Frequently Asked Questions
Everything you need to know about the Law of Cosines Calculator.
What is the Law of Cosines?
A formula relating a triangle's sides and one angle: c² = a² + b² − 2ab·cos C, generalizing the Pythagorean theorem.
When do I use SAS mode?
When you know two sides and the angle between them, and want the third side.
When do I use SSS mode?
When you know all three sides and want to find one or more angles.
Does it show the steps?
Yes, the formula substitution and algebra (including the arccos step for SSS) are shown.
What if my sides can't form a triangle?
The triangle inequality is checked first, and invalid combinations are flagged with an explanation.
Does it calculate area?
Yes, via Heron's formula or the ½ab·sinC formula once an angle is known.
Can I use degrees or radians?
Yes, a toggle switches units for all values.
How is this different from the Law of Sines?
Law of Cosines works with SAS/SSS data; Law of Sines is for ASA/AAS/SSA — use whichever matches what you know.
Is it free?
Completely free, no sign-up or limits.
Does it work on mobile?
Yes, fully responsive.