Quick answer: how do you calculate BMI?
BMI is calculated by dividing weight in kilograms by height in metres squared: BMI = kg ÷ m². For example, 75 kg and 1.75 m gives a BMI of about 24.5. Use consistent units and remember that BMI is a screening measure, not a direct measurement of body fat.
Learn the BMI formula step by step in metric units, see an example, and understand the mistakes that can distort the result.
The BMI formula
The metric formula is BMI = kg ÷ m². Square the height in metres before dividing the weight.
Worked example: BMI for 75 kg and 175 cm
- Convert height: 175 cm = 1.75 m.
- Square the height: 1.75 × 1.75 = 3.0625 m².
- Divide weight by squared height: 75 ÷ 3.0625 ≈ 24.5.
The resulting adult BMI is approximately 24.5. The calculation is the same regardless of gender, but BMI alone cannot tell you a person’s body composition or overall health. For adults aged 20 or older, use it alongside other relevant health information; children and teens require age- and sex-specific interpretation.
Open the calculator: BMI Calculator →
Calculate BMI directly from kilograms and centimetres
If your height is already in centimetres, you do not have to convert it to metres first. The equivalent formula is BMI = weight (kg) ÷ [height (cm) × height (cm)] × 10,000. For a person weighing 75 kg and standing 175 cm tall: 75 ÷ (175 × 175) × 10,000 ≈ 24.5. The multiplier of 10,000 accounts for squaring the centimetre-to-metre conversion. Both metric methods produce the same result when used correctly. See the CDC BMI calculation formulas.
Is BMI calculated differently for men and women?
The BMI formula is the same for adult men and women: weight divided by squared height. However, it does not measure muscle, fat distribution or individual health risks. It can be less informative for very muscular adults. It is also not suitable for interpreting children and teenagers using adult categories; they require age- and sex-specific BMI-for-age assessment.
Use consistent units
Do not mix kilograms with centimetres. Convert 175 cm to 1.75 m before using the metric BMI formula.
Calculate BMI using pounds and inches
If you use U.S. customary units, the formula is BMI = weight in pounds ÷ (height in inches × height in inches) × 703. For example, a person weighing 165 lb and measuring 69 inches tall has a BMI of approximately 24.4: first square 69 to get 4,761, then calculate 165 ÷ 4,761 × 703.
The metric and U.S. formulas describe the same calculation when the measurements are converted correctly. These examples illustrate the arithmetic for adults; they do not diagnose a medical condition. Adult BMI categories are intended for people aged 20 or older. Children and teenagers require age- and sex-specific interpretation. For the official formulas, see the CDC calculation reference.
Check measurement quality
Small errors in height can noticeably change BMI because height is squared. Use a recent, reasonably accurate height and weight. If tracking over time, measure under similar conditions.
Common calculation mistakes
Typical errors include forgetting to square height, entering centimetres as metres, using a child’s result with adult categories, or treating the final number as a direct body-fat percentage.
Adult BMI chart: how to read the result
For adults aged 20 or older, CDC uses the following BMI screening categories. A category alone cannot diagnose a condition, and a person’s individual circumstances may change how useful BMI is.
| Adult BMI | Screening category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to less than 25 | Healthy weight |
| 25 to less than 30 | Overweight |
| 30 or greater | Obesity |
At exactly 1.75 m in height, the BMI formula gives about 24.5 at 75 kg. This falls within the adult healthy-weight screening interval. It does not tell you how much of that weight is muscle or whether an individual has a particular health risk. For children and teens aged 2–19, CDC uses BMI-for-age percentiles instead of this adult table. Source: CDC Adult BMI Categories.
Interpretation matters more than extra decimals
Reporting BMI to one decimal place is usually more meaningful than displaying many decimals. The category and broader health context matter more than tiny numerical differences caused by rounding.
Official reference
CDC publishes the adult BMI formula and categories and describes BMI as a screening measure.
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Frequently asked questions
Why is height squared?
BMI was defined as weight relative to the square of height to scale body mass for adult height.
Can I calculate BMI using kilograms and centimetres?
Yes. Divide weight in kilograms by height in centimetres squared, then multiply by 10,000. For example, 75 ÷ (175 × 175) × 10,000 ≈ 24.5. Converting 175 cm to 1.75 m and using the metres formula gives the same result.
How many decimals should BMI have?
One decimal place is generally enough for everyday use.
Is BMI the same as body-fat percentage?
No. BMI does not directly measure body fat.
What if my manual answer differs from the calculator?
Check unit conversion, height squaring and rounding.
Content updated: . Examples are illustrative and official references are linked in the article. If you notice an error, contact Toolszzz. Read our disclaimer.