Supplementary Angles Calculator - Find the Missing Angle

Two angles are called supplementary when their measures add up to exactly 180 degrees. If angle A measures 60°, then its supplement angle B measures 120°, because 60 + 120 = 180. The supplementary angles calculator automates this trivially simple but constantly needed subtraction so you never have to reach for pencil and paper. Supplementary angles appear throughout geometry, trigonometry, architecture, and everyday practical tasks. In a triangle, the interior angle and the exterior angle at any vertex are always supplementary — they form a straight line. When a transversal crosses two parallel lines, co-interior angles (also called consecutive interior angles or same-side interior angles) are supplementary. This property is used constantly in proofs involving parallel lines. In construction and design, supplementary angles arise whenever two surfaces meet to form a straight edge. If one side of a joint cuts at 35°, the mating piece must be cut at 145° so the two pieces lie flat together. Cabinet makers, metalworkers, tile setters, and frame builders all use supplementary angle relationships daily without necessarily calling them by that name. It is important to distinguish supplementary angles from complementary angles. Complementary angles sum to 90°, not 180°. A common memory trick: the letter C in Complementary comes before S in Supplementary, just as 90 comes before 180. Supplementary angles also differ from a linear pair, though a linear pair is always supplementary — a linear pair specifically requires the two angles to be adjacent and to share a common ray, forming a straight line. Supplementary angles do not need to be adjacent. Two non-adjacent angles are called supplementary if their measures add to 180°, and they can appear anywhere in a figure as long as the sum holds. For example, in a regular hexagon, any two non-adjacent interior angles whose positions are directly across from each other are supplementary to the angles that fill the gap between them in various configurations. The formula is simply B = 180° − A. Enter your known angle in degrees, and the calculator returns the supplement immediately. You can also enter decimal angles such as 45.5° or 123.75° — the formula works equally well for any value strictly between 0° and 180°. Angles of exactly 0° or 180° are degenerate cases that do not form meaningful supplementary pairs, so the calculator restricts input to that open interval.