Machinists, mechanics and home-remodelling enthusiasts use feeler gauges to measure tight-fitting gaps between two surfaces. Although a standard caliper is used for most measurements in many settings, occasionally a gap is too small or difficult to access with a caliper. Feeler gauges typically come in sets of a certain range of sizes. The gauges themselves are flat pieces of finely calibrated metal blades. Each blade's thickness measurement is stamped directly on the blade itself. Feeler gauges come in both metric and English versions. You'll need to calculate the conversion if using a metric feeler gauge for English measurements.
Remove a blade from the feeler gauge.
Look for a measurement value stamped on the blade. The measurement on metric feeler gauges is typically to hundredths or thousandths of a millimetre and signified by an "mm" stamp next to the measurement.
Convert the number to its English counterpart if necessary. Multiply the millimetre value by 25.4 to determine the size of the blade in inches.