Wheel diameter, the distance of the line that goes through the center of the wheel, is necessary to know if you want to match the right tire to the right wheel. Wheels and tires are matched if the wheel diameter is the same as the tire's rim diameter. You can calculate wheel diameters with online wheel/tire calculators. These calculators will automatically calculate the wheel diameter given the tire size and rim size. You can also use the radius, the circumference or the area of the wheel to calculate the diameter.
Things You'll Need
 Calculator
 Pencil
 Paper

Calculate the wheel diameter given the radius. Multiply the radius by 2 to obtain the diameter of the wheel. A wheel with a radius of 2 inches will have a diameter of 4 inches

Calculate the wheel diameter given the circumference. Divide the circumference by 3.14. The result will be the diameter. For example, the wheel diameter for a wheel with a circumference of 10 inches is 3.18 inches.

Calculate the wheel diameter given the area. Divide the area by 3.14. Take the square root of that result. Multiply the result by 2 to obtain the diameter. For a wheel with an area of 10 square inches, the diameter is 3.57 inches
Tips & Warnings
 For cars, the wheel is different than the tire. The wheel is the rim. The wheel diameter is the diameter of the rim minus 2 times the height of the flanges. The flanges of a rim extend over the sides of the rim. Examine wheel and tire schematics provided by manufacturers so you can clearly understand the difference between a wheel and a tire.
 The circumference of a wheel can be determined by rolling the wheel on the ground for one revolution. Place the wheel on a line such that the wheel is upright and ready to roll. Mark the point on the line where the base of the wheel touches the ground. Roll the wheel for one revolution and then mark that endpoint on the line. Measure the distance between the beginning point and endpoint. This is the circumference of the wheel with the flange included.
 Consider that flanges on the end of the wheel will result in a circumference that is somewhat longer than the actual circumference. Correct for this by measuring the height of the flange. Next multiply the flange height by 3.14 and then subtract this result from the measured circumference. Divide this number by 3.14 to obtain the diameter of the wheel
 The center of the wheel is the point within the wheel that is equidistant to all the points on the circumference of a wheel.
 Remember that the diameter, radius, circumference and area measurements of a wheel are in a twodimensional plane that is perpendicular to the wheel axle. The center of the wheel plane is at the midpoint of the axle.
References
 Photo Credit Wheel image by Dusi from Fotolia.com