Drill Bit & Screw Size Guide
Pick the screw type, size and job to get the drill diameter, the nearest imperial bit, the tap drill and the clearance hole — with full charts for machine screws, wood screws and wall plugs.
The drill has to be smaller than the thread and larger than the shank, and the right number changes with the material and the job. Pick a screw type, a size and what you are doing; the page returns the drill diameter, the nearest imperial bit, and the matching tap, bolt or wall plug.
Screw and job
Choose the type first; the size and job lists update to match.
Drill diameter
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Nearest imperial bit
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Reference
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Matches
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Metric machine screws
| Size | Pitch | Tap drill | Clearance hole | Socket-head counterbore |
|---|---|---|---|---|
| M3 | 0.5 | 2.5 mm | 3.4 mm | 6.0 mm |
| M4 | 0.7 | 3.3 mm | 4.5 mm | 8.0 mm |
| M5 | 0.8 | 4.2 mm | 5.5 mm | 10.0 mm |
| M6 | 1.0 | 5.0 mm | 6.6 mm | 12.0 mm |
| M8 | 1.25 | 6.8 mm | 9.0 mm | 16.0 mm |
| M10 | 1.5 | 8.5 mm | 11.0 mm | 20.0 mm |
| M12 | 1.75 | 10.2 mm | 13.5 mm | 24.0 mm |
Wood screws
| Gauge | Major diameter | Pilot — softwood | Pilot — hardwood | Clearance hole |
|---|---|---|---|---|
| #4 | 2.8 mm | 1.6 mm | 2.0 mm | 3.0 mm |
| #6 | 3.5 mm | 2.0 mm | 2.4 mm | 3.5 mm |
| #8 | 4.2 mm | 2.4 mm | 2.8 mm | 4.5 mm |
| #10 | 4.8 mm | 2.8 mm | 3.2 mm | 5.0 mm |
| #12 | 5.5 mm | 3.2 mm | 3.6 mm | 5.5 mm |
| #14 | 6.3 mm | 3.6 mm | 4.0 mm | 6.5 mm |
Plastic wall plugs
| Plug diameter | Drill | Screw | Typical use |
|---|---|---|---|
| 5 mm | 5 mm | M3–M4 | Frames, small hooks |
| 6 mm | 6 mm | M4 | Curtain rods, hooks |
| 8 mm | 8 mm | M5 | Wall cabinets, towel rails |
| 10 mm | 10 mm | M6 | TV brackets (with metal anchors) |
| 12 mm | 12 mm | M8 | Wall units, water heaters (concrete) |
| 14 mm | 14 mm | M10 | Heavy-duty brackets |
Four rules that cover most jobs
- The bit is always smaller than the thread: a tapped hole needs metal left for the threads, so the tap drill is the major diameter minus the pitch.
- Wood screws do not need tapping, but they do need a pilot hole to stop splitting: drill finer in softwood and coarser in hardwood.
- A clearance hole lets the screw shank pass freely — usually 0.2–0.5 mm larger than the major diameter.
- A wall plug hole must equal the plug diameter exactly. Oversize and the anchor spins; undersize and it will not go in.
- The material decides the bit: brad-point for wood, cobalt or HSS for metal, masonry with a hammer drill for concrete, and a glass bit at low speed for tile.
Practical notes
- Drill a small pilot first, then open it up. It is far easier to control than one pass with a large bit.
- Metal: low speed and a drop of cutting oil. Blue chips and smoke mean the bit is overheating.
- Tile: no hammer action, low speed, keep water on the bit. Break through the glaze first, then switch to the masonry bit behind it.
- For wall units, water heaters and TV brackets, use metal expansion anchors. Plastic plugs are for light loads only.
Four common mistakes
- Using a wood bit on tile. It skates across the glaze and cracks the edge.
- Drilling a tap hole at the major diameter. The tap cuts no threads, then jams and snaps.
- Using one size up on a wall plug. The plug turns in the hole and pulls out under load.
- Running a metal bit fast and dry. The tip anneals, turns blue and stops cutting.
Sizes are generic reference values. Follow the screw or plug packaging when it differs; for load-bearing installations use the product’s specified anchor and confirm the wall type before drilling.