Pick the pilot hole for the wood that receives a screw and the clearance hole for the piece being fastened. The calculator works from published shank and root diameters and names the nearest standard drill.
A screw joint has two parts. The top piece — the one being fastened — gets a clearance hole the size of the screw’s shank, so the threads slide through and the head can pull the two pieces tight. The bottom piece gets a pilot hole smaller than the threads, so the threads cut into solid wood without splitting it.
Skip the clearance hole and the threads grip both pieces, so the joint never fully closes. Skip the pilot and hardwood splits, screws snap, and brass screws twist off. The calculator sizes both from the screw’s gauge and the receiving wood.
Clearance equals the shank (major) diameter. The pilot is a fraction of the root, the solid core inside the threads: about 70% of the root in softwood, which compresses easily, and about 90% in hardwood, which does not. Each theoretical size is then matched to the closest drill in the numbered, letter and fractional series.
clearance = shank · softwood pilot = 0.70 × root · hardwood pilot = 0.90 × root
| Screw | Shank (in) | Root (in) | Softwood pilot | Hardwood pilot | Clearance drill |
|---|---|---|---|---|---|
| #4 | 0.112 | 0.075 | 0.0525 → #55 | 0.0675 → #51 | #33 |
| #6 | 0.138 | 0.094 | 0.0658 → #51 | 0.0846 → #44 | #29 |
| #8 | 0.164 | 0.112 | 0.0784 → #47 | 0.1008 → #38 | #19 |
| #10 | 0.190 | 0.130 | 0.0910 → #43 | 0.1170 → #32 | #11 |
| #12 | 0.216 | 0.148 | 0.1036 → #37 | 0.1332 → #29 | 7/32 in |
| #14 | 0.242 | 0.165 | 0.1155 → #32 | 0.1485 → #25 | C |
Pilot values are theoretical inches → the nearest drill the calculator picks. Many printed charts run a size or so larger, especially in softwood; both work, and the difference is grip versus splitting risk. Test in an offcut when it matters.
#8 screws into a pine cleat. Softwood pilot 0.70 × 0.112 = 0.0784 in → #47 drill (0.0785 in). Clearance in the part being attached: #19 (0.166 in). If you only own fractional bits, 5/64 in (0.078) for the pilot and 11/64 in (0.172) for the clearance are close.
#8 screws into white oak. Hardwood pilot 0.90 × 0.112 = 0.1008 in → #38 (0.1015 in). A 7/64 in bit (0.109) is the nearest common fractional size and is easier to find.
#10 screws attaching a tabletop button to maple. Pilot #32 (0.116 in), clearance in the button #11. Slot the clearance hole if the button needs to let the top move.
A common rule of thumb is to get at least half, and ideally about two-thirds, of the screw’s length into the receiving piece, without coming within about ⅛ in of its far face. Fastening ¾ in stock to a 1½ in frame, a 1¼ in screw puts only ½ in into the frame; a 2 in screw puts 1¼ in in, which is the better choice.
End grain holds screws poorly — far worse than face or edge grain — so screws into the end of a board need more length, or a cross-grain dowel buried in the joint for the threads to bite into. In particleboard and MDF, use screws designed for those materials and keep well away from edges.
Many self-drilling screws are designed to go in without one in softwood. Near edges, in hardwood, or with small or brittle screws, a pilot is still a good idea.
A size code for the shank diameter. Each step adds 0.013 in: a #8 is 0.164 in, a #10 is 0.190 in.
Use the closest fractional size. The calculator shows the theoretical size, so you can see which way you are rounding.
Plywood faces split less than solid wood, but the glue lines are hard. The softwood pilot is usually fine for softwood ply; use the hardwood figure for Baltic birch and hardwood ply.