Home / Topics / Measurement & Math Tools
Screw gauge
Beginner
Micrometer using a screw thread to measure small thicknesses.
LC = pitch / circular divisionsA screw gauge turns a precision screw so one full rotation advances the spindle by the pitch (often 0.5 or 1 mm). The circular scale divides the pitch, giving a least count = pitch ÷ number of circular divisions. Reading = main (linear) scale + circular scale × LC.
Pitch 0.5 mm, 50 circular divisions: LC = 0.01 mm. Linear 3.5 mm and circular 27 give 3.5 + 0.27 = 3.77 mm.
Always stop tightening with the ratchet to avoid squeezing the object, and correct for zero error, which can be positive or negative.
In more detail
A screw gauge uses a fine screw thread to convert a small rotation into a small linear movement. Its least count is the pitch divided by the number of circular scale divisions.
Real-world uses
- Physics labs use screw gauges to measure wire diameters and sheet thickness.
- Manufacturers check the thickness of foil and sheet metal with micrometers.
- Wire diameters, sheet thicknesses and the walls of thin tubes are commonly checked with a screw gauge in workshops and quality control.
Open the interactive lesson, with simulation and quiz →
Frequently asked questions
Why is a screw gauge more precise than a ruler?
The thread magnifies small movements, so a quarter turn moves the spindle by only a fraction of a millimetre.
What is zero error on a screw gauge?
When closed, the reading should be zero. A non-zero reading is a zero error that you must subtract from every measurement.
What should I do if my screw gauge reads a little off with nothing in the jaws?
Note the zero error and subtract it from every reading. Many instruments are checked this way before use.
Related topics in Measurement & Math Tools
Scientific notation · Length measurement · Vernier calliper · Measuring time · Mass vs weight · Significant figures