Formulas, without the guesswork.
Choose a relationship from the measurements you know. The variable names below belong to each calculator; a letter can have a different role in another method. π is evaluated using the browser’s full numerical constant, rather than replacing it with 3.14.
Two diagonals
Inputs: Diagonal p (length units); Diagonal q (length units).
Side and interior angle
Inputs: Side a (length units); Interior angle θ (degrees).
Side and height
Inputs: Side a (length units); Perpendicular height h (length units).
Area and interior angle
Inputs: Area A (square units); Interior angle θ (degrees).
Reading the notation
√ means a square root, ∛ means a cube root, and a superscript ² or ³ means a square or cube. Sine, cosine and tangent use angles converted from entered degrees. Where an inverse trigonometric function is used, its displayed angle is converted back to degrees. The ellipse perimeter is approximate; other formula labels describe the ideal mathematical model.
Perpendicular height is at most a side length. Diagonals need not have equal lengths.
Reference reading: Rhombus at Wolfram MathWorld.