WOOD SCREWS

			Shank Diameter		Root Diameter
Screw Number	Max. Head Diameter	Basic Decimal Size	Nearest Fractional Equivelant	Average Decimal Size	Nearest Fractional Equivelant	Threads Per Inch
0	.119	.060	1/16	.040	3/64	32.00
1	.146	.073	5/64	.046	3/64	28.00
2	.172	.086	3/32	.054	1/16	26.00
3	.199	.099	7/64	.065	1/16	24.00
4	.225	.112	7/64	.075	5/64	22.00
5	.252	.125	1/8	.085	5/64	20.00
6	.279	.138	9/64	.094	3/32	18.00
7	.305	.151	5/32	.102	7/64	16.00
8	.332	.164	5/32	.112	7/64	15.00
9	.358	.177	11/64	.122	1/8	14.00
10	.385	.190	3/16	.130	1/8	13.00
11	.411	.203	13/64	.139	9/64	12.00
12	.438	.216	7/32	.148	9/64	11.00
14	.491	.242	1/4	.165	5/32	10.00
16	.544	.268	17/64	.184	3/16	9.00
18	.597	.294	19/64	.204	13/64	8.00
20	.650	.320	5/16	.233	7/32	8.00
24	.756	.372	3/8	.260	1/4	7.00

The following table can be found all over the internet. I find it a little recommending drills that are too small. I have included alterniate tables I have found as well.

Pilot Hole Sizes For Wood Screws

Screw Gauge #	Shank Hole	Pilot Soft Wood	Pilot Hard Wood
0	1/16	1/64	1/32
1	5/64	1/32	1/32
2	3/32	1/32	3/64
3	7/64	3/64	1/16
4	7/64	3/64	1/16
5	1/8	1/16	5/64
6	9/64	1/16	5/64
7	5/32	1/16	3/32
8	11/64	5/64	3/32
9	3/16	5/64	7/64
10	3/16	3/32	7/64
11	13/64	3/32	1/8
12	7/32	7/64	1/8
14	1/4	7/64	9/64
16	17/64	9/64	5/32
18	19/64	9/64	3/16
20	21/64	11/64	13/64

Wood Screw Pilot Holes from screwChart.pdf

Screw Size	Hardwood	Softwood
2	1/16	1/16
4	5/64	1/16
6	3/32	5/64
8	7/64	3/32
10	1/8	7/64
12	9/64	1/8
14	5/32	9/64

Well, here is yet another table I found that gives even larger sizes for the pilot holes:

Screw Size	Hard Wood	Soft Wood	Countersink Size
2	1/16	1/16	1/4
3	5/64	1/16	1/4
4	5/64	1/16	1/4
5	3/32	5/64	5/16
6	7/64	3/32	5/16
7	7/64	3/32	5/16
8	1/8	7/64	3/8
9	9/64	1/8	3/8
10	9/64	1/8	7/16
12	5/32	9/64	7/16
14	11/64	5/32	1/2
16	3/16	11/64	9/16
18	7/32	13/64	5/8
20	15/64	7/32	3/4
24	17/64	1/4	3/4

Here is one that goes the other way. I think it is the same as the one above.

Screw Size #

Pilot Hole For Thread

Hardwood

1/16

5/64

3/32

7/64

1/8

9/64

5/32

11/64

3/16

7/32

Softwood

1/16

5/64

3/32

7/64

1/8

9/64

5/32

11/64

13/64

Pilot Hole For Shank

3/32

7/64

1/8

9/64

5/32

11/64

3/16

13/64

7/32

1/4

9/32

5/16

Countersink

C10

C12

C14

C16

C18

Given all this information, which should you pick? Well, looking at #14 in soft wood we see from the tables that a pilot hold should be 7/64, 9/64, or 5/32. Which is correct? My dad taught me that in soft wood the pilot hole should be slightly less than the root diameter, which is 5/32. He also said that in hardwood, it should be about mid way between the root and body diameters. This is way larger than the chart and may explain why I could barily drive some screws into teak using the recommended values. Anyway, the last table gives the largest pilot hole you would want to use and IMHO. The first table, the most widely circulated one, gives one way smaller. I used the 9/64 bit and it was tight but OK. By the way, to make things more complicated the actual screw root measurement was larger than given in the charts. Turns out there is an official standards body, the American Wood Council that issues recommendations: 90% of the root diameter for hardwood, 70% for softwood. These correspond to the tight numbers from my first chart, go figure. What am I going to do given all this? I will just measure the root and pick a drill slightly smaller for softwood and slightly larger for hardwood. If you don't have a caliper, hold the screw and drill up to the light like you did before you found these tables.

	Flat Head Machine Screws Wood Screws * Tapping Screws
SLOTTED		PHILLIPS

	Oval Head Machine Screws Tapping Screws Wood Screws*
SLOTTED		PHILLIPS

	Round Head Machine Screws Wood Screws*
SLOTTED		PHILLIPS