A screw gauge, also known as a micrometer screw gauge, is a precision instrument used to measure very small lengths or diameters — typically in the range of 0.01 mm. The least count (LC) of a screw gauge is the minimum value it can measure accurately.

Least Count of Screw Gauge: Formula

Least Count (LC) = Pitch / Number of divisions on the circular scale
or
Least Count (LC) = Number of divisions on the circular scale / Pitch

• Pitch: Distance moved by the spindle in one full rotation.
• Circular scale divisions (CSD): The number of markings on the rotating thimble/circular scale.

Example: Standard Screw Gauge

Let’s say the pitch is 0.5 mm and there are 50 divisions on the circular scale. Then:

LC = 0.5 / 50 = 0.01 mm
LC = 50 / 0.5 = 0.01 mm

So, this screw gauge has a least count of 0.01 mm, or 10 microns.

What Does This Mean Practically?

If you're measuring a wire or a thin metal sheet, your screw gauge can detect even a 0.01 mm difference. That’s like measuring something thinner than a strand of hair, which is about 0.06 mm in diameter.

Table: Least Count of Common Screw Gauges

Why Is Least Count Important in Screw Gauges?

• Precision in small measurements: Especially when working with engineering tolerances or microscopic components.
• Avoiding parallax and reading errors: A finer least count reduces chances of rounding off.
• Maintaining Quality Control: Many industries require components within ±0.01 mm tolerances — screw gauges enable this.

Real-World Application

If a screw gauge with a least count of 0.01 mm reads a measurement of:

• Main scale: 5 mm
• Thimble scale: 27 divisions

Then total reading:
= 5 + (27 × 0.01) = 5.27 mm

This accuracy is often used in:

• Measuring cylinder diameters
• Checking the thickness of metal plates
• Verifying wire diameters in electronics

Avoid These Mistakes

• Not checking for zero error: Always ensure the tool starts at zero.
• Misreading the scale: Understand both main and circular scales.
• Assuming all screw gauges have same LC: Check the scale and pitch before measuring.

The least count of a standard screw gauge is 0.01 mm, though it can vary based on the number of divisions and pitch. Knowing the LC is essential for accurate and consistent measurements, especially in high-precision tasks.