Body Mass Index (BMI) Formula: How to Calculate

The body mass index (BMI) formula is weight in kilograms divided by height in meters squared (kg/m²). You enter your weight and height, and the calculator returns your BMI value, which is then placed into standard categories ranging from underweight to obese.

Formula

The standard BMI formula is:

BMI = weight (kg) / [height (m)]²

Where:

• weight is measured in kilograms (kg)
• height is measured in meters (m); if you have centimeters, divide by 100 first

For example, a person who weighs 70 kg and is 1.70 m tall has a BMI of 70 / (1.70 × 1.70) = 24.2 kg/m².

Worked Example

Using the default values from the calculator above (weight = 70 kg, height = 170 cm):

1. Convert height to meters: 170 cm ÷ 100 = 1.70 m
2. Square the height: 1.70 × 1.70 = 2.89 m²
3. Divide weight by height squared: 70 ÷ 2.89 = 24.22 kg/m²
4. Rounded to one decimal place: 24.2 kg/m²

This value falls in the “normal weight” category for most adults.

How to Read Your BMI Result

For adults aged 20 and older, the World Health Organization (WHO) and the U.S. National Institutes of Health (NIH) use these standard categories:

• Underweight: BMI less than 18.5
• Normal weight: BMI 18.5 to 24.9
• Overweight: BMI 25.0 to 29.9
• Obesity class I: BMI 30.0 to 34.9
• Obesity class II: BMI 35.0 to 39.9
• Obesity class III: BMI 40.0 and above

These cutoffs apply to most adults but do not account for muscle mass, bone density, age, or sex. A very muscular athlete may have a high BMI without excess body fat, while an older adult with low muscle mass may have a normal BMI but still have high body fat percentage.

Common Mistakes and Unit Confusion

The most frequent error is mixing units. The formula requires kilograms and meters, but many people measure weight in pounds and height in feet/inches. To use the formula directly, convert first:

• 1 pound = 0.4536 kg
• 1 inch = 0.0254 m (or 2.54 cm)
• 1 foot = 0.3048 m

For example, a person who weighs 154 lb and is 5’9″ (69 inches) tall would convert to 69.9 kg and 1.75 m, giving a BMI of about 22.8.

Another common mistake is using height in centimeters without converting to meters. If you plug 170 into the formula as if it were meters, you get an absurdly small BMI. Always divide centimeters by 100 first.

Rounding early can also cause small errors. Keep at least two decimal places during calculations and round only the final result.

BMI in Children and Teens

For children and teenagers (ages 2–19), BMI is calculated with the same formula, but the interpretation is different. Instead of fixed adult cutoffs, BMI is compared to age- and sex-specific percentiles from growth charts published by the WHO or the U.S. Centers for Disease Control and Prevention (CDC).

For example, a 10-year-old boy with a BMI of 20 might be at the 85th percentile, which is considered overweight, whereas the same BMI for a 40-year-old man would be in the normal range. Because body composition changes rapidly during growth, a single BMI number is not meaningful for a child without the percentile context.

Limitations of the BMI Formula

BMI is a screening tool, not a diagnostic test. It does not directly measure body fat, and it cannot tell where fat is distributed. Visceral fat (fat around the organs) is more strongly linked to metabolic disease than subcutaneous fat, but BMI does not distinguish them.

Other limitations:

• Muscle mass: Muscle is denser than fat, so a muscular person can have a high BMI with low body fat.
• Age: BMI tends to overestimate body fat in older adults who have lost muscle and underestimate it in younger, fitter adults.
• Ethnicity: Some populations have different health risks at the same BMI. For example, Asian populations may have higher diabetes risk at lower BMI cutoffs, which is why some countries use lower thresholds (e.g., 23 for overweight).
• Pregnancy: BMI is not applicable during pregnancy because weight gain is expected and necessary.

For a fuller picture, clinicians often combine BMI with waist circumference, skinfold measurements, or bioelectrical impedance analysis.

Alternatives to BMI

Several other methods estimate body fat or health risk:

• Waist-to-hip ratio: Waist circumference divided by hip circumference. A ratio above 0.90 for men or 0.85 for women indicates higher risk.
• Waist-to-height ratio: Waist divided by height. A ratio above 0.5 is associated with increased cardiometabolic risk.
• Body fat percentage: Measured via skinfold calipers, bioelectrical impedance, DEXA scan, or hydrostatic weighing. Each method has its own error margins.
• Relative Fat Mass (RFM): A newer formula using height and waist circumference, which some studies suggest is more accurate than BMI for estimating body fat.

No single number perfectly captures health. BMI remains popular because it is cheap, quick, and requires only a scale and a stadiometer, but it should be interpreted alongside other factors.

Frequently Asked Questions

Is BMI the same for men and women?

The formula is the same, and the same cutoffs are used for both sexes in adults. However, women generally have a higher body fat percentage than men at the same BMI, so the health implications may differ slightly. Some researchers have proposed sex-specific cutoffs, but they are not yet widely adopted.

What is a healthy BMI for a 70-year-old?

For older adults, some experts suggest a slightly higher BMI range (about 24–27) may be associated with lower mortality risk, possibly because a little extra weight provides a reserve during illness. The standard “normal” range of 18.5–24.9 still applies, but it may not be optimal for everyone over 65.

Can BMI be wrong for athletes?

Yes. Athletes with high muscle mass often have a BMI in the overweight or obese range despite having very low body fat. For example, a rugby player or bodybuilder may have a BMI of 30 but a body fat percentage below 15%. In such cases, waist circumference or body fat percentage is a better metric.

How do I calculate BMI in pounds and inches?

You can use the conversion factors: BMI = (weight in lb × 703) / (height in inches)². The factor 703 converts the units to kg/m². For example, a person weighing 180 lb and 70 inches tall has a BMI of (180 × 703) / (70²) = 25.8.

Does BMI account for age?

No, the adult formula does not include age. For children, age is used to determine the percentile, but for adults, age is not part of the calculation. This is a known limitation, as body composition changes with age.

What BMI is considered overweight?

For most adults, a BMI of 25.0 to 29.9 is considered overweight. A BMI of 30 or higher is classified as obese. These cutoffs are based on large population studies linking BMI to mortality and disease risk.

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in w | Weight | kg | 70 | 0
in h | Height | cm | 170 | 0
out bmi | Body Mass Index | kg/m² | w / ((h/100)^2) | 1