Clearance hole or pilot hole
- Clearance hole: in the piece being fastened, sized so the screw's shank passes through. Fuller calls it the screw shank hole size.
- Pilot hole: in the piece the screw goes into, sized for the threaded part so the threads cut into wood without splitting it. Fuller calls it the pilot drill size.
- Countersink: a cone that seats a flat head flush. Counterbore: a flat-bottomed cylinder that sinks the head below the surface, to be filled with a plug. Fuller sizes both by gauge.
Reading Fuller's sizes
For a #8 screw, Fuller's chart gives an 11/64 in clearance hole, a straight pilot of 1/8 in in hardwood or 7/64 in in softwood, and — for its own taper point drills — 11/64 in in hardwood or 5/32 in in softwood. The tapered sizes are larger because they are measured at the wide end of a drill that narrows toward its point; they apply only to tapered drills. For an ordinary straight bit, use the straight columns.
The Wood Handbook's rule
The Wood Handbook gives no table of sizes. It states the lead holes its strength figures assume: for withdrawal, a lead hole about 70% of the thread's root diameter in softwoods and about 90% in hardwoods; for lateral loads, a lead hole for the shank the same as the shank or slightly smaller, and for the threads the same as the root in dense species or slightly smaller in low-density ones. Applying it means knowing the root diameter, which the Handbook says averages about two-thirds of the shank but does not list by gauge — so measure the screw you are using.
Lead hole for the threads
softwood pilot ≈ 0.70 × root diameter
hardwood pilot ≈ 0.90 × root diameter
Wood Handbook, Chapter 8, for withdrawal. Worked example: a screw whose root measures 0.110 in gives about 0.077 in in softwood — nearest bit 5/64 in (0.078) — and about 0.099 in in hardwood, between 3/32 in (0.094) and 7/64 in (0.109).
| Point | Wood Handbook | W. L. Fuller |
|---|---|---|
| What it gives | A rule, as a share of the root diameter | Drill sizes by gauge |
| Hole for the shank | Same as the shank or slightly smaller (lateral loads) | A shank hole at or a little over the shank at most gauges — 11/64 in (0.172) for a #8, whose shank is 0.164 in — but 7/64 in (0.109) for a #4, just under its 0.112 in shank |
| Wood classes | Softwoods and hardwoods; dense and low-density species | Hardwood and softwood columns |
The Handbook's shank rule is about joint strength under side load; Fuller's shank hole lets the shank pass freely. A dense softwood or a light hardwood falls between the columns: test in scrap.
Testing in scrap
Drill both holes in offcuts of the same wood
Clearance in one, pilot in the other, at the sizes chosen.
Drive the screw
Turn it in — the Handbook says never to start or drive a screw with a hammer.
Read the result
A split means the pilot is too small or too near an edge; a screw that turns without tightening means it is too large. Adjust one bit size and test again.
Screw diameters and heads by gauge are on the wood screw size chart, a bit's decimal size on the drill bit size chart, and countersinks and the other wood bits in the guide to wood drill bits. Self-drilling screws, the Handbook notes, are made to need no predrilling; for construction, cabinet or pocket-hole screws, follow the screw maker's instructions.
Before drilling for a screw
- Gauge known, and the screw a traditional wood screw
- Clearance size for the top piece, pilot size for the bottom
- Straight or tapered column chosen to match the bit
- Hardwood or softwood column chosen, or the size tested
- Countersink or counterbore if the head must sit flush or below
Sources
Hole sizes are W. L. Fuller's chart as printed in its catalog (PDF dated 2006), transcribed and checked row by row; decimals are computed. The Wood Handbook gives a rule, not sizes. Where the two differ, both are shown. Sizes are for traditional wood screws; tap drills for machine screws and holes for construction, structural or pocket-hole screws are not covered.
- 1.
Countersinks, Counterbores, Tapered Drills, Plug Cutters catalog (PDF dated 2006), page 4: Recommendations for Wood Screw Predrill Sizes and Lengths. W. L. Fuller, Inc., 2006.
Used for: Predrill recommendations by gauge, #2 to #18: actual head size, minimum and recommended counterbore, screw shank hole size, straight pilot drill sizes for hardwood and softwood, tapered pilot drill sizes for hardwood and softwood, and countersink numbers; the tapered drill and countersink system, which predrills, clears the shank, countersinks and counterbores in one pass.
- 2.
Chapter 8: Fastenings. USDA Forest Service, Forest Products Laboratory — Wood Handbook, FPL-GTR-282, 2021.
Used for: Withdrawal equations applicable when lead holes are about 70% of the root diameter in softwoods and about 90% in hardwoods; for lateral loads, the shank's lead hole the same as the shank or slightly smaller and the thread's lead hole the same as the root in dense species or slightly smaller in low-density ones; root diameter averaging about two-thirds of the shank; Table 8–9 shank diameters; screws turned in, never hammered; self-drilling screws needing no predrilling.
- 3.
ANSI Wood Screws per ANSI B18.6.1 (flat, pan and oval head size table). Engineers Edge.
Used for: Basic screw diameters by gauge, ASME B18.6.1.
About this chart
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