Law of Cosines Calculator
Solve a triangle from two sides and the included angle using the cosine rule.
Pythagoras is a special case
Set C to 90° and cos C becomes zero, leaving c² = a² + b². The cosine rule is Pythagoras generalised to any triangle, with the −2ab·cos C term correcting for how far the angle departs from a right angle.
The formula
c² = a² + b² − 2ab·cos C
Every result on this page comes from this formula. Nothing is rounded until the final display, so figures match a spreadsheet to the cent.
How to use the Law of Cosines Calculator
- Fill in each field with your own figures — the defaults are only there as an example.
- The result updates as you type, so there is no button to press.
- Check the breakdown under the main result for the supporting numbers.
- Use the copy button to take the result with you, or share the link to reload the same inputs.
Law of Cosines Calculator — frequently asked questions
Is this law of cosines calculator free to use?
Yes. Every calculator on the site is free, needs no sign-up, and runs entirely in your browser, so your figures never leave your device.
How accurate is the result?
The maths is exact for the formula shown on the page. Accuracy in the real world depends on your inputs and on assumptions the formula cannot see, so treat the output as a well-grounded estimate rather than a quote.
Can I use it on my phone?
Yes. The page is responsive, weighs a few kilobytes and works offline once loaded, so it behaves the same on a phone as on a desktop.
What people search for
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About this calculator
Written and reviewed by The Samsung Calculator editorial team. Every calculator is written against a published formula, reviewed against at least one independent reference implementation, and dated when it changes.
Last reviewed August 23, 2026 · Calculation runs client-side · No data collected · Report an error