Triangle calculators

Triangle Calculator

Solve sides, angles, area, and perimeter from the values you know.

Missing sidesAnglesArea & perimeterStep-by-step formula path

Calculate triangle values

Enter the known values for this triangle case. Results are educational estimates for geometry and measurement problems.

Result

What this calculator does

This calculator is designed for common triangle homework, geometry, construction, and measurement problems where you know some sides or angles and need the rest.

SSS, SAS, ASA, AASUse the right case for the information you know.
Steps matterThe page explains which formula was used.
Related pagesUse a dedicated page for missing sides, right triangles, or angles.
For classroom work, check the required rounding rules and whether angles are in degrees or radians. This page uses degrees.

How to choose the right triangle calculation

A triangle problem is usually determined by what you already know. If you know three side lengths, use the SSS path. If you know two sides and the included angle, use the SAS path. If you know two angles and one side, use ASA or AAS. If the triangle has a 90° angle, use the right triangle path.

You knowUse this caseWhat the calculator finds
Three sidesSSSAngles, area, perimeter, triangle type
Two sides and included angleSAS / Law of CosinesMissing side, remaining angles, area
Two angles and one sideASA / AAS / Law of SinesThird angle and missing sides
Two sides of a right triangleRight triangleMissing side, acute angles, area, perimeter
Base and heightAreaTriangle area only

Quick answers for common triangle questions

How do I find a missing side?For a right triangle, use the Pythagorean theorem. For two sides and an included angle, use the law of cosines. For an angle-side pair, use the law of sines.
How do I find a missing angle?If two angles are known, subtract them from 180°. If three sides are known, use the law of cosines for each angle.
Why does SSA sometimes give two answers?Side-side-angle can be ambiguous because the same side and angle information may describe two different triangles.
This calculator uses degrees. If your homework, exam, or engineering task uses radians, convert the angle before comparing results.

Common mistakes to avoid

  • Using the wrong angle: In SAS, the angle must be between the two known sides.
  • Forgetting triangle inequality: Three side lengths only form a triangle when each pair of sides is longer than the remaining side.
  • Mixing side labels: Side a is opposite angle A, side b is opposite angle B, and side c is opposite angle C.
  • Confusing area formulas: Base × height ÷ 2 needs a perpendicular height, not a slanted side.

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Triangle CalculatorsTriangle CalculatorTriangle Solver with StepsFind Missing SideRight Triangle with StepsTriangle Angle CalculatorTriangle Area CalculatorLaw of CosinesLaw of Sines

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