To read the measurements of a feeler gauge, the user needs to take note of the numbers on the blade itself and whether the numbers are measured in hundredths of a millimeter or thousandths of an inch. For example, a marking on a feeler gauge of 0.040 indicates forty-thousandths of an inch thick. If this same feeler was marked in hundredths of a millimeter, it would read 1.02.
It is not uncommon for feeler gauges to have measurements for both millimeters and inches stamped onto its blades. Feeler gauges usually come with many different metal blades attached. These blades are not sharp and are used to measure gap distance. As such, a feeler gauge is useful when the gap between two surfaces needs to be exact, such as when setting the gap for a spark plug.
There are two main styles of feeler gauges. The first is a parallel feeler gauge, which has blades that stay the same width throughout the length of the blades. The next is a tapered feeler gauge, which has an end that comes to a finer point. Both gauges are used in the same way, and despite the change in width at the ends, they are both the same thickness.