**The formula for calculating the length of one side of a right-angled triangle when the length of the other two sides is known is a ^{2} + b^{2} = c^{2}.** This is known as the Pythagorean theorem.

In this formula, c represents the longest side of a triangle, known as the hypotenuse. The a and b represent the two other sides. For right-angled triangles, the square of the hypotenuse is always equal to the sum of the squares of the other two sides. The square is calculated by multiplying the value of one side by itself. As long as the length of any two of the sides is known, the third can be calculated using the Pythagorean theorem.