Drill bit size chart and tap and drill chart

Three whole charts: drill bit diameters, UNC threads and metric threads — in inches and millimetres, and a chart of wood screw pilot holes further down. The filter at the top narrows the charts to the rows you want.

Drill bit diameters

Number, letter and fractional drill bits. Millimetres are computed from inches by multiplying by 25.4 and rounded to 0.001 mm.
Drill Series Ø, inch Ø, mm


Checked: . These values are for reference. For critical joints follow the manufacturer's specification.

UNC threads (inch, coarse pitch)

Tap drill at 75 % and 50 % of thread, and clearance holes. Millimetres are computed from inches by multiplying by 25.4.
Thread Threads per inch Major Ø, inch Major Ø, mm Tap drill 75 % Ø, inch Ø, mm Tap drill 50 % Ø, inch Ø, mm Clearance, close Ø, inch Ø, mm Clearance, free Ø, inch Ø, mm


Checked: . These values are for reference. For critical joints follow the manufacturer's specification.

Metric threads

Tap drill and clearance holes. These values are given in millimetres in the source itself; inches are computed by dividing by 25.4.
Thread Pitch, mm Series Tap drill, mm Same, inch Close, mm Medium, mm Free, mm


Checked: . These values are for reference. For critical joints follow the manufacturer's specification.

Where the references disagree on drills and threads

Each line is one row of the charts above: the column the references disagree on, the value the chart gives, and what each reference prints there, written the way that reference writes it.

Number drill sizes

A number drill is called by a gauge number, and the numbering runs the other way round from the size: the higher the number, the smaller the drill. The number on the shank tells you nothing about the diameter, so the inch and millimetre columns are the whole point of looking it up. Number drills cover the small end of the range, and most tap drills for the smaller inch threads are number drills.

Letter drill sizes

Letter drills pick up where the number series leaves off and run from A, the smallest of them, to Z, the largest, one step per letter. Like the numbers, a letter carries no size in itself; it has to be read off the chart. These are the drills the inch threads of a quarter inch and up are tapped and cleared with.

Fractional drill sizes

Fractional drills are the inch sizes of the ordinary index — sixteenths, eighths, quarters and the sixty-fourths in between. Their step is 1/64 inch, which is coarse next to the number and letter series, so a fraction is often near a tap drill rather than on it. Each one stands here in decimal inches and in millimetres, which is what it takes to compare a fraction with a number or a letter.

Metric tap drill sizes

A metric thread is written as its nominal diameter and its pitch: M6x1.0 is six millimetres across with a one millimetre pitch. The tap drill is stated in millimetres, the way the sources state it, with the inch equivalent computed beside it and the clearance holes — close, medium and free — in the same row. A coarse and a fine pitch of the same diameter are separate rows, because they call for different drills.

Wood screw pilot holes

A wood screw wants two holes: a pilot hole for the threaded part, so the wood does not split, and a clearance hole in the top piece, so the screw draws the two pieces together instead of jacking them apart. Hardwood needs a bigger pilot than softwood, because it does not give way. The references agree on the clearance hole and much less on the pilot, so where they part ways the chart gives you their spread rather than one pick.

Wood screw pilot and clearance holes

Gauge-numbered wood screws. A range means the references print different drills for that hole; a dash means only one reference prints it. Millimetres are computed from inches by multiplying by 25.4.
Screw Screw Ø, inch Screw Ø, mm Pilot, softwood Ø, inch Ø, mm Pilot, hardwood Ø, inch Ø, mm Clearance hole Ø, inch Ø, mm


Checked: . These values are for reference. For critical joints follow the manufacturer's specification.

Where the references disagree

Each line is one hole of one screw and what every reference prints for it.