When to use this calculator
Use this when the longest side and one perpendicular side are known. The Pythagorean relationship only applies to a right triangle.
Explore the triangle calculators for other questions in this topic. For a related calculation, use the Triangle Altitude Calculator: Recover a triangle’s perpendicular height from area and base.
PUTTING IT INTO PRACTICE
Right Triangle Leg: a worked example
The values below are an illustrative example, not a recommendation. Replace them with measurements or assumptions that match your question.
| Input | Example value |
|---|---|
| Hypotenuse | 5 |
| Known leg | 3 |
| Result | 4 units |
- Check the input labels and enter the example values above.
- Apply the relationship: Missing leg = √(hypotenuse² − known leg²).
- The result is 4 units, rounded for display.
How to interpret your result
The hypotenuse must be longer than the known leg.
Use base and perpendicular height for the simplest area calculation. With three known sides, Heron’s formula avoids measuring height. Triangle side lengths must satisfy the triangle inequality before any area result is meaningful.
Frequently asked questions
Can I use the sloping side as triangle height?
Only if it is perpendicular to the chosen base. A general sloping side is not the perpendicular height.
When is the right triangle leg calculator useful?
Use this when the longest side and one perpendicular side are known. The Pythagorean relationship only applies to a right triangle.
How do I use the right triangle leg calculator?
Enter hypotenuse, known leg. The result updates automatically. Use the units shown beside each input; the example values illustrate the method.
Which formula does this calculator use?
Missing leg = √(hypotenuse² − known leg²).
What should I check before using the result?
The hypotenuse must be longer than the known leg.
How are results rounded?
Scalar numbers display up to six decimal places, with scientific notation for very large or small values. Text, fraction, date and schedule outputs use the format described on the tool. Intermediate calculations use JavaScript floating-point arithmetic. Keep sensible precision for your measurements.
Further reading
For background on this topic, see OpenStax: non-right triangles ↗. This is a topic reference, not an external review or endorsement of this tool.
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