Convert between common PCB units: mil, mm, inch, μm and copper weight.
A unit conversion is a mathematical translation between different measurement systems, and copper thickness conversion refers to the extra PCB-specific step of translating foil weight conventions into physical thickness values used for design review. Those definitions matter because the same number can imply a different manufacturing outcome if the unit label is dropped.
Engineers usually move between these units when reconciling a fabricator quote, a design rule sheet, a stackup proposal, and a component datasheet. The goal is not just numerical equivalence. It is also to maintain the same engineering intent across documents created for different audiences.
| From | To | Factor |
|---|---|---|
| 1 mil | mm | 0.0254 |
| 1 mm | mil | 39.3701 |
| 1 inch | mm | 25.4 |
| 1 mm | μm | 1000 |
| Weight (oz/ft²) | Thickness (μm) | Thickness (mil) |
|---|---|---|
| 0.5 oz | 17.5 μm | 0.69 mil |
| 1 oz | 35 μm | 1.37 mil |
| 2 oz | 70 μm | 2.76 mil |
| 3 oz | 105 μm | 4.13 mil |
A mil (also called thou) is 1/1000 of an inch, equal to 0.0254 mm. It's commonly used in PCB design for trace widths, spacing, and drill sizes.
1oz copper refers to the weight of copper per square foot, which equals approximately 35 μm (1.37 mil) thickness. This is the standard way to specify copper weight on PCBs.
PCB design uses both imperial (mil, inch) and metric (mm, μm) units. American datasheets typically use mil, while Asian manufacturers often use mm.
Micrometers are useful when comparing copper foil thickness, plating expectations, or fabrication capabilities where the process data is more naturally expressed in metric thickness values.
Rounded conversions are fine for discussion and planning, but fabrication release data should preserve the precision needed by your drawing, stackup, or electrical requirement.