked Questions

    For adults, a BMI from 18.5 to 24.9 is classed as a healthy weight. That said, a “good” BMI for you depends on your age, muscle mass, ancestry and medical history.

    Not the adult version. Children and teens are assessed using BMI-for-age percentiles, which compare a child with others of the same age and sex. Your child’s doctor can help interpret the result.

    Checking every few months is enough for most adults. Daily weight changes come from water, food and hormones, so they are not meaningful for BMI.

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