Drill Bit / Screw Size Reference
Inch fractional and screw-size drill bits cross-referenced to mm.
Overview
The Drill Bit and Screw Size Reference cross-references inch fractional drills, number / letter drill bits, and the recommended pilot- and clearance-hole sizes for common wood screw and machine screw gauges, all alongside the metric equivalents in millimetres. It is the table you keep next to the drill press when the box says "use a 13/64" and your bit index is mostly metric.
The tool is for woodworkers, machinists and DIYers staring at a screw chart trying to figure out the right pilot hole. Long-tail queries it answers include "what drill bit for #8 wood screw", "13/64 inch in mm", "tap drill size for M6", and "letter drill bit P in mm".
How it works
The cross-reference is a fixed table that pairs each fractional size (1/64 through 1/2 in 64ths), each number drill (1 to 80) and each letter drill (A through Z) with its decimal-inch value and its closest metric millimetre equivalent. Pilot and clearance hole guidance for screws follows the recommendations published by the major fastener manufacturers.
For metric tap drills, the formula is straightforward: tap drill diameter = nominal diameter − pitch. The table pre-computes the standard ISO coarse and fine pitches so you don't have to look them up.
Examples
1/4" → 6.35 mm → drill F (.257") for clearance
#8 wood screw → pilot 3/32" (2.4 mm), clearance 11/64" (4.4 mm)
M6 × 1.0 tap → 5.0 mm drill
Letter J drill → .277" / 7.04 mm
FAQ
What's the difference between pilot and clearance holes?
A pilot hole guides the screw's threads and is slightly smaller than the screw's root diameter. A clearance hole is in the piece you're attaching and is slightly larger than the screw's outer thread diameter so it can pass freely.
Why three different drill numbering systems?
Fractional drills go up in 1/64" steps and cover the broad range. Number drills (1-80) and letter drills (A-Z) fill in the gaps with finer increments for tapping and reaming.
How does this work for hardwood vs softwood?
Hardwoods need a slightly larger pilot hole — typically the next size up from the standard recommendation — because hardwood resists thread cutting more aggressively and is prone to splitting.
Is the metric equivalent exact?
The decimal-inch value is exact; the millimetre value is rounded to the nearest 0.01 mm. For most workshop use this is more accuracy than a drill chuck can deliver.
What about self-tapping screws?
Self-tapping fasteners cut their own threads and typically don't need a pilot in thin sheet metal. For thicker stock, a pilot at about 90% of the root diameter is recommended.