A mil is one thousandth of an inch (0.001 inch). It is also called a "thou" (from "thousandth"). One mil equals 25.4 micrometers or 0.0254 millimeters.

Is a mil the same as a millimeter?

No. A mil is about 39 times smaller than a millimeter. 1 mm ≈ 39.37 mil, and 1 mil = 0.0254 mm.

How thick is a standard PCB in mils?

A standard 4-layer FR4 PCB is 62 mil (1.57 mm). Thin boards may be 31 mil, and thicker ones 93 or 125 mil.

Why do engineers use mils instead of millimeters?

Mils are the historic US manufacturing standard for thin dimensions. PCB design tools, wire gauges, and coating specs in the US have used mils for decades. International contexts prefer µm or mm.

How many mils is 1 mm?

One millimeter equals approximately 39.37 mils. That is why thin metric specs can become large mil values quickly.

What coating thickness is typical in mils?

Automotive paint: 4–5 mil; powder coat: 2–5 mil; conformal coating: 1–5 mil; anodizing: 0.5–3 mil.

Mil (Thousandths of an Inch) Conversion Calculator

Convert mils to inches, mm, µm, cm, meters, and feet. Includes thickness reference for PCBs, coatings, foils, and common materials.

Value

From

Decimals

10.00 Mils (thou) = 10.00 mil

10 mil

0255075100 mil

Mils (thou)

10.0000 mil

1 mil = 0.001 in = 25.4 µm

Inches

0.0100 in

1 in = 1000 mil

Millimeters

0.2540 mm

1 mm = 39.3701 mil

Micrometers (µm)

254.0005 µm

1 um = 0.03937 mil

Centimeters

0.0254 cm

1 cm = 393.701 mil

Meters

0.0003 m

1 m = 39370.079 mil

Feet

0.0008 ft

1 ft = 12000 mil

Common Thicknesses in Mils

Material	mil	µm	mm	in
Plastic wrap	0.47	11.9	0.0119	0.0005
Aluminum foil	0.63	16	0.016	0.0006
Kapton tape	0.98	24.9	0.0249	0.001
1-oz copper foil	1.38	35.1	0.0351	0.0014
Conformal coating	2	50.8	0.0508	0.002
Automotive paint	4.7	119.4	0.1194	0.0047
Powder coat	3	76.2	0.0762	0.003
Standard PCB FR4	62	1574.8	1.5748	0.062
Business card	14	355.6	0.3556	0.014
Sheet of paper	4	101.6	0.1016	0.004
Credit card	30	762	0.762	0.03
US quarter (thickness)	69	1752.6	1.7526	0.069

Planning notes, formulas, and examples

About the Mil (Thousandths of an Inch) Conversion Calculator

A mil — also known as a "thou" — is one thousandth of an inch (0.001 in, or 25.4 µm). It is the standard unit for measuring thin dimensions in American manufacturing: PCB board and copper thicknesses, coating and paint layers, wire diameters, plastic films, and machining tolerances across many production workflows and quality-control checks in industrial settings.

This mil conversion calculator converts between mils and six other length units in one step: inches, millimeters, micrometers (µm), centimeters, meters, and feet. Enter a value in any unit and review all the others together. Preset buttons load common industrial values like 62 mil (standard PCB thickness), and a 12-row reference table lists everyday materials and their mil thicknesses.

Whether you are a PCB designer, coating engineer, machinist, or packaging specialist, the page keeps the same thickness expressed in the unit systems used across design, purchasing, and production — without separate tables or repeated 25.4 arithmetic.

When This Page Helps

US manufacturing specs, PCB design tools, and coating standards all speak in mils, while metric-country suppliers use µm and mm. Converting between them requires dividing or multiplying by 25.4 — easy to miscalculate under pressure. Keeping seven related units on the same screen makes it easier to sanity-check a thickness against known material references before it reaches production or procurement.

How to Use the Inputs

Enter a numeric value in the "Value" field.
Select the source unit (mil, inches, mm, µm, cm, m, or ft) from the dropdown.
Adjust decimal precision for your tolerance requirements.
Click a preset for common values like 62 mil (PCB), 1 mil, or 100 µm.
Read all seven unit equivalents in the output cards.
Check the thickness reference table to see what real-world material matches your value.

Formula used

All conversions route through mils: mil = value × (from-unit factor). Key: 1 mil = 0.001 in = 0.0254 mm = 25.4 µm. 1 in = 1,000 mil; 1 mm ≈ 39.37 mil.

Example Calculation

Result: 0.062 in / 1.5748 mm / 1574.8 µm

62 mil is the standard thickness of a 4-layer FR4 PCB. That equals 0.062 inches, 1.575 mm, or 1,574.8 µm.

Tips & Best Practices

Quick conversion: multiply mil by 25 for a µm estimate (exact: 25.4).
PCB designers: 1 oz copper = 1.38 mil, 2 oz = 2.76 mil.
Paint-gauge readings in mils can be compared to this table to detect repaints or excess coating.
Wire gauge (AWG) can be cross-referenced in mils: 30 AWG ≈ 10 mil diameter.
The smallest standard drill bit in PCB work is typically 6 mil (150 µm).
When working with international suppliers, convert your mil spec to µm or mm to avoid confusion.

Mil in PCB Design

The mil is deeply embedded in PCB design culture. Trace widths (6, 8, 10 mil), via drill sizes (10–20 mil), pad diameters, and board thickness (62 mil) are all historically specified in mils. Modern design tools like Altium and KiCad support both mil and mm, but many legacy designs and manufacturer capabilities are still quoted in mils.

Mil in Coating & Painting

Industrial paint and powder-coat thickness is measured in mils using dry-film thickness (DFT) gauges. Automotive OEM paint is typically 4.5–5.5 mil total (primer + base + clear). A repainted panel may show 8–12 mil, flagging collision repair. Converting between mil and µm readings is essential when working with European-calibrated gauges.

Historical Context

The word "thou" comes from "thousandth," referring to one thousandth of an inch. US machinists have used this unit since the 19th century. Although SI (metric) is the global standard, mils remain entrenched in US precision manufacturing, especially where legacy tooling and documentation exist.

Sources & Methodology

Last updated: March 8, 2026

Frequently Asked Questions

A mil is one thousandth of an inch (0.001 inch). It is also called a "thou" (from "thousandth"). One mil equals 25.4 micrometers or 0.0254 millimeters.