The diameter is the width of a circle, from one side to the other via the center. Circles are 2-dimensional shapes with a flat surface, enabling you to measure them easily, but 3-dimensional round objects are much harder to measure. Simple external calipers consist of two curved and pivoted legs that span the opposite sides of an object. They are cheap and easy to use, but the accuracy of readings depends on the care and skill of the user.
Open the calipers to a width slightly greater than the object they are measuring. If they feel loose and do not stay open, tighten the pivot screw until you feel slight resistance when opening and closing them.
Close the calipers onto the object at the point you estimate to be the widest part. Slide the calipers across the object surface then around its surface, seeking the widest point. Keep adjusting the calipers by lightly tapping them to match the size of the object. Continue moving the calipers until you can no longer find positions that are wider than the existing caliper setting.
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Remove the calipers from the object and lay them on a flat surface. Measure the distance between the caliper points. This distance is the diameter of the round object.
Take care not to move the caliper tips when removing them from the object.
Non-spherical round objects, such as an egg, have more than one diameter.
More expensive analog and digital calipers include a dial or display to directly show the measurements.
Loose calipers will slip and give false readings.