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Interior and Exterior Triangle Angles Calculator.

Find the third interior angle and each supplementary exterior angle.

On-device calculationNo signup
01

Set your values

Results update as you type.

Interior A: 55 °

Interior A

0.000000°
Interior B: 65 °

Interior B

0.000000°
Interior C: 60 °

Interior C

0.000000°
Exterior at A: 125 °

Exterior at A

0.000000°
Exterior at B: 115 °

Exterior at B

0.000000°
Exterior at C: 120 °

Exterior at C

0.000000°

Results update automatically as you type.

Use Cases

Solve geometry homework problems

Quickly find the missing interior angle and all exterior angles for triangle problems, saving time and reducing errors.

Example: Given angles 50° and 60°, find the third angle and exterior angles.

Verify angle measurements in design

Check if a set of triangle angles is consistent and compute exterior angles for construction or design projects.

Example: Confirm that 30°, 70°, and 80° form a valid triangle and find exterior angles.

Frequently Asked Questions

How do I use the Interior and Exterior Triangle Angles Calculator?
Enter the values for two interior angles (A and B) in degrees. The calculator will compute the third interior angle (C) using the sum of interior angles (180°) and then determine all three supplementary exterior angles.
What is the relationship between interior and exterior angles?
Each exterior angle is supplementary to its adjacent interior angle, meaning they add up to 180°. The calculator uses this to find the exterior angles once the interior angles are known.
Can I use this calculator for any triangle?
Yes, as long as you provide two interior angles that sum to less than 180°. The calculator works for any triangle type—acute, obtuse, or right—as long as the inputs are valid.

Tips & Common Mistakes

Tips

  • Ensure the two interior angles you enter sum to less than 180°, otherwise the triangle is invalid.
  • Use the calculator to check your work: the sum of all three interior angles should always be 180°.
  • Remember that each exterior angle is 180° minus the adjacent interior angle, so they are always between 0° and 180°.
  • For a quick mental check, the sum of the three exterior angles (one at each vertex) is always 360°.

Common Mistakes to Avoid

  • Entering angles in radians instead of degrees—this calculator expects degree values.
  • Providing two angles that sum to 180° or more, which makes a triangle impossible.
  • Confusing interior and exterior angles: the calculator outputs both, but they are different values.

Last updated: August 13, 2026