Pilot Hole & Drill Size Calculator

The unit above decides which figure is shown large for each hole. Both units are always listed in the rows underneath.

Where the figure above comes from: the measured value

The source table gives one value per screw size and does not divide it by material or by species. Its scope is softwood construction carpentry, so that single value is read here as the measured softwood practice value: 7/64 in, that is 0.1094 in or 2.78 mm for a #8 screw. It is what the table records, not what a design calculation would allow, and it is the figure to drill from unless your own design data says otherwise.

The root diameter that the hardwood rule is scaled from

• First estimate: the industry practice that the National Bureau of Standards recorded in 1977 puts the root diameter at the body diameter multiplied by 0.67, which for this screw is 0.1099 in, that is 2.79 mm.
• Second estimate: the lead hole value in the source table itself, taken as the root diameter. That is the cabinetmaking habit which the 1926 National Bureau of Standards paper records and recommends discontinuing.

Two estimates with different origins, shown side by side. They are not averaged and not merged into one root diameter, and neither of them is a standard value.

How every figure on this page is produced

There is no hand-typed number anywhere on this page. The body diameter, the lead hole and the clearance hole are read from the screw-size row of the source table, where the lead hole is printed as STARTER HOLE and the clearance hole as PILOT HOLE, the reverse of today's usage. Inch and millimetre figures are the same value converted at 25.4 mm to the inch, and the decimal inch column printed in the source table is deliberately not reproduced: it carries last-digit differences and one misplaced row. The nearest drills are found by comparing the hole diameter against a 169-row imperial drill series and a 169-size metric series, both from public-domain tables. The quantity you type never enters the calculation.

Known gaps in the data behind this calculator

Stated rather than hidden, so that you can judge how far to rely on a figure:

• The metric drill series comes from a single public-domain source, US Army TC 9-524 Table 4-1, which lists 169 sizes. Its metric column and its printed decimal-inch column are two readings of the same table rather than two independent sources, so the one cannot check the other, and 6 of the sizes rest on the metric column alone. The table is also the commonly available subset that the manual lists, not the complete series of any standard, and it has no row-by-row sizes above 25.00 mm. ISO 235, DIN 338 and BS 328 are named here only so that their existence is on record.
• The thread root diameter has no standard value that may be quoted from a public-domain source, so no absolute hole size can be derived from the load-design rule on its own: the two estimates above are separate conventions, and the hardwood figure on this page is scaled from the second of them. That is why the hardwood figure is marked inferred, and why the measured value is the one to drill from unless your own design data says otherwise.