# When pre-drilling for screws, how do you determine the correct bit diameter?

Suppose I want to pre-drill for, say, a 5/16 screw. How do I determine the correct drill bit diameter?

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:

Wood Screw Diameter:

• Be useful to have these measurements in modern metric sizes
– user12845
May 4, 2013 at 16:16

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!

• 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, 2011 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. May 5, 2013 at 2:54
• @EricGunnerson - basically what I was taught. Hard materials need the thread to bite in to full root depth, soft materials need compression to add a little density to allow the thread to have something to bite into with the objective that you don't split the material. Mar 10, 2014 at 14:50
• If you have a drill sizing template, you can use that to get the diameter of anything. Otherwise, your drill index is also an excellent gauge. Sep 1, 2014 at 22:08
• I screw the screw into the drill index hole of the "appropriate" bit to make sure that it holds in that hole; as a final check, after sizing them up visually. Feb 12, 2015 at 15:05

Here is my chart drawn up from over 30yrs onsite experience... (sorry its in millimetres for those stateside!)

More detail here screws explained chart. (NB based on UK/EU experience)

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).

• 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? Sep 8, 2011 at 17:47
• Yeah, that'd be appropriate. Sep 8, 2011 at 18:24

The best charts I have found for both tapered and straight wood screws is at http://www.wlfuller.com/html/wood_screw_chart.html - the fuller company has been around forever and it shows with the customer service and the accuracy of the advice.