I am working on a DIY electronics project with the instructions posted online and they call for screws and nuts in imperial (US) format. Because I live in Europe, I need the screw and nut sizes in metric format. I have absolutely no idea about anything regarding this and I am turning to you guys for help. I tried doing some conversions myself but I am unsure about it.

Here are the screws and nuts I need in imperial (US) sizes:

  1. 4-40 thread, 3/8" long (https://www.mcmaster.com/90272A108/)
  2. 4-40 thread, 1/2" long (https://www.mcmaster.com/90272A110/)
  3. 4-40 thread, 5/8" long (https://www.mcmaster.com/90272A112/)
  4. 18-8 Stainless Steel, Number 2 size, 1/4" long (https://www.mcmaster.com/92470A095/)
  5. 4-40 thread size hex nut (https://www.mcmaster.com/90480A005/)

What are the equivalent screw and nut sizes in metric format? I figured they are M3 and this is as close as I've gotten with my own conversions but I'm unsure:

  1. M3x10 Phillips
  2. M3x12 Phillips
  3. M3x16 Phillips
  4. ???
  5. M3 hex nut

I would be very grateful for your answers and suggestions, especially with the thread forming screw (number 4 on the list). Also, any good ideas where I could buy those online in Europe would be greatly appreciated.

  • Many sites online will sell you what you ask for in imperial.
    – Solar Mike
    Commented Nov 24, 2021 at 12:03
  • @SolarMike do you have any examples of such sites?
    – brozo
    Commented Nov 24, 2021 at 12:10
  • 1
    @brozo You can definitely use a normal threaded fastener in PLA if you tap the thread with a proper tap and use 100% infill in the area that's going to take the screw, some people say they just make the hole an appropriate size and put a screw in; make a practice piece and try it. Ebay is fine for things like M3 screws and nuts, you could get a pack with a variety of thread lengths. You might be interested in heat-set inserts. Commented Nov 24, 2021 at 12:55
  • 2
    Generally you're going to find what you found - close but not exactly the same size, and in most cases that will be fine, and using local fasteners is likely to be less annoying overall. If you are printing parts with nut recesses, you might need to alter the print, or use heat and/or epoxy to set the nuts firmly. Presumably there are sheet metal screws in the metric parts of the world...
    – Ecnerwal
    Commented Nov 24, 2021 at 13:49
  • 1
    I have the same problem so I bit the bullet so to say and purchased several assortments of metric screws, different sizes and heads. I have good luck with Aliexpress and they are not expensive. They just take a while to arrive.
    – Gil
    Commented Nov 24, 2021 at 15:55

1 Answer 1


#2 machine screw has been internationalized as an m2.2-0.45, as a sheetmetal screw as you pictured it would be a m2.2x6.5.

18-8 grade fastener has 18% chromium and 8% nickel content. Always baffled me why they would use a description that resembles a unrelated standard.

  • 1
    This is really helpful! That screw size was the only one I was really stuck on. I think I might go with the M3 sizes for the rest.
    – brozo
    Commented Nov 24, 2021 at 16:44
  • 1
    If you are going to be mounting parts that come with imperial threads you may want to consider a kit of mixed 4-40's amazon.co.uk/Guard4U-280Pcs-Stainless-Button-Assortment/dp/… Commented Nov 24, 2021 at 18:10
  • Thanks for the suggestions! I will actually try with a screw assortment pack and see how it goes. You can never have too many screws lying around, am I right? :D
    – brozo
    Commented Nov 24, 2021 at 19:54
  • 3
    Spoken like someone who has not wasted hours trying to find the right screw in a selection of random screws lying around. Organized? Sure. "Assortment lying around" - The horror...
    – Ecnerwal
    Commented Nov 25, 2021 at 0:20
  • I have a drawer of random screws which I've had for 20 years. I don't think I've ever found one in there which would fit what I needed... why I'm hanging onto them I have no idea!
    – LarryBud
    Commented Dec 24, 2021 at 1:58

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