Right Angled Triangle Angle Calculator
Right Triangle Angles Calculator works out Leg a and Leg b from Leg a (opposite angle A), Leg b (opposite angle B) and Hypotenuse c. The default figures on this page give angle A of 36.869898; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
5 steps
- Solved from two legs (Pythagoras).
- Legs a = 3, b = 4; hypotenuse c = sqrt(a^2 + b^2) = 5.
- Angle A = arctan(a/b) = 36.86989764584402 deg, angle B = 90 - A = 53.13010235415598 deg.
- Area = a*b/2 = 6; perimeter = a + b + c = 12.
- Altitude to the hypotenuse = a*b/c = 2.4; the right angle's vertex projects onto the hypotenuse dividing it into a^2/c and b^2/c.
Its 10 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
4 sources
What the Right Triangle Angles Calculator calculates
Solves a right triangle from any two of leg a, leg b, hypotenuse, an acute angle, the area or the perimeter, returning all sides, angles, area, altitude to the hypotenuse, inradius and circumradius.
How to use this Right Triangle Angles Calculator
Enter Leg a (opposite angle A), Leg b (opposite angle B), Hypotenuse c and Area. Leg a updates as you type. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Leg a, Leg b and Hypotenuse c.
The method follows Euclid, Elements, Book I Proposition 47 (the Pythagorean theorem), NIST Digital Library of Mathematical Functions, 4.14 Definitions and periodicity (right-triangle ratios), Weisstein, Eric W. "Right Triangle", MathWorld and OSHA 29 CFR 1926.1053(b)(5)(i) — non-self-supporting ladders are set with the base about one quarter of the working length from the support.
What you can compare
Change one input at a time to see how it affects Leg a, Leg b and Hypotenuse c. The inputs you can vary are Leg a (opposite angle A), Leg b (opposite angle B), Hypotenuse c, Area, Perimeter and Angle unit.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How to calculate angles in a right triangle?
Solves a right triangle from any two of leg a, leg b, hypotenuse, an acute angle, the area or the perimeter, returning all sides, angles, area, altitude to the hypotenuse, inradius and circumradius. Open “How this is worked out” beside the result to see each step for your own figures.
What is the 3/4/5 rule for right angles?
On the figures this page opens with — leg a (opposite angle A) 3 and leg b (opposite angle B) 4 — the angle A is 36.869898. No single number fits everyone: change any figure above and the answer updates as you type, with each step of the arithmetic beside it.
How do I find the 3rd side of a right triangle?
It needs Leg a (opposite angle A), Leg b (opposite angle B), Hypotenuse c, Area, Perimeter and Angle unit. Nothing else is required, and the figures stay in your browser.
What information do I need for the Right Triangle Angles Calculator?
You need Leg a (opposite angle A), Leg b (opposite angle B), Hypotenuse c, Area, Perimeter and Angle unit. The page opens with default figures that give angle A of 36.869898, so you can see the scale before entering your own.
What does this Right Triangle Angles Calculator show?
It shows Leg a, Leg b, Hypotenuse c, Angle A, Angle B, Angle C (right angle), Angle A in degrees and Angle A in radians, with the working and any warnings for your entries.
Can I check how the Right Triangle Angles Calculator result was calculated?
Yes. The “How this is worked out” section shows the calculation steps, and the sources section identifies the references used for the method.