Missing Angle of a Triangle Calculator
Find the missing third angle of a triangle from any two known interior angles and show the 180-degree arithmetic step by step.
LOCAL TOOL: Angles and calculations stay entirely in your browser.
How do you find the missing angle of a triangle?
The three interior angles of every triangle add to 180°. Add the two known angles, then subtract that total from 180°.
Missing angle = 180° − known angle A − known angle B
Example: if two angles are 40° and 65°, then 40° + 65° = 105°. The missing third angle is 180° − 105° = 75°.
Check the completed triangle.
After finding the missing angle, add all three angles. A valid triangle must total exactly 180°. Common mistakes include using 360°, entering two known angles that already total 180° or more, or confusing an angle problem with a missing-side problem.
If you need to find a missing side of a right triangle instead, use the Pythagorean Theorem Calculator. For general shape measurements, try the Area & Perimeter Calculator.
Triangle missing-angle questions
Do triangle angles always add to 180°?
Yes. The three interior angles of a triangle always total 180°.
Can a triangle have two 90° angles?
No. Two 90° angles already total 180°, leaving no positive third interior angle.
What if the two known angles add to more than 180°?
Those values cannot be the interior angles of the same triangle.
Does this calculator find triangle side lengths?
No. This tool is intentionally focused on missing interior angles and the triangle angle-sum rule.