How this calculator works
Parallel bases make each leg a transversal. The interior angles at the ends of that leg add to 180 degrees. In a general trapezoid, the two angles on the longer base need not be equal, so two measurements are needed. For an isosceles trapezoid they are equal and one angle is enough. The table follows the two separate legs to avoid pairing opposite corners by mistake. This page uses unequal bases and excludes parallelograms.
Formula
A + D = 180°; B + C = 180°. Isosceles case: B = A, C = D = 180°-A.
Worked example
If A = 60° and B = 80° on the longer base, D = 120° and C = 100°. The four angles add to 360°, and each leg joins a pair totalling 180°.
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Frequently asked questions
Can I solve a general trapezoid from one angle?
One angle determines its partner on the same leg. It does not determine the other leg pair without another condition.
Are opposite angles supplementary?
That is guaranteed for an isosceles trapezoid. In a general trapezoid the guaranteed pairs are the angles sharing each leg.
Can a longer-base angle be obtuse?
Yes, if the other longer-base angle is small enough that their sum stays below 180 degrees. The shorter base can extend beyond one end of the longer base.