How many micrometres are in one thousandth of an inch?

One thou, also called a mil, is 0.001 inch, which is 25.4 micrometres. Machinists often speak in tenths, meaning one ten-thousandth of an inch, which is 2.54 micrometres. A tolerance of ±0.01 mm is therefore about ±0.4 thou.

How do I convert surface feet per minute to metres per minute?

Multiply SFM by 0.3048 to get m/min. Going the other way, multiply m/min by 3.28084. A cutting speed of 200 m/min for aluminium is about 656 SFM.

How do I convert feed rate between mm/min and inches per minute?

Divide mm/min by 25.4 to get inches per minute. Feed per tooth converts the same way, so a chip load of 0.05 mm/tooth is about 0.002 inch per tooth.

How do I convert hardness and tensile strength?

Hardness scales such as Rockwell and Brinell relate to tensile strength only through empirical curves, and the relationship differs by material family and heat treatment. There is no exact conversion, so this tool deliberately does not provide one — use the material certificate or a published table for the specific alloy and condition.

CNC Unit Converter

One place for the conversions that come up on drawings, inspection reports and machine control data: length and tolerance, surface roughness, cutting speed, feed, mass, pressure and temperature.

Conversion

Quantity

Value

Convert from

Pure arithmetic conversions. Hardness and tensile strength are deliberately excluded because no exact conversion exists — those relationships are empirical and material-specific.

Result

Result—All equivalents—Context—

Length and tolerance, roughness, speed, feed, mass, pressure and temperature convert exactly by definition or by standard factor. Temperature uses the standard offset formulae.

Copy-ready text

Reference

The conversions shops use most

From	To	Multiply by	Rule of thumb
mm	inch	0.03937	25 mm is very close to 1 inch
inch	mm	25.4	Exact by definition
mm	thou (0.001 in)	39.37	0.01 mm is about 0.4 thou
µm	µin	39.37	Ra 1.6 µm is about 63 µin
m/min	SFM	3.28084	200 m/min is about 656 SFM
SFM	m/min	0.3048	100 SFM is about 30 m/min
mm/min	inch/min	0.03937	1000 mm/min is 39 ipm
kg	lb	2.20462	1 kg is about 2.2 lb
MPa	psi	145.038	1 MPa is 145 psi
bar	psi	14.5038	1 bar is about 14.5 psi

Tolerances are where unit mistakes actually cost money. A drawing dimension in inches with a tolerance in millimetres, or a program written in one unit and verified in another, is a classic source of oversize parts. Pick one unit system for a job and stay in it from drawing to inspection.

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Basis & references: NIST — SI units and conversion factors · ISO 80000 — Quantities and units

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