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Angle Calculator

Find missing angles using angle relationships. Choose what type of angle problem you have, enter what you know, and get the missing angle with a clear explanation of why.

change_history What type of angle problem?

Why?

Angle Types Quick Reference

Acute
< 90°
Sharp
Right
= 90°
Square corner
Obtuse
90°–180°
Blunt
Straight
= 180°
Flat line
Reflex
> 180°
More than half
Full
= 360°
Full turn

"The angles of any triangle always add up to exactly 180°. The angles of any quadrilateral always add up to 360°. The angles around any point always add up to 360°. These rules are always true — no exceptions!"

Key angle rules

Complementary: Two angles that add to 90°. If one is 35°, the other is 90° − 35° = 55°.

Supplementary: Two angles that add to 180°. If one is 110°, the other is 180° − 110° = 70°.

Triangle: All three angles always add to 180°. If two angles are 60° and 80°, the third is 180° − 60° − 80° = 40°.

Vertically opposite: When two lines cross, the opposite angles are equal. If one is 65°, the one directly across is also 65°.

Alternate angles (Z-angles): When a line crosses two parallel lines, alternate angles are equal. Co-interior (C-angles): they add to 180°.

Quick Questions

How do I know which angle rule to use?

Look at what the question gives you. If you see two angles on a straight line → supplementary (180°). Two angles in a corner → complementary (90°). Angles inside a triangle → they sum to 180°. Parallel lines with a transversal → look for Z or C shapes. If angles are across from each other at a crossing → vertically opposite (equal).

What is an exterior angle?

The exterior angle of a triangle is formed by extending one side outside the triangle. The exterior angle always equals the sum of the two opposite interior angles. So if a triangle has angles 50° and 70°, the exterior angle next to the 60° angle is 50° + 70° = 120°.

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