While finding the diameter of a round object that is flat (circular) can be easily done by measuring the distance from one side to the other, the diameter of a spherical object can't be measured directly. However, thanks to the discovery of the mathematical constant pi, a simple calculation gives the diameter.
Place a drafter's triangle inside the object so the right angle is against the edge at any location. Make two marks on the object where the sides of the triangle extend past the edges of the circle. Use a straight edge to make a straight line between the two marks.
2
Choose another place to set the triangle that is about 1/4 of the way around the object and repeat Step 1. The place where the two lines intersect is the center of the circle.
3
Measure the distance between the center and any edge to find the radius. The diameter of the object is twice the radius.
Finding the Diameter of a Spherical Object
1
Wrap a string around the object, being careful to use the pressure of a finger or thumb to hold only the very end of the string against the object.
2
Pull the string taut, allowing it to adjust itself around the sphere. If the string becomes taut without slipping off the sphere, use the finger and thumb of your other hand to mark the location where the string meets the end.
3
Remove the string and measure the length from the end to where you are holding it. If the string is too long to measure while holding it, mark the string at your thumb and finger and then measure.
4
Divide the length of the string by 3.14 (pi). The result is the diameter of the object.
Tips & Warnings
Pi is an irrational number, which means that its exact value can't be determined. Consequently, you can never know the exact diameter of a round object, or any number calculated using pi. Practically, however, there is a limit to the accuracy of common measuring devices such as rulers and measuring tapes, so it's not necessary to use a more exact value of pi than 3.14.
The string will slip off if it's not placed correctly to measure the sphere's circumference. You must take care not to hold more than the end of the string against the object, preventing it from slipping off if it's in the wrong place.
Don't use something with a measurable diameter of its own (such as a rope or thick cord) to measure a sphere or your calculation of its diameter will be incorrect.
