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.
Three sides (SSS)
Inputs: Side a (length units); Side b (length units); Side c (length units).
Base and perpendicular height
Inputs: Base b (length units); Perpendicular height h (length units).
Two sides and included angle
Inputs: Side a (length units); Side b (length units); Included angle θ (degrees).
Third interior angle
Inputs: Angle α (degrees); Angle β (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.
The included angle in SAS lies between the two entered sides. A sloping side is usually not the perpendicular height.
Reference reading: Triangle at Wolfram MathWorld.