Screw Size and Pilot Hole Chart
A pilot hole is a narrow hole drilled into the wood the screw threads bite into, and a clearance hole is a wider hole the unthreaded shank passes through. For a #8 wood screw (0.164 in, 4.17 mm), Gardner Screw Corporation’s chart gives a 0.116 in pilot hole in hard wood, 0.076 in in soft wood, and a clearance hole of about 11/64". Pick any gauge from #2 to #18 below.
Pilot hole finder
Pick the screw gauge printed on the box and the wood you are screwing into. The sizes come from the maker’s chart below, and the choice is saved in the page address. Nothing leaves your browser.
How to read the chart
Each gauge has three hole sizes. Drill the pilot hole into the piece that holds the threads. If you are pulling two boards together, drill the clearance hole through the top board so the screw can slide through and clamp the joint.
millimetres = inches × 25.4 nearest fraction = round (inches × 64) ÷ 64, reduced pilot share = pilot hole ÷ screw diameter
- 25.4
- millimetres in an inch, exact (source 3)
- pilot share
- a check on the chart, not a rule: for #8 it is 71% in hard wood and 46% in soft wood
Worked example
Given: a #8 wood screw; joining two oak boards (hard wood).
- Screw diameter:
0.164 in × 25.4 = 4.17 mm - Pilot hole into the second board:
0.116 in × 25.4 = 2.95 mm - Clearance hole through the first board:
11/64" -, drill No. 18
Answer: Drill 0.116 in (about 3 mm; chart drill No. 32) into the second board and 11/64" - through the first. In pine the pilot drops to 0.076 in (about 1.93 mm).
Full wood screw drilling chart
| Gauge | Screw diameter | Pilot, hard wood | Pilot, soft wood | Clearance hole (shank) |
|---|---|---|---|---|
| #2 | 0.086 in · 2.18 mm | 0.052 in · 1.32 mm · 3/64" + · No. 55 | 0.035 in · 0.89 mm · 1/32" + · No. 65 | 3/32" · No. 42 |
| #3 | 0.099 in · 2.51 mm | 0.063 in · 1.60 mm · 1/16" + · No. 52 | 0.042 in · 1.07 mm · 3/64" - · No. 58 | 7/64" - · No. 37 |
| #4 | 0.112 in · 2.84 mm | 0.070 in · 1.78 mm · 5/64" - · No. 50 | 0.050 in · 1.27 mm · 3/64" + · No. 55 | 7/64" + · No. 32 |
| #5 | 0.125 in · 3.17 mm | 0.082 in · 2.08 mm · 5/64" + · No. 45 | 0.057 in · 1.45 mm · 1/16" - · No. 53 | 1/8" + · No. 30 |
| #6 | 0.138 in · 3.51 mm | 0.093 in · 2.36 mm · 3/32" · No. 42 | 0.062 in · 1.57 mm · 1/16" · No. 52 | 9/64" + · No. 27 |
| #7 | 0.151 in · 3.84 mm | 0.106 in · 2.69 mm · 7/64" - · No. 36 | 0.066 in · 1.68 mm · 1/16" + · No. 51 | 5/32" · No. 22 |
| #8 | 0.164 in · 4.17 mm | 0.116 in · 2.95 mm · 7/64" + · No. 32 | 0.076 in · 1.93 mm · 5/64" - · No. 48 | 11/64" - · No. 18 |
| #9 | 0.177 in · 4.50 mm | 0.128 in · 3.25 mm · 1/8" + · No. 30 | 0.082 in · 2.08 mm · 5/64" + · No. 45 | 3/16" - · No. 14 |
| #10 | 0.190 in · 4.83 mm | 0.140 in · 3.56 mm · 9/64" · No. 28 | 0.088 in · 2.24 mm · 3/32" - · No. 43 | 3/16" + · No. 10 |
| #12 | 0.216 in · 5.49 mm | 0.152 in · 3.86 mm · 5/32" - · No. 24 | 0.102 in · 2.59 mm · 7/64" - · No. 38 | 7/32" + · No. 2 |
| #14 | 0.242 in · 6.15 mm | 0.166 in · 4.22 mm · 11/64" - · No. 19 | 0.116 in · 2.95 mm · 7/64" + · No. 32 | 1/4" - · No. D |
| #16 | 0.268 in · 6.81 mm | 0.189 in · 4.80 mm · 3/16" + · No. 12 | 0.132 in · 3.35 mm · 1/8" + · No. 29 | 17/64" + · No. I |
| #18 | 0.294 in · 7.47 mm | 0.209 in · 5.31 mm · 13/64" + · No. 4 | 0.145 in · 3.68 mm · 9/64" + · No. 26 | 5/16" - · No. N |
Source 1. Each pilot cell shows the decimal size, millimetres, nearest fraction and drill number as the maker printed them (a + or − after a fraction is the maker’s mark, so rely on the decimal size). Millimetres are our conversion (source 3).
Assumptions and limits
- The chart is Gardner Screw Corporation’s recommendation for its Hi-Torque wood screws, for average conditions. Other makers’ screws, such as thread-cutting structural screws, may have their own instructions.
- Wood varies by species, moisture and grain. Test on a scrap of the same wood first, and go up a drill size if the wood is very hard.
- The diameters are the chart’s decimal screw sizes; the screw’s actual thread diameter may differ slightly by maker.
- Millimetre figures are conversions from inches, not metric screw sizes (such as M4 or 5 mm screws).
- Source data as of 2026-11-13.
Frequently asked questions
What size pilot hole do I need for a #8 screw?
Gardner Screw Corporation’s recommended pilot hole for a #8 wood screw is 0.116 in (2.95 mm, drill No. 32) in hard wood and 0.076 in (1.93 mm, drill No. 48) in soft wood. The shank clearance hole is 11/64" - (drill No. 18).
What is the difference between a pilot hole and a clearance hole?
The pilot hole goes into the piece the screw threads grip, and is smaller than the screw’s outer diameter so the threads can cut into the wood. The clearance hole goes through the piece you are pulling tight, and is about as wide as the screw’s shank so the screw slides through and draws the pieces together. Gardner’s chart lists both.
Do I always need to drill a pilot hole?
No. Gardner says every recommendation is for average conditions and a craftsman may need to allow for the wood. Some modern screws are designed to be driven without pre-drilling: GRK says its RSS structural screw does not require predrilling, but suggests it in very hard, dense wood such as mahogany if the screw will not fully drive, or if torque is splitting the wood or stripping the recess. Always check the screw’s own instructions.
What happens if the pilot hole is too big or too small?
Gardner’s catalog says a hole that is too large gives too little gripping surface, and an undersized pilot hole weakens the fastening because the wood fibers are crushed and packed into the thread root, which sets up stresses that tend to split or crack the wood.
Why is the pilot hole bigger for hardwood?
Dense wood resists the threads, so the chart calls for a larger hole there. For a #8 screw the hard wood pilot is 71% of the screw diameter and the soft wood pilot is 46% (0.116 and 0.076 in against 0.164 in). Test on a scrap of the same wood first.
How do I convert these sizes to millimetres?
Multiply inches by 25.4. The calculator shows millimetres for you. If you only have metric bits, choose the closest size to the pilot hole figure and test on scrap.
What does the number after the hash mean on a screw?
It is the screw gauge, a number for the thread diameter. Gauge #8 is 0.164 in (4.17 mm) and #10 is 0.190 in. Each step up adds about 0.013 in to the diameter on this chart.
What are the drill numbers in the chart?
They are number and letter wire-gauge drill bit sizes, which are common in sets sold for machinists. A fractional bit near the decimal size also works. Each row gives the decimal hole size, the nearest fraction and the drill number exactly as the maker printed them.
Sources
- Gardner Screw Corporation, Hi-Torque Wood Screws: Recommended Drill and Bit Sizes (catalog, 01B-09/07) (s3.amazonaws.com) Accessed 2026-11-13.
- GRK Fasteners, GRK Installation Technical Bulletin (predrilling in hard wood) (grkfasteners.ca) Accessed 2026-11-13.
- NIST SP 811, Appendix B.8: inch = 25.4 mm (exact) (nist.gov) Accessed 2026-11-13.