What is the Law of Cosines?

It is a mathematical bridge that connects the three sides of any triangle to one of its angles. Unlike simpler geometry rules that only work for perfect right angles, this law works on every single triangle in existence. It is incredibly handy for DIY builders, surveyors, and anyone trying to map out tricky spaces.

What inputs do I need to get started?

You only need one of two setups to use this tool: either the lengths of all three sides, or the lengths of two sides plus the angle tucked between them. Just plug those numbers in, make sure you are using the same unit of measurement for all your sides, and let us do the heavy lifting!

How often should I recalculate my project measurements?

It's a great habit to recalculate whenever you make a change to your design layout or if you realize your physical measurements were slightly off. A tiny shift in a corner angle can change a diagonal side length by several inches, so running the numbers again keeps your project on track.

What are the most common mistakes people make?

The biggest slip-ups are forgetting to set your calculator to 'Degrees' mode and rounding your decimal numbers too early in the math. To keep things simple, just paste your raw measurements into our calculator, and we will handle the precision decimals for you!

Can the Law of Cosines help me find all sides and angles of a triangle?

Absolutely! Once you use the formula to find one missing side or angle, you will have enough information to easily solve for the remaining parts of the triangle. It's like finding the first domino—once it falls, the rest of the triangle's secrets are revealed!

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Law of Cosines Calculator

Enter sides a and b and the included angle C to find side c.

Side a

Side b

Angle C (°)

Deep Dive

Read the full guide on how to use this calculator effectively

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Reviewed October 2026

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Law of Cosines Calculator

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