¿Qué es un/una Mil (Thou) (mil)?
Formal Definition
The mil (also called thou) is a unit of length equal to one thousandth of an inch (0.001 inch). One mil equals exactly 0.0254 millimeters or 25.4 micrometers. The name "mil" is a contraction of the Latin "millesimum" (thousandth), and "thou" is short for "thousandth of an inch."
Practical Scale
A mil is extremely small — about one-quarter the thickness of a sheet of standard copy paper (typically 3 to 4 mils thick). A human hair is about 2 to 4 mils in diameter. A standard plastic garbage bag is about 0.9 to 1.5 mils thick. The mil is used primarily for measuring thin materials, coatings, and in electronics manufacturing.
Important Distinction
The mil should not be confused with the millimeter (mm) or the milliradian (also abbreviated "mil" in military contexts). One mil (0.001 inch) equals 0.0254 mm — about 1/40 of a millimeter. This potential confusion between "mil" and "millimeter" is a significant source of errors in international engineering.
Etymology
Latin Origin
The word "mil" comes from the Latin "millesimum," meaning one thousandth. It directly describes the unit: one thousandth of an inch. The alternative name "thou" is simply a shortening of "thousandth" — "one thou" means one thousandth of an inch.
British vs. American Terminology
British engineers historically preferred the term "thou" (plural "thou" or "thous"), while American engineers more commonly use "mil." In spoken language, a machinist might say "three thou" (British) or "three mils" (American) to mean 0.003 inches. Both terms are understood internationally in English-speaking engineering contexts.
The Circular Mil
A related unit is the circular mil, used in the American wire gauge system to measure the cross-sectional area of electrical conductors. One circular mil is the area of a circle with a diameter of one mil (0.001 inch). This unit is specific to the American electrical industry and has no metric equivalent in common use.
Precise Definition
Definition
The mil is defined as exactly one thousandth of an international inch: 1 mil = 0.001 in = 0.0254 mm = 25.4 μm. Since the international inch is defined as exactly 25.4 mm, the mil's metric equivalent is exact, not approximate.
Measurement Methods
Mils are measured using precision instruments such as dial indicators, micrometers, and ultrasonic thickness gauges. Coating thickness gauges — both magnetic and eddy-current types — measure film and coating thicknesses in mils with accuracy of ±0.1 mil. For PCB manufacturing, optical and laser measurement systems provide sub-mil accuracy.
The Mil in PCB Design
In printed circuit board (PCB) design, the mil is the fundamental unit. Trace widths, spacing, pad sizes, and via diameters are all specified in mils. Common PCB grid spacings are 25 mils, 50 mils, and 100 mils (0.1 inch). The standard pitch for through-hole IC packages is 100 mils (2.54 mm).
Historia
Industrial Origin
The mil emerged as a practical unit during the Industrial Revolution as manufacturing precision increased. When machinists needed to express dimensions finer than 1/64 of an inch (the finest common fraction), decimal inches and the mil became practical alternatives. By the early 20th century, the mil was well established in American manufacturing.
PCB and Electronics
The mil became the standard unit for PCB design in the 1960s and 1970s, when the US electronics industry established design rules in thousandths of an inch. The 100-mil (0.1 inch, 2.54 mm) grid became the standard for through-hole component spacing, and this legacy persists in modern PCB design software.
Coating Industry
The paint and coatings industry adopted the mil as its standard thickness unit in the United States. Specifications like SSPC (Society for Protective Coatings) standards express dry film thickness in mils. This convention remains dominant in the American coating and painting industry.
Modern Status
The mil remains widely used in the United States for coating thicknesses, plastic film thicknesses, PCB design, and precision manufacturing. It has no ISO or SI recognition and is used primarily in countries that employ the inch-based measurement system. International engineering increasingly uses micrometers for the same measurements.
Uso actual
PCB Manufacturing
The mil is the default unit in PCB design software and manufacturing. Trace widths are specified in mils (typically 4 to 20 mils for signal traces). Clearances between traces are specified in mils. Via drill sizes are in mils. Even engineers working in metric countries often use mils for PCB design because of the American-origin standards and software.
Coatings and Paint
In the US coating industry, film thickness is universally specified in mils. Automotive paint systems typically specify 1 to 2 mils of primer, 1 to 2 mils of basecoat, and 1.5 to 2 mils of clearcoat. Industrial protective coatings may specify 3 to 10 mils per coat. Coating inspectors measure dry film thickness in mils.
Plastic Films
Plastic film thickness in the US is expressed in mils. Cling wrap is about 0.5 mils. A standard garbage bag is 0.9 to 1.5 mils. Heavy-duty trash bags are 2 to 3 mils. Vapor barriers for construction are 6 to 20 mils. This convention is deeply entrenched in the American plastics industry.
Wire and Cable
Insulation thickness on electrical wire in the US is often specified in mils. The circular mil is used for wire cross-section area. These units are part of the American Wire Gauge (AWG) system that dominates US electrical work.
Everyday Use
Plastic Bags and Film
Consumers encounter mil specifications when buying plastic products. A 3-mil trash bag is heavier duty than a 1-mil bag. Laminating pouches are sold in 3-mil, 5-mil, and 10-mil thicknesses. Zip-lock bags are typically 1.5 to 4 mils thick.
Paper and Printing
Paper thickness (caliper) in the US printing industry is measured in mils. Standard copy paper is about 3.5 to 4 mils thick. Business cards are typically 12 to 16 mils. Playing cards are about 12 mils. Cardstock ranges from 10 to 14 mils.
Home Improvement
Homeowners encounter mils when dealing with paint specifications and vapor barriers. "Apply 2 mils dry film thickness" is a common paint instruction. Polyethylene sheeting for vapor barriers is sold by mil thickness: 4-mil, 6-mil, and 10-mil are common.
In Science & Industry
Materials Testing
In materials testing, the mil is used in the US for measuring coating thickness, film thickness, and thin specimen dimensions. ASTM standards for coating adhesion, flexibility, and corrosion protection reference thicknesses in mils.
Electronics Research
Research in electronics and PCB manufacturing uses mils for all board-level dimensions. Academic papers from US institutions typically use mils for PCB-related measurements, though international journals may require conversion to micrometers or millimeters.
Corrosion Science
Corrosion rates in the US are sometimes expressed in mils per year (mpy) — the thickness of metal lost to corrosion per year. A corrosion rate of 1 mpy means the metal surface recedes by 0.001 inches per year. This unit is standard in the American corrosion engineering field.
Interesting Facts
The standard through-hole IC pin pitch of 100 mils (2.54 mm) has been one of the most influential dimensions in electronics history. Billions of components have been designed to this spacing since the 1960s.
A human hair is about 2 to 4 mils in diameter. A sheet of copy paper is about 3.5 to 4 mils thick. These common references help engineers visualize mil-scale dimensions.
The PCB industry uses mils despite the global electronics supply chain being otherwise metric, creating a unique dual-unit environment where component datasheets are in millimeters but board designs are in mils.
Corrosion engineers measure metal loss in mils per year (mpy). Carbon steel in seawater corrodes at about 5 to 10 mpy, while stainless steel corrodes at less than 0.1 mpy.
A 6-mil polyethylene vapor barrier (0.006 inches, 0.15 mm) is standard for residential construction in the US. It is thin enough to handle easily but thick enough to resist punctures and moisture transmission.
The confusion between "mil" (0.001 inch = 25.4 μm) and "millimeter" (1000 μm) has caused numerous engineering errors. The factor of approximately 40 between the two units makes mistakes catastrophic.
Playing cards are typically about 12 mils (0.3 mm) thick. Casino-quality cards may be slightly thicker at 14 to 16 mils. This seemingly tiny difference (2-4 mils) is noticeable to experienced card handlers.
Regional Variations
United States
The mil is standard in American manufacturing, coatings, plastics, and electronics. It is deeply embedded in US industrial practice and shows no signs of being replaced by micrometers in domestic applications.
United Kingdom
British engineers use the term "thou" more commonly than "mil" but refer to the same unit. British manufacturing has largely transitioned to metric (micrometers), but "thou" persists in workshop language among older machinists.
International
Outside the US and UK, the mil is rarely used. International engineering uses micrometers (μm) for the same measurements. PCB design is the exception: mils are used globally in PCB layout software regardless of the designer's country, though metric units are increasingly offered as an alternative.