The rules are holes can be no more than 40% the width of a stud on a load bearing wall and no more than 60% on non-load bearing.

This works out to roughly the following:

Load Bearing:
1.4" on a 2x4
2.2" on a 2x6

Non-load Bearing:
2.1" on a 2x4
3.25" on a 2x6

There's also code for how close a hole can be to the edge of a stud and when to use a nail plate... I don't remember what these are, but just use common sense and have never had a problem.

If you're running through ceiling or floor joists, be sure you know the rules for this.  I've never seen an inspector give too much greif over holes drilled in wall studs, but I've been on several jobs where the inspector had problems with how electricians and plumbers drilled through joists. 

Don't forget, that's outer diameter.

I'm working on a beach house in Morro Bay where the Central Vac plumbing had to be engineered and signed off by the inspector before a single hole was drilled.
Major PITA.
This is not the first time I've discovered the rules change a little when your back yard is the Pacific Ocean.

This is what I had remembered as well, but my math came out just a bit different. I figured a 2x4 is 3-3/4" wide so at 40% that would be 1-1/2" MAX hole.

We did a job in SF where there was one small section on EVERY floor, down the hallway that EVERYBODY had to run through to get from the front to the back of the house on every floor (4 floors)...It was ridiculouse, Central vac, plumbing, electricial, sprinklers and a big bundle of lv cables. The joists were probably 6' wide.

 I figured a 2x4 is 3-3/4" wide so at 40% that would be 1-1/2" MAX hole.


2x4 is 3 1/2" wide

that explains it. its 1-3/4" the other way right? So when you run 1-1/4" conduit, what size hold saw/auger bit are you using? 1-3/8's?