Draw the vertical leg of a right triangle. Draw the horizontal leg of a right triangle so that one end of this line touches the vertical leg of the triangle. Draw a straight line connecting the unconnected ends of the vertical and horizontal legs.
Measure the vertical leg of the triangle and write down the measurement next to this triangle leg. Measure the horizontal leg of the triangle and write this measurement down next to this triangle leg.
Multiply the length of the vertical leg by itself and write this number down. For example, if the vertical leg is 3 inches long, write down the number 9 (which is equal to 3 x 3, also known as 3 squared).
Multiply the length of the horizontal leg by itself and write this number down. For example, if the horizontal leg is 4 inches long, write down the number 16 (which is equal to 4 squared).
Sum the squared length of the vertical leg with the squared length of the horizontal leg. For the example given above, 3 squared equals 9 and 4 squared equals 16; therefore, 9 + 16 = 25.
Find the square root of the sum that you calculated in the previous step. For the example, you would obtain the square root of 25, which is equal to 5.