Supplementary Angle Calculator
Two angles are supplementary when their measures sum to exactly 180 degrees. This calculator finds the supplement of any angle in the range 0 to 180 degrees. Enter an angle in degrees, and the calculator instantly shows its supplement. A reference table displays common supplementary pairs: 0 and 180, 30 and 150, 45 and 135, 60 and 120, 90 and 90. Supplementary angles frequently appear in geometry, particularly when two lines form a straight line (called a linear pair). When two straight lines intersect, the adjacent angles on either side are supplementary. This concept is essential for solving geometry problems involving parallel lines, transversals, and polygon angle calculations. Use this calculator in geometry, construction, surveying, and any context where you need to find angles that form a straight line.
Supplementary angle formula
supplement (degrees) = 180 - angle
Common supplementary angle pairs
| Angle (degrees) | Supplement (degrees) | Sum |
|---|---|---|
| 0 | 180 | 180 |
| 30 | 150 | 180 |
| 45 | 135 | 180 |
| 60 | 120 | 180 |
| 90 | 90 | 180 |
| 120 | 60 | 180 |
| 150 | 30 | 180 |
| 180 | 0 | 180 |
Supplementary angle calculator: frequently asked questions
What are supplementary angles?
Supplementary angles are two angles whose measures sum to exactly 180 degrees. For example, 60 degrees and 120 degrees are supplementary because 60 + 120 = 180. Supplementary angles often appear on a straight line or as linear pairs in geometry.
What is the supplement of an angle?
The supplement of an angle is the angle you must add to it to reach 180 degrees. If your angle is x degrees, its supplement is (180 - x) degrees. For example, the supplement of 45 degrees is 135 degrees, because 45 + 135 = 180.
What is the difference between supplementary and complementary angles?
Complementary angles sum to 90 degrees. Supplementary angles sum to 180 degrees. For example, 30 and 60 degrees are complementary, but 30 and 150 degrees are supplementary. These concepts describe different angle relationships in geometry.
What are linear pairs?
A linear pair is two adjacent angles that form a straight line. The two angles in a linear pair are always supplementary because they sum to 180 degrees. When two lines intersect, they create two linear pairs of vertically opposite angles.
Why are supplementary angles important in geometry?
Supplementary angles appear whenever two lines form a straight line. In polygons, consecutive angles at a vertex often have supplementary relationships. Understanding supplementary angles helps solve problems involving parallel lines, transversals, and angle measures in complex geometric figures.
Official sources
- Geometry concepts: National Institute of Standards and Technology.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.