Millimeters to Thou

How Many Thou in a Millimeter?

One millimeter equals approximately 39.37 thou (mils). A thou, also called a mil, is one-thousandth of an inch (0.001 inches = 0.0254mm). To convert millimeters to thou, divide the mm value by 0.0254, or equivalently multiply by 39.3701. In 3D printing, thou are sometimes used in precision manufacturing contexts, especially when discussing tolerances with US-based engineers or when specifying clearances for parts that interface with machined components. While most of the 3D printing community uses millimeters or microns, understanding thou helps when reading datasheets for American-made hardware, bearings, or electronic enclosures. It is also useful when translating printer accuracy claims into the language a machinist or toolmaker is more likely to use during fit and tolerance discussions. That makes collaboration with traditional shops much smoother. It also gives a better feel for inch-based tolerancing habits. Without that translation, printer accuracy claims can sound misleadingly precise in cross-team conversations and design reviews. That perspective is useful when an imperial drawing makes a printed tolerance sound tighter or looser than it really is.

How to Convert Millimeter to Thou (Mil)

1. Start with your measurement in millimeters.
2. Multiply the mm value by 39.3701 to get thou (mils).
3. Alternatively, divide by 0.0254.
4. Quick reference: 0.1mm = 3.94 thou, 0.2mm = 7.87 thou, 0.4mm = 15.75 thou, 1mm = 39.37 thou.
5. For practical estimation, 1 thou is very close to 25 microns (25.4µm exactly).

Real-World Examples

History of Millimeter and Thou (Mil)

The thou (plural: thou) derives from "thousandth of an inch" and has been a standard unit in American and British precision engineering since the Industrial Revolution. The word "mil" is an alternative name used primarily in the United States. Confusingly, "mil" is also used to mean millimeter in some European contexts, and "mils" can refer to milliradians in military use. In 3D printing, thou occasionally appear in discussions about dimensional accuracy when comparing 3D printed parts against traditionally manufactured ones. The connection between thou and microns (1 thou = 25.4µm) provides a bridge between imperial precision engineering and metric 3D printing specifications.

Common Mistakes to Avoid

• Confusing "mil" (thou, 0.001 inch = 0.0254mm) with "mm" (millimeter = 1mm). They sound similar but differ by a factor of about 39. Reading "20 mil" as "20mm" would make a dimension 39 times too large.
• Confusing "mil" (thou) with "millimeter." In plastics and packaging, film thickness is often given in mils (thou). A 10-mil plastic sheet is 0.254mm thick, not 10mm thick. In 3D printing, build plate adhesion sheets (like PEI) are often specified in mil thickness.
• Using thou for everyday 3D printing settings. While thou are valid units, entering them into a slicer requires conversion to mm first. Mixing unit systems increases the chance of errors.
• Rounding too aggressively when converting printer tolerances. A 0.15mm adjustment is 5.9 thou, not 5 thou. On press fits and shim clearances, that missing fraction of a thou can matter.

Frequently Asked Questions