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Open calculator →Right Triangle Calculator
Use the calculator for your own numbers and compare another example instantly.
Choose a trig ratio
Relative to the angle you want, identify the opposite side, adjacent side and hypotenuse. Then choose sine, cosine or tangent based on the sides you know.
sin θ = opposite/hypotenuse; cos θ = adjacent/hypotenuse; tan θ = opposite/adjacent
Worked example
If opposite = 3 and adjacent = 4, tan θ = 3/4. Then θ = arctan(0.75) ≈ 36.87°.
The two acute angles add to 90°
After finding one acute angle, the other is 90° minus that angle because the triangle already contains one 90° angle.
Common mistakes
- Using degrees when the calculator is set to radians.
- Mixing up opposite and adjacent sides.
- Forgetting to use an inverse trig function to solve for the angle.
Frequently asked questions
How do I find an angle from two sides?
Use the trig ratio that matches those sides, then apply the inverse trig function.
Do the two acute angles always add to 90°?
Yes, in a right triangle.
What mode should my calculator use?
Use degree mode when you want the answer in degrees.