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Woodworking Math

Pilot hole calculator

Pick the screw and the wood to get the pilot hole, the clearance hole for the piece on top, and how big and deep to countersink a flat head. Each hole is the median of the published charts, with the span they cover.

Wood

Sizes are for a straight drill bit. Self-drilling construction screws come with their maker’s own advice; follow that for those screws.

Pilot hole, hardwood

7/64 or 1/8 in

Median 0.117 in (2.98 mm); charts span 0.094–0.125 in

Median of 5 sources

Clearance hole (top piece)

11/64 in

Median 0.172 in (4.37 mm); all charts agree

Median of 3 sources

Countersink for a flat head

0.332 in across, 0.191 in deep at 82°

8.43 mm across, 4.85 mm deep. Other heads and bits

Why: Published charts put a #8 pilot in hardwood at a median of 0.117 in, so drill 7/64 or 1/8 in.

Why these numbers?

  1. The situation

    A #8 wood screw into hardwood, drilled with a straight bit.

    From your input: screw size and wood

  2. What that means

    The lead hole for the threads should be about 70% of the thread’s root diameter in softwoods and about 90% in hardwoods: denser wood needs a bigger hole.

    One source: USDA Forest Products Laboratory, the federal wood research laboratory , sources [1]

  3. So

    No standard sets the drill, so we take the median of the published charts: 0.117 in, with the charts spanning 0.094 in to 0.125 in.

    5 sources agree , sources [2, 3, 4, 5, 6]

  4. The number

    0.117 in falls halfway between 7/64 in and 1/8 in; by the Wood Handbook’s rule, the denser the wood, the bigger the lead hole.

    Arithmetic: exact by definition

How we source these numbers

Three holes for one screw

The pilot hole goes in the piece the screw threads into. It lets the threads cut their own path without splitting the wood, and it is smaller than the screw, so the threads still bite. Dense hardwood takes a bigger pilot than softwood: the USDA Wood Handbook puts the lead hole at about 90% of the thread’s root diameter in hardwoods and about 70% in softwoods.

The clearance hole goes through the top piece. It is about the size of the screw’s shank, so the screw slides through and pulls the top piece down tight instead of threading into it. The countersink is a cone for a flat head to sit in, cut at 82° for inch wood screws.

No standard sets pilot hole sizes, and the published charts differ by a drill size or two, which is why the answer is a median and why a test hole in a scrap of the same wood is worth the minute it takes. Where the median falls exactly between two drills, both are named. For self-drilling construction screws, follow the screw maker’s own advice.

Every size at once is on the pilot hole chart; lag screws are on the lag screw chart.

Sources

  1. USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-190), ch. 8 Fastenings: lead holes and Table 8–9 screw shank diameters (as republished by CED Engineering, course S03-019), read 2026-11-07
  2. Engineers Edge, ANSI flat, pan and oval head wood screws per ANSI B18.6.1, size table; pilot hole drill sizes for wood screws, read 2026-11-07
  3. Engineering ToolBox, US wood screws: pilot holes, shank clearance holes and head diameters, read 2026-11-07
  4. McFeely’s, Drilling chart for screw sizes (straight pilot drill, shank hole, head size), read 2026-11-07
  5. MISUMI USA, Wood screw pilot hole sizes chart (straight bit columns), read 2026-11-07
  6. Bob Vila, How to drill a pilot hole: pilot hole sizes for wood screws, read 2026-11-07
  7. WoodBin, Wood screw pilot holes (shank hole column), read 2026-11-07
  8. Aspen Fasteners, Wood screws spec sheet: standard wood screw dimensions; 82° head angle, read 2026-11-07
  9. AmesWeb, Flat head wood screw sizes (ANSI B18.6.1-1981): conical head angle of approximately 82°, read 2026-11-07

How we source these numbers