Triangle calculator

Solve a triangle from three sides (3-4-5), two sides and the included angle, or one side and two angles, with a labelled sketch.

Runs in your browser
No upload
Instant result

Assumptions & formula

Formula and assumptions

c²=a²+b²−2ab cos(C); a/sin(A)=b/sin(B)=c/sin(C); S=√s(s−a)(s−b)(s−c)

Results

Enter values and press Calculate to see the result here.

How to use this calculator

Set inputs and units
Choose SSS, SAS or ASA and one length unit, then enter the sides and angles shown for that mode.
Calculate
Check this worked example: SSS: a=3 m,b=4 m,c=5 m → area=6 m², perimeter=12 m, angleC=90°
Results
Match opposite sides and angles. The schematic is not a surveyed drawing to scale.

Questions and limits

Is SSA supported?

No. The accepted modes avoid the ambiguous side-side-angle case. ASA uses the side you enter and two angles.

Which input errors should I correct?

All sides must be positive and satisfy the triangle inequality. The angles you enter must leave a positive third angle.

What does this calculation leave out?

Triangles on a Euclidean plane only. Near-degenerate precision errors are rejected, not repaired into a triangle the tool has guessed.