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Screw Gauge Least Count: Formula, Diagram & How to Read
Quick answer
The least count of a screw gauge is normally 0.01 mm (0.001 cm). It is calculated as Least Count = Pitch ÷ Number of divisions on the circular scale. For a pitch of 1 mm and 100 circular-scale divisions, LC = 1 ÷ 100 = 0.01 mm.
A screw gauge (also called a micrometer screw gauge) is a precision instrument used to measure very small lengths — such as the diameter of a wire or the thickness of a thin sheet — far more accurately than a ruler or even a vernier caliper.
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Screw Gauge (Micrometer) — 0.01 mm Least Count
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What Is a Screw Gauge?
A screw gauge is a mechanical measuring instrument based on the screw and nut principle. When the screw makes one complete rotation, it advances by a fixed distance called the pitch. By dividing this small movement across many divisions on a circular scale, it can measure lengths as small as 0.01 mm. It is used to measure:
- The diameter of a wire
- The thickness of a metal sheet or paper
- The diameter of small spheres or rods
Parts of a Screw Gauge
U-shaped frame — holds all the parts together
Anvil & spindle — the object is held between these faces
Main scale (sleeve) — a linear mm scale
Circular scale (thimble) — usually 50 or 100 divisions
Ratchet — gives the same gentle pressure each time
Lock nut — holds the reading in place
What Is Least Count?
The least count is the smallest length an instrument can measure accurately. The smaller it is, the more precise the tool. A screw gauge (0.01 mm) is finer than a vernier caliper (0.1 mm or 0.02 mm).
Least Count of Screw Gauge Formula
Least Count = Pitch ÷ No. of circular scale divisions
Pitch = distance moved on the main scale in one full rotation
Example 1
Pitch = 1 mm, 100 divisions.
LC = 1 ÷ 100 = 0.01 mm
Example 2
0.5 mm per rotation, 50 divisions.
LC = 0.5 ÷ 50 = 0.01 mm
How to Read a Screw Gauge (Step by Step)
Total Reading = MSR + (CSR × Least Count)
MSR = Main Scale Reading · CSR = Circular Scale Reading
- Take the main scale reading (MSR) — the last visible mark (in mm).
- Take the circular scale reading (CSR) — the division on the reference line.
- Multiply the CSR by the least count.
- Add the two values.
Example: MSR = 4 mm, CSR = 35, LC = 0.01 mm → Total = 4 + (35 × 0.01) = 4.35 mm.
Zero Error and Zero Correction
- No zero error — the circular scale zero meets the reference line.
- Positive zero error — zero is below the line; subtract it from every reading.
- Negative zero error — zero is above the line; add the correction.
Correct reading = Observed reading − Zero error (with sign).
Screw Gauge vs Vernier Caliper
| Screw Gauge | Vernier Caliper | |
|---|---|---|
| Typical least count | 0.01 mm | 0.1 mm or 0.02 mm |
| Best for | Very small thicknesses / diameters | Lengths & diameters |
| Principle | Screw & pitch | Vernier scale |
See our full Vernier Caliper Least Count guide.
Frequently Asked Questions
What is the least count of a screw gauge?
Usually 0.01 mm (0.001 cm). It equals the pitch divided by the number of divisions on the circular scale.
What is the formula for the least count of a screw gauge?
Least Count = Pitch divided by the number of divisions on the circular scale.
What is the pitch of a screw gauge?
The distance the spindle moves along the main scale in one complete rotation, commonly 1 mm or 0.5 mm.
How do you read a screw gauge?
Total reading = Main Scale Reading + (Circular Scale Reading x Least Count), after correcting for zero error.
Why is a screw gauge more accurate than a vernier caliper?
Its least count (0.01 mm) is smaller, so it measures finer dimensions like wire diameter and sheet thickness.
Need a lab-quality screw gauge?
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