How to Calculate BMI: A Complete Step-by-Step Guide

BMI in Plain Language

Body mass index (BMI) compares weight with height. It gives adults a quick screening number that can be tracked over time or discussed with a healthcare professional. It does not reveal how much of your weight is fat, muscle, bone, or water.

That distinction matters. Two people can have the same BMI and very different body composition or health profiles. Treat BMI as one useful clue—not a diagnosis and not a grade for your body.

BMI Formula in Kilograms and Metres

For metric measurements, divide weight in kilograms by height in metres squared:

BMI = weight (kg) ÷ height (m)²

Example: a person who weighs 72 kg and is 1.75 m tall has a BMI of 72 ÷ (1.75 × 1.75) = 23.5. If height is recorded in centimetres, divide it by 100 first. For example, 175 cm becomes 1.75 m.

You can also use: weight (kg) ÷ height (cm)² × 10,000. Both versions produce the same result.

BMI Formula in Pounds and Inches

For US customary measurements, use total height in inches:

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

Example: 165 lb at 5 ft 9 in. Convert height first: (5 × 12) + 9 = 69 inches. Then calculate (165 ÷ 69²) × 703 = 24.4.

Small measurement errors can change a result near a category boundary. Measure without shoes, use a level floor, and enter current rather than remembered measurements.

Adult BMI Chart and Meaning

The US Centers for Disease Control and Prevention uses the following categories for adults aged 20 and older. These cutoffs are the same for adult men and women.

A category describes a statistical range, not an individual diagnosis. A value close to a boundary should not be treated as meaningfully different from a value just across it.

What BMI Can and Cannot Tell You

BMI is inexpensive, repeatable, and useful for population screening. At an individual level, it cannot locate body fat or distinguish fat mass from lean mass. It may overestimate body fatness in muscular people and miss elevated body fat in someone with relatively little muscle.

Age, pregnancy, medical conditions, disability, and differences in body composition can change how useful the number is. A fuller assessment may include waist circumference, blood pressure, laboratory results, movement habits, symptoms, medications, and family history.

BMI for Children, Teens, and Pregnancy

Do not interpret a child’s BMI with the adult chart. From ages 2 through 19, the CDC compares BMI with age- and sex-specific growth charts and reports a percentile. Growth pattern over time is often more informative than a single reading.

Standard adult BMI categories are also not designed to evaluate weight gain during pregnancy. A clinician can use pre-pregnancy BMI and pregnancy-specific guidance to discuss an appropriate range.

How to Use Your Result Responsibly

Recheck the inputs before reacting to the number. If you monitor BMI, use measurements taken under similar conditions and focus on the trend. Avoid choosing an extreme diet solely because a calculator placed you just outside a category.

Speak with a qualified professional if weight is changing without intention, eating or exercise feels difficult to control, you are pregnant, you are assessing a child, or you have symptoms or a condition affected by nutrition or weight.

How to Apply This how to calculate BMI Guide

When applying how to calculate BMI, personal context matters: measurement quality matters before category labels are discussed. Use a current weight, measure height without shoes, apply an age-appropriate method, and pay more attention to a repeated trend than to a tiny change around a category boundary.

A workable how to calculate BMI plan should follow this principle: a useful next step should improve health rather than simply force the number downward. Depending on the person, that may mean maintaining weight, improving food quality, building strength, increasing daily movement, investigating unintended change, or getting tailored clinical support.

For how to calculate BMI, choose one primary outcome and use the same method long enough to see a meaningful pattern. Review progress with more than the scale. Energy, strength, waist trend, blood pressure, relevant laboratory markers, sleep, menstrual function, mobility, and the sustainability of the routine can show whether a plan is helping.

The aim is not to follow every possible tip. It is to use the article’s main evidence—starting with “Metric formula: weight in kilograms divided by height in metres squared”—to make one realistic decision, observe the result, and adjust without losing sight of health, function, and sustainability.

Sources and Further Reading

• CDC: formulas for calculating BMI
• CDC: adult BMI categories and interpretation
• CDC: BMI-for-age categories for children and teens

Editorial note: This educational guide is not a substitute for individual medical advice.