How this calculator works
Opposite sides match in length. The parallelogram law recovers the second side from two diagonals, with a triangle check to exclude flat configurations. In height mode the side is the height divided by the sine of its inclination.
Formula
P = 2(a+b); b = sqrt((d²+e²)/2 - a²); b = h/sin θ; b = P/2-a.
Worked example
A base of 8 m and a height of 2.5 m at 30° give b = 2.5/0.5 = 5 m, so P = 26 m. If a = 4 m and the diagonals are 10 m and 6 m, b = √52 m and P ≈ 22.4222 m.
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Frequently asked questions
Why not add base and height?
Height runs perpendicular to the base and generally lies inside or outside the shape rather than along its boundary.
Can any side fit two diagonals?
No. Twice that side must lie strictly between the difference and sum of the diagonals.
Does the angle affect perimeter?
It does not when both sides are fixed. It matters when recovering a side from a height.