I 'm very new in PCB designer CAD. I have a simple PCB design in Altium which I need to create gerber and drill files before sending it to a milling machine. When I go to:

Files-->Fabrication Outputs-->Gerber Files

Here I need to select one of the formats from 2:3, 2:4, 2:5 as below:

enter image description here

But I cannot decide which format to chose. How can I figure out what my resolution is and how can I associate it with these formats?

Edit: I found only this info:

If you are using one of the higher resolutions, check that the PCB manufacturer supports that format. The 2:4 and 2:5 formats only need to be chosen if there are holes on a grid finer than 1 mil.

What does it mean "finer than 1 mil" ? I use mm. I have holes on the PCB but when I double click on them it shows hole size and location in mm not the resolution. How can I figure out the resolution?


2 Answers 2


1 mil is one thousands of an inch or 25.4mm/1000 = 0.0254mm.

I usually generate my Gerbers in mm and always use the format with the greatest precision. Never had any issues.

  • \$\begingroup\$ I also don't have GKO and GTP files when I follow tutorial and generate gerber files. \$\endgroup\$
    – user1245
    Jan 10, 2018 at 10:46
  • \$\begingroup\$ @user134429 Have you selected them in the layers tab? \$\endgroup\$
    – Manu3l0us
    Jan 10, 2018 at 11:02
  • \$\begingroup\$ I didnt add any other to default settings. So I have no keepout layer and the mechanical file shows nothing just an empty sheet \$\endgroup\$
    – user1245
    Jan 10, 2018 at 11:32
  • \$\begingroup\$ would you mind if I send you gerber files in rar? \$\endgroup\$
    – user1245
    Jan 10, 2018 at 11:37
  • \$\begingroup\$ @user134429 I have no access to a machine with the tools on it at the moment... \$\endgroup\$
    – Manu3l0us
    Jan 10, 2018 at 11:55

I always use the format with the highest precision, regardless of whether I use inches or millimeters. I have never had a supplier complain. The resolution is simply how precise you want the positioning of your design to be, and it is an attribute of the entire PCB design, not of any holes or vias. In the image shown in your question, 2:5 stands for 2 digits before the decimal and 5 digits after the decimal, or XX.XXXXX inches. 2:3 is lower resolution because it only goes out to three decimal places: XX.XXX inches. The same concept can be applied to millimeters.

When you go to export your Gerbers make sure to click the "Layers" tab after selecting your units and resolution (shown in your initial post). In the "Layers" tab you need to select all of the layers you want to export to the Gerbers. The most important layers are as follows:

GTO - Top Overlay (silkscreen)

GTP - Top Paste

GTS - Top Solder(mask)

GTL - Top Layer

GBL - Bottom Layer

GBS - Bottom Solder(mask)

GBP - Bottom Paste

GBO - Bottom Overlay (silkscreen)

You may want to add other layers if they exist in the design, such as:

GKO - Keep-Out Layer

G1 - Inner Layer 1

G2 - Inner Layer 2

GP1 - Inner Plane 1

GP2 - Inner Plane 2

Also make sure in the right-hand panel to select your board outline. You want this outline to be shown on all of the Gerbers. I generally create my own layer called "BOARD_OUTLINE". Read the Altium documentation to learn how to use mechanical layers. Once I create this layer I draw the board outline in it and make sure to add it to all plots. The below image illustrates this.

enter image description here

  • \$\begingroup\$ Do you know how can I make a single layer board in Altium. It only has min two layer board. I also need to figure out how can I auto route only in bottom layer. \$\endgroup\$
    – user1245
    Jan 10, 2018 at 13:07
  • 1
    \$\begingroup\$ 1) Make a "2-layer" board but don't put any copper on the top layer. This is the equivalent of a single-layer board. 2) DO NOT USE THE AUTOROUTER. The autorouter makes a mess of things unless you take hours and hours to set it up just right. You'd be much better off routing by hand, it will be well worth the time and effort. I have been designing PCBs for a long time and I have NEVER found an autorouter to be worth the trouble. \$\endgroup\$
    – DerStrom8
    Jan 10, 2018 at 13:24

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