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PCB design in Mils (Inch) or MM's?

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Newbie level 6
Dec 3, 2002
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Hi All,

I was wondering if you guys/girls :lol: make your boards in milimeters or in inches or maybe a combination of both worlds.
And what are your experiences and maybe do you have tips?


Most use mils since for conventional DIP IC's thats the way to go. But Ifyou are using the newer 2mm pitch parts, then use mm

Being a nationalist, and as the common measure standard in my country is based on the meter, I of course converted every inch-based value to millimeters and used it in my CAD-tool, which was Cadstar at that time. If I only knew better at that time...

That particular tool (as well as several others, I am sure!), internally converts the values given i millimeters to inch-based values (mils), and since this inevitably gives round-off errors I had a tremendous work in getting everything to look as I wanted.

My advise is that, even if you think and walk i millimeters, to use inch-based values (mils) whenever you are going to do any work in an EDA-CAD program, be it schematics or layouts (especially layouts!).

Also, circuit-board makers more likely than not have their drills in mils, not in millimeters. So even then, when you are sending them the drill-file for the circuit-board, they are rounding off the drill sizes to mils, and which can result in too small clearance sizes for the pads and numerous other unforeseen errors.


I always use mils. But I'm in North America and the mm never really arrived here. All vendors I deal with for PCB work use mils.


Here in my company ( center/south Europe ) we use always mils and this is appreciated ( sometimes forced ) from the PCB constructors.
We tried for some special applications to use mm, but we have always found problems with the companies selling PCB.

BTW I know all the companies in my area use mils.


The units of measure must be set up so that they are the same as that required for manufacturing the board (photoplotter, drill equip., etc.)


Most use Mils - which is 1/1000 of an inch.
You will find it very easy once you start working with mils. The reason for that is because most PCB compnents library have there component in mils and also the datasheets are in mils as a more convinient way.
I would suggest you although you have use it to start using mils and not mm - milimeters. The reason for that it is because it will give you good hubits from the beginning. And also it depends I think if you want to be in this buisness for long time or it is a short project never the less if it is for short or for long you may never know when you need it so it is better for you to use mils.


mil is more natural since most components' footprint are defined in mils.

1000 mils = 1 inch.
25.41 milimeters (mm.) = 1 inch.
39.354584 mils = 1 mm.
draftly 40 mils = 1 mm.

I never used mils until started to do pcbs :) but is worth using them, i find them more comfortable to work with, and now i even find it easier. But there is the problem when talking with people that works in mm, have to convert everything in the head, but still is good using mils (unless footprints are changed to mm) :D

I have been using mils since I started to layout pcb's. I come from a metric country. I used both systems, now I moved to USA and I switched to metric and I'm helping my mechanical guys to do that. All the stuff is made over seas now and they use metric. If you need to start your own library I will suggest you to do it in metric units. In a couple of years everything will be metric, even in the USA. Look at IPC, evrything is metric in their books. I understand that the change in the USA is going to be really hard.


It should not matter what units you use, the cad systems take into account the conversion and you should be able to work in both or switch from one to the other to input library shapes.
The front end tools that are used by the board fabs will do likewise.

Phil 8)

All Industry is migrating to mm

See: hxxp://

The selection of the units is also a kind of selection what accuracy you need.
selecting MM,MILS,UM (micrometer) and the appropriate amount of decimals will give you either a smooth working or all the time a lot of rounding errors which can impact your work a lot.

for instance working in Allegro with MILS and 4 decimals will all the time you do an autovoid of shapes give you nice results, Allegro is converting every X/Y location into an internal database value (independent of MIL,MM,UM) wich cause very often round off errors...


Dear friends

I have seen do project for IBM, Applied Material, American Predator, HP,... all of company use mils in their design, but in Europe use mm, Japan use Micron and mm. everything depend on your customer, but when U use the Unit, U need take care the the tolerance with Unit U use.

Best Regards

my national standard saya mm but all the pcb manufactures insist on mils

Depend on the footprint pitch.If the picth use mm I will use mm or I use mil.

I use MM for placement of critical items, such as board outline, mounting holes etc so as not to get wrong values as the mechanical drawings are all in mm now.

I also use MM when creating a footprint (unless the spec is only in thou).

Then I use thou when placing and routing.

As for the industry moving over to MM, oh yeah, like I'll believe that when it actually happens. Just because industry stds want us to does not mean that we will.


I use mil,i think use which unit is no impadent!

mils is so standart because of all components sizes and pins are related mils.

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