Four sides, four angles, endless variety — the quadrilateral family stretches from squares to kites to lopsided trapezoids, and each member has its own area recipe. Rectangles and parallelograms use base × height (the height measured perpendicular to the base, not along the slanted side). Trapezoids average the two parallel sides first: (a + b) ÷ 2 × height. Kites and rhombuses use half the product of the diagonals: (d₁ × d₂) ÷ 2. And any quadrilateral at all surrenders to triangulation — split it along a diagonal into two triangles and add their areas.

Flooring estimators measure rooms this way, since few rooms are perfect rectangles; splitting an L-shaped living room into two rectangles is triangulation in disguise. The universal trap is the slanted side: area needs the perpendicular height, and using the slant edge instead inflates the answer. This calculator handles the multiplicative recipes: for a rhombus with diagonals 10 and 6, type half the first diagonal (5) in the Variable A box and the second diagonal (6) in the Variable B box — the Result box shows 30, the area, since (10 × 6) ÷ 2 = 5 × 6. For rectangles, enter base in A and perpendicular height in B directly.

With the pre-filled defaults — Variable A 10, Variable B 5 — the Quadrilateral Calculator returns Result 50.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.