Triangle Side Calculator Step by Step
Find the missing side of any triangle instantly. Step-by-step solutions for right and oblique triangles.
Known Sides
Known Angles (Degrees)
Why Use Our Side Calculator?
Instant Solution
Just enter what you know, and we'll find the missing side length immediately.
Step-by-Step
See the formulas used (Law of Sines/Cosines) so you can learn how to do it yourself.
Accurate
Precise calculations for homework, construction, or design projects.
How to Calculate a Missing Triangle Side
The method to find a missing side depends on what information you already have:
1. Right Triangles
If one angle is 90°, use the Pythagorean Theorem: $a^2 + b^2 = c^2$.
2. Oblique Triangles (No 90° angle)
- SAS (Side-Angle-Side): Use Law of Cosines.
- AAS or ASA (Angle-Angle-Side): Use Law of Sines.
Frequently Asked Questions
How do I find the third side of a triangle?
If you know two sides and the included angle, use the Law of Cosines (c² = a² + b² - 2ab*cos(C)). If it is a right triangle, use the Pythagorean theorem (a² + b² = c²).
Can I find a side knowing only angles?
No. Knowing only angles tells you the shape (similarity) but not the size. You need at least one side length to calculate the others.
What if I have two angles and a side?
First find the third angle (angles sum to 180°). Then use the Law of Sines (a/sin(A) = b/sin(B)) to find the other sides.