Rpm, or rotations per minute, measures the rotational speed of an object. If you want to convert a rotational speed to a linear speed, such as miles per hour, you need to know the diameter of the circle the object spins around. The larger the diameter, the larger the circumference, meaning the longer the distance covered. When converting, you need to make sure you convert the units properly, which is easier when using a calculator.

Enter the diameter in feet. For example, if the diameter equals 2 feet, enter "2."

Push the multiplication sign.

Enter "3.14" to multiply by pi and change the diameter to a circumference.

Push the multiplication sign.

Enter the rotational speed. In this example, if the rotational speed equals 100 rpm, enter "100."

Push the multiplication sign.

Enter "60" because you need to multiply by 60 to convert from feet per minute to feet per hour.

Push the division sign.

Enter "5,280" and push the equal sign to find the speed in mph. You divide by 5,280 to convert from feet per hour to miles per hour. In this example, when you push the equals sign, your calculator displays 7.14 miles per hour.