Mil (thou): Thousandth of an Inch Length Unit
The mil (also called a thou) is an imperial length unit equal to one thousandth of an inch. It remains common in electronics, coating thickness, sheet metal, and machining because it offers a convenient step between inches and micrometres. This article defines the mil, traces its industrial adoption, and shows how to convert mil values into SI units for international specifications.
Definition and SI Conversion
By definition, 1 mil = 0.001 in. Using the exact inch-to-metre conversion, the mil equals 25.4 micrometres (µm) or 0.0254 millimetres (mm). These conversions allow engineers to reconcile imperial thickness tolerances with SI documentation.
1 mil = 0.001 in = 25.4 µm = 0.0254 mm. When reporting results, specify whether “mil” refers to a thousandth of an inch (common in the United States) rather than the millimetre-based “mill” occasionally used in other contexts. Explicitly stating “thousandth of an inch” removes ambiguity in procurement documents and quality reports.
Historical Development
Machining and early manufacturing
The mil gained traction in the nineteenth century as machinists sought a practical increment for tolerances smaller than a full inch or fractional inch. With the rise of precision gauges and micrometers, “thou” became a shop-floor shorthand that could be read directly from dial indicators, lathes, and milling machines. The unit persists in maintenance manuals and tooling catalogs for legacy equipment.
Electronics and materials
Printed circuit board thickness, copper foil, solder mask, and dielectric layers were historically specified in mils. Similarly, protective coatings, films, and packaging materials often list thickness in mils because the values are easy to communicate without decimals. Even as metrication progresses, many standards still provide mil equivalents to preserve compatibility with existing tooling.
Key Concepts and Measurement Practice
Tolerances and stack-up
Small mil-level tolerances can accumulate in assemblies, making tolerance stack-up analysis critical. A laminated structure with ten layers each held to ±1 mil can vary by ±10 mil in total thickness. Converting these values to microns enables compatibility with SI-based simulation tools and helps reconcile supplier specifications across regions.
Instrumentation and calibration
Calipers, micrometers, and coating gauges often display mils or micrometres. Calibration should trace back to SI length standards so that a stated 3 mil coating corresponds to 76.2 µm within defined uncertainty. Document the measurement method (contact gauge, ultrasonic, or eddy current) to interpret variability correctly.
Applications and Importance
Electronics. PCB designers use mil grids to lay out traces and vias; a 10 mil trace width is still a common design rule. Coatings. Paint and galvanisation thickness are frequently specified in mils to align with inspection gauges. Packaging and films. Polyethylene film thickness is quoted in mils for quick comparison of barrier performance. Translating these values into SI units supports global sourcing, compliance, and product documentation.
Why the Mil Remains Relevant
The mil persists because it fits comfortably between the inch and micrometre, making it intuitive for shop-floor work. Engineers who can move between mils and SI units avoid specification errors and ensure that precision manufacturing requirements remain clear across international teams.