Triangle Degree Calculator — Find the Missing Angle
The three interior angles of any triangle always sum to exactly 180°. Enter two known angles and get the third instantly — plus the triangle type (acute, right, obtuse, equilateral, isosceles, or scalene).
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Third interior angle: C = 180° − A − B
- 1
Sum of known angles
60 + 80 = 140 - 2
Missing angle C
180° − 140 = 40
How does this calculator work?
The three interior angles of any triangle sum to 180°. Enter any two angles A and B to get the missing angle C = 180° − A − B. Both angles must be positive and their sum must be less than 180°. The calculator also classifies the triangle as acute, right, or obtuse and as equilateral, isosceles, or scalene.
Formula
How this is calculated
The interior angle sum theorem states that the three interior angles of any Euclidean triangle always add up to exactly 180°. This is provable by drawing a line through one vertex parallel to the opposite side and using the properties of parallel lines — the alternating interior angles show that the three angles of the triangle equal a straight angle (180°).
Given two angles A and B, the missing angle C is simply C = 180° − A − B. The only constraint is that both A and B must be strictly positive (greater than 0°) and their sum must be strictly less than 180°; otherwise no valid interior angle C > 0° exists and the triangle degenerates into a straight line.
Once all three angles are known, the triangle can be classified in two ways. By angles: a right triangle has one 90° angle; an obtuse triangle has one angle greater than 90°; and an acute triangle has all three angles less than 90°. By sides: an equilateral triangle has all three angles equal (each 60°); an isosceles triangle has at least two equal angles; and a scalene triangle has all three angles distinct. Note that side lengths are not determined by angles alone — AAA fixes the shape but not the scale.
Frequently asked questions
Draw a line through one vertex parallel to the opposite side. The two base angles of the triangle equal the alternating interior angles formed with the parallel line (by the parallel line theorem). Together with the apex angle they span exactly a straight angle — 180°. This holds in Euclidean (flat) geometry.
A triangle cannot be formed: the missing angle C = 180° − A − B would be zero or negative. Every interior angle of a valid triangle must be strictly greater than 0°. Check that you have not entered exterior angles by mistake.
No — enter interior angles only. The exterior angle of a triangle is the supplement of its interior angle (exterior + interior = 180°). If you know an exterior angle, subtract it from 180° first to get the interior angle, then enter that.
Also known as
TG we-Calculate Editorial Team. (2026). Triangle Degree Calculator — Find the Missing Angle [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/triangle-degree-calculator
TG we-Calculate Editorial Team. "Triangle Degree Calculator — Find the Missing Angle." TG we-Calculate. 2026. https://we-calculate.com/calculator/triangle-degree-calculator.
TG we-Calculate Editorial Team, "Triangle Degree Calculator — Find the Missing Angle," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/triangle-degree-calculator
@misc{wecalculate_triangle_degree_calculator, title = {Triangle Degree Calculator — Find the Missing Angle}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/triangle-degree-calculator}}, year = {2026}, note = {TG we-Calculate} }
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