Why are the mounting hole footprints all metric?

Hi, it looks to me like the footprint library only has metric mounting holes, with diameters in millimeters and metric sizes like M3, M4, M5, and so on. The CAD programs I’m familiar with have both U.S. and metric sizes for things like machine screws.

I could make my own footprint for a #8 machine screw hole (say), but I’m new to PCB design and I’m wondering if I’m going against a common practice. Is there a convention of sticking to metric sized fasteners in electronics? Or perhaps I missed something?

I’ll venture a guess here. KiCad library content is build by user contributions, where the contributions are checked by librarians. So either no users from US contributed imperial mounting holes, or they did but the footprint did not make it into the library as the standards (KLC) for acceptance are somewhat strict and contributors did not have the motivation to make an effort.

And what is the diameter for #8 machine screw?
For metric ones diameter is in marking, no special tables needed. (locating #8 diameter)
Even inch is based on metric measurement 1"=25,4mm :slight_smile:

It would probably be this standard: https://en.wikipedia.org/wiki/Unified_Thread_Standard.

If you make your own footprints, who cares? You decide what parts you use and can make corresponding footprints.

I asked because I was wondering if the KiCAD library’s designers (or someone else) might know something I don’t. (Since this is my first board, it seems rather likely.)

If you are making your own mounting holes, you don’t want to make a hole that is the exact major diameter of the fastener you plan on using. If you do then your fastener will bind on the sides of the hole. For reference, see the clearance hole specifications in the following Imperial tap and clearance table:
That page has a link to a pdf with both Imperial and Metric tap and clearance charts.

I find that when I’m making a mounting hole footprint for my designs I also want to take into account not only the drill for the fasteners, but also a clearance area around the hole so washers, screw heads, standoffs, etc aren’t pinching down on any copper (over time with vibration and heat/cool cycles the fastener may wear through the solder mask and short out to copper on the board). I also take into account the most likely tool, and especially for hex-head screws/standoffs I also leave an additional component clearance around the mounting hole so I don’t hit any components when attaching the fasteners.


Yes, good explanation. That’s what I did by hand.

In OnShape (the CAD program I’m using), when creating a hole and setting the diameter, you can pick “ANSI”, “Clearance”, and a size from a menu. (It basically has something like that table built-in.)

Using KiCAD I’m picking a hole size somewhere between normal and loose fit (converted to mm) and hoping for the best. The available sizes are so close together that it seems like it should work.

Sounds like a candidate for a new footprint generator script.


1 Like

I think the why has been covered (noone has submitted or none have been accepted) but that shouldn’t stop you from making one & equally creating an Imperial_mount library

Out of all the footprint’s this should be easy to create

  1. launch KiCAD
  2. launch footprint editor
  3. file -> new library & name it something sensible
  4. browse to the mounting_hole library and find the nearest metric hole
  5. right click -> save as -> choose your library and footprint name
  6. browse to your library & load the saved hole (double click)
  7. double click on the gold hole outline
  8. change holesize & annular ring & save

OR… go with metric with the nearest larger holesize. Most of my official work has to use 4-40 but I still use M3 to test pillars

This topic was automatically closed 90 days after the last reply. New replies are no longer allowed.