Observe the dimensions of the figure you're given carefully. You may be presented with a full circle or a semicircle to work on. Note the units used to express the measurements. They are sometimes in centimeters, meters, millimeters or even feet. The units used do not affect the calculation process, but it's important to include the same units in the final answer.
Determine the diameter of the circle from the diagram. If you are given the radius only, multiply its value by two to get the diameter. Decide on the value of pi that you will use for the calculation. Pi is a constant that is multiplied by the diameter to obtain the circumference. Pi can be approximated as 22/7 or 3.142. Some calculators, however, are made with a pi button.
Use the calculator to multiply the diameter by the value of pi and write down the value obtained on the screen to four significant figures, which is widely accepted. This product is the length of the entire curved line of the circle.
Take the circumference obtained to calculate the length of sections expressed in terms of the circle. For example, suppose you are asked to determine the length covered by a bicycle wheel of a given diameter that rotates in a stated number of times. Multiply the number of rotations made by the circular wheel with the circumference of the wheel to determine the distance covered.