BMI calculator for metric and imperial units.
BMI is a screening measure based on weight and height. It can be useful as a quick check, but it does not directly measure body fat or diagnose health conditions.
Metric BMI
Imperial BMI
Formula and categories
Metric formula: BMI = kg / m^2.
Imperial formula: BMI = 703 * lb / in^2.
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal weight |
| 25.0 to 29.9 | Overweight |
| 30.0 and above | Obesity |
BMI can misclassify people with high muscle mass, older adults, athletes, or people with different body compositions. It is a screening tool, not a diagnosis.
Use cases
- Quick self-screening for health education.
- Classroom and public-health examples.
- Converting between metric and imperial tracking habits.
Worked examples
- 68 kg and 1.72 m: 68 / 1.72^2 = 22.99, which is in the normal range.
- 150 lb and 68 in: 703 * 150 / 68^2 = 22.8.
- 90 kg and 1.70 m: 90 / 1.70^2 = 31.14, which falls in the obesity category.
Common mistakes
- Entering centimeters where meters are expected in the formula.
- Forgetting the 703 factor in the imperial formula.
- Treating BMI as a full health diagnosis instead of one screening signal.
Why this calculator method works in practice
Students often treat calculators as answer machines, but the strongest results come from using them as verification tools. Start by identifying the problem type, then confirm units and assumptions before entering values. This step matters because the same numbers can represent different scenarios depending on units, rounding rules, and interpretation context. In homework and exam settings, many wrong answers are caused by setup errors rather than arithmetic errors.
A practical workflow is: define the target quantity, list known inputs, choose the matching formula, estimate a reasonable range, then calculate. After calculation, compare output against your estimate and course context. If the result is far outside expectations, revisit assumptions first. This method is especially helpful when working across algebra, statistics, percentage changes, and geometric formulas where sign, order, and units can change the meaning of the same expression.
For long-term improvement, pair calculator use with explanation. Write one sentence describing why the chosen method fits the question and one sentence explaining what the result means. This habit turns isolated computations into transferable reasoning skills. It also improves consistency between classwork, quizzes, and self-study because you are validating process quality, not only the final number.
FAQ
Is BMI always accurate?
No. It is a rough screening measure and should be interpreted with other health information.
Which unit system should I use?
Use whichever matches your source data. The metric formula is usually simpler for manual calculation.
How should I check my result before submitting?
Verify units, confirm the formula matches the question, and estimate whether the final value is reasonable for the scenario.
How should I check my result before submitting?
Verify units, confirm the formula matches the question, and estimate whether the final value is reasonable for the scenario.
Practice with Examples
Click any example below to automatically fill in the calculator and see the BMI result.
Related reading
See BMI as a Screening Tool for a longer explanation of what the number can and cannot tell you.