How to find LC of screw gauge

To find the least count (LC) of a screw gauge, use the formula: LC = Pitch ÷ Number of circular scale divisions. First, determine the pitch by rotating the thimble until the spindle advances exactly one full millimeter on the main scale while counting the complete rotations required. If two full rotations advance the spindle 1 mm, the pitch is 0.5 mm (most common). Then count the total number of divisions marked around the circumference of the thimble's circular scale—typically 50 or 100 divisions.

For a standard screw gauge with 0.5 mm pitch and 50 thimble divisions, the calculation is LC = 0.5 mm ÷ 50 = 0.01 mm. You can verify this experimentally by rotating the thimble exactly one division and observing the spindle movement against a precision scale it should move by the calculated least count value. Some screw gauges have the least count stamped on the frame or specified in accompanying documentation, but knowing how to calculate it yourself is essential when working with unmarked instruments or verifying an instrument's specification. If the screw gauge includes an additional vernier scale on the barrel (uncommon but found on precision instruments), the effective least count becomes LC = (Pitch ÷ Circular divisions) ÷ Vernier divisions, potentially achieving 0.001 mm resolution for ultra-precision measurements.