This free online geometry calculator will use the Pythagorean Theorem to solve for the missing length of a right triangle given the lengths of the other two sides.
Plus, unlike other online calculators, this calculator will show its work and draw the shape of the right triangle based on the results.
Finally, this page also includes a mini calculator that checks to see if the given lengths of three sides of a triangle form a right triangle (Converse of Pythagorean Theorem).
If you are unfamiliar with the Pythagorean Theorem, you may want to read the following brief explanation before using the calculator.
The Pythagorean Theorem is the relationship between the lengths of the two legs of a right triangle and its hypotenuse. The relationship is expressed as follows:
a^{2} + b^{2} = c^{2} 
where ... 
a = the length of the vertical side. b = the length of the base. c = the length of the side opposite of the 90° angle. 
Based on this relationship, we can isolate each unknown length to solve for it.

If you already know the lengths of all three sides of a triangle, the Converse of the Pythagorean Theorem can be used to determine whether or not the triangle is a right triangle.
If a^{2} + b^{2} = c^{2} is true, then the triangle is a right triangle. 
If a^{2} + b^{2} = c^{2} is false, then the triangle is not a right triangle. 
You can conduct your own test here:
With that, let's use the Pythagorean Theorem Calculator to find the missing side of a right triangle.

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