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Total noob question here:

This can apply in general, but for a specific case, suppose I have a 5/16 bolt for which I want to drill a hole. What's the best drill bit to use to make the hole big enough, but not so big that the threads on the bolt don't catch the wood?

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3 Answers

up vote 16 down vote accepted

Here are some charts from Bolt Depot. Bolt Depot is a handy resource, and has lots of information about all different types of fasteners.

Pilot hole size:

Pilot hole chart

Wood Screw Diameter:

enter image description here

*Major thread diameter is measured on the outside of the threads.

enter image description here

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+1: This is a great resource. – Doresoom Sep 8 '11 at 18:55
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nice chart, I gotta bookmark that site. +++ – shirlock homes Sep 8 '11 at 22:06
Be useful to have these measurements in modern metric sizes – Graham May 4 at 16:16
up vote 9 down vote

A real simple technique I use is to hold up the drill bit and the screw. Hold the shaft of the drill bit up in front of the screw. You should only be able to see the threads of the screw (and maybe a bit of the screw). If you can't see the threads the drill bit is too big. If you can see too much of the screw, the bit is too small.
I hope this helps!

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That's how I learned to do it as well. I've always been told to use a drill bit the same diameter as the screw's shaft. – DA01 Sep 9 '11 at 1:40
I like this advice, but it depends on the kind of wood that you are fastening to. If you are screwing into a dense wood, you want to just have the threads cutting into the wood. If the wood is less dense, you will want a slightly smaller hole. – Eric Gunnerson May 5 at 2:54
up vote 1 down vote

It depends on if you want it to be loose or not. If you want it to be very loose (in the case of a bolt where you're going to have washers on both sides and a nut on the far side threads, then feel free to upsize by a fraction -- in this case, 6/16 or 3/8. If you're using a screw where you want the threads to be firmly bedded, use the smallest drill bit you have in a soft material like wood -- and in harder substances like metal or concrete, a drill bit that is one fraction lower (e.g. 4/16" or 1/4" in this case).

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What if it's the former case (a bolt with washers and a nut) but I don't want it very loose. Should I go with 5/16? – ageektrapped Sep 8 '11 at 17:47
Yeah, that'd be appropriate. – Karl Katzke Sep 8 '11 at 18:24

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