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Health and fitness formula

How to calculate BMI

BMI is a screening estimate that compares body weight with height.

See the formulaOpen calculator
Direct answer
Formula
BMI = weight_kg / height_m^2

Use the table below to match each symbol with the right input. Keep units consistent before you start.

BMI formula
Start here when you only need the equation and variable names.
Formula
BMI = weight_kg / height_m^2

BMI is a screening estimate that compares body weight with height.

Variable meanings
Check each symbol, meaning, and unit before you calculate.
SymbolNameDescriptionUnit
BMIBody mass indexWeight-to-height index.kg/m^2
weight_kgWeightBody weight.kg
height_mHeightBody height.m
When to use this formula
Check that your situation matches the formula before you trust the result.
  • Treat it as a quick population-style weight category estimate, not as a diagnosis or a full body composition review.
Step-by-step method
Follow these steps when you are solving it by hand.
  1. 1Convert weight to kilograms.
  2. 2Convert height to meters.
  3. 3Square the height.
  4. 4Divide weight by squared height.
Examples
These sample numbers show the order of operations and units.
Metric BMI
weight
70 kg
height
1.75 m
  1. 1.BMI = 70 / 1.75^2
  2. 2.BMI = 70 / 3.0625
  3. 3.BMI = 22.9
Result
The BMI estimate is 22.9.
Imperial BMI
weight
180 lb
height
70 in
  1. 1.BMI = 180 / 70^2 * 703
  2. 2.BMI = 25.8
Result
The BMI estimate is 25.8.
Mistakes to avoid
Small input or unit errors can change the answer a lot.
  • Treating the bmi result as a diagnosis or a personal training plan.
  • Mixing inches, centimeters, pounds, and kilograms without converting first.
  • Using stale measurements after weight, activity, or training level changes.
  • Ignoring the method assumptions behind the estimate.
Open the BMI Calculator
Open the calculator for the answer without rewriting the formula.

Enter your values in the related calculator, then compare the output with the hand method above.

Open BMI Calculator
FAQs
Short answers for common formula questions.
Is the BMI formula a medical test?

No. It gives an estimate from a few inputs. Treat it as a planning or screening number, and ask a qualified clinician about symptoms, medication, pregnancy, eating disorders, or medical conditions.

Why can my BMI result differ from a wearable or lab test?

Different methods use different assumptions. Small measurement changes, activity estimates, and rounding can move the result.

Can I use the BMI Calculator instead?

Yes. The BMI Calculator handles the arithmetic and helps catch unit conversion mistakes.