Engineering Tools · Pilot Hole & Drill Bit Reference
#18 Screw Pilot Hole & Drill Bit Size
A #18 wood screw has a body diameter of 0.2940 in (7.47 mm). The lead hole that the threads bite into and the clearance hole that the screw passes through are listed below with the closest imperial and metric drill sizes, and with the basis of every figure disclosed. The figures are field practice values from public-domain United States Government sources.
A #18 wood screw takes a 13/64 in lead hole, that is 0.2031 in or 5.16 mm. The closest imperial drill bits are 13/64 (0.2031 in) and #6 (0.2040 in); the closest metric drills are 5.20 mm and 5.10 mm. Practice value from the source table, not a standard value.
Work out your own order
Open the pilot hole calculator with this screw size and material already filled in: it returns the hole diameter, the closest imperial and metric drill sizes, and a printable list you can keep with the job.
Opens with screw size 18 and material softwood pre-filled. Change the material there to see the hardwood basis as well.
#18 Screw Hole and Drill Sizes
The three rows are the screw body, the lead hole (the threads bite into it, and the source table calls it the STARTER HOLE) and the clearance hole (the screw passes through it, and the source table calls it the PILOT HOLE). Fractional and millimetre figures come from the source table; decimal inch figures are the same fractions converted at 25.4 mm per inch. The decimal inch column printed in the source table is deliberately not reproduced: it carries last-digit differences and one misplaced row, so every inch figure here is convertible from the millimetre value instead.
| Parameter | Source-table column name | Fraction (in) | Decimal (in) | Millimetre (mm) | Source-table bit | Closest imperial drill bits | Closest metric drills |
|---|---|---|---|---|---|---|---|
| Screw body diameterNominal body diameter of the screw, and the dimension the hole sizes are compared against. | NOMINAL SCREW BODY DIAMETER | 19/64 | 0.2940 | 7.47 | not listed in the source table | not listed in the source table | not listed in the source table |
| Lead hole (threads bite here)Drill this in the piece that carries the load, to roughly half to two thirds of the thread length. | STARTER HOLE DRILL SIZE | 13/64 | 0.2031 | 5.16 | No. 4 (4/16 in) | 13/64 · 0.2031 in · delta 0.0000#6 · 0.2040 in · delta 0.0009#7 · 0.2010 in · delta 0.0021 | 5.20 mm · delta 0.0016 in 5.10 mm · delta 0.0023 in 5.25 mm · delta 0.0036 in |
| Clearance hole (screw passes through)Drill this in the piece being fixed, so the screw passes through without biting. | PILOT HOLE DRILL SIZE | 19/64 | 0.2969 | 7.54 | No. 5 (5/16 in) | 19/64 · 0.2969 in · delta 0.0000M · 0.2950 in · delta 0.0019N · 0.3020 in · delta 0.0051 | 7.50 mm · delta 0.0016 in 7.60 mm · delta 0.0023 in 7.40 mm · delta 0.0056 in |
Name reversal in the source: FM 5-426 prints PILOT HOLE for the hole the screw passes through, and STARTER HOLE for the hole the threads bite into, which is the opposite of today's usage. This page uses the modern names (clearance hole and lead hole) and prints the original column name beside each row so the two cannot be read the wrong way round.
delta is the difference between the drill and the hole in the row above: a negative delta means the drill is smaller than the hole, a positive delta means it is larger. Candidates are ordered by how close they are, so the first is the nearest available drill.
Where two designations share the same diameter (the letter series E and the fraction 1/4), both are kept as separate catalogue entries and neither replaces the other.
Where the single value in the table comes from
FM 5-426 gives one value per screw size and does not divide it by material or species. The manual states three purposes for the holes: to locate the screw accurately, to prevent splitting, and to save time. Its scope is softwood construction carpentry, so the single value is best read as a field practice value for softwood work rather than as a checked design allowance. That is why the table above has one column and not a material matrix.
Softwood (measured) against hardwood (inferred)
The source table gives one value per screw size and does not divide by material or by species, so that value is the softwood column. The hardwood column applies the Forest Products Laboratory hardwood proportion to the same value and is an inference, not a measurement. Use the softwood column as the practice value; treat the hardwood column as a rule demonstration only.
| Screw size | Softwood, source-table value measured practice value | Hardwood, inferred inferred · not measured · not a design value |
|---|---|---|
| #1 | no lead hole value in the source table | no lead hole value in the source table |
| #2 | 0.0625 in (1.59 mm) | 0.0562 in (1.43 mm) · inferred · not measured · not a design value |
| #3 | 0.0625 in (1.59 mm) | 0.0562 in (1.43 mm) · inferred · not measured · not a design value |
| #4 | 0.0781 in (1.98 mm) | 0.0703 in (1.79 mm) · inferred · not measured · not a design value |
| #5 | 0.0781 in (1.98 mm) | 0.0703 in (1.79 mm) · inferred · not measured · not a design value |
| #6 | 0.0938 in (2.38 mm) | 0.0844 in (2.14 mm) · inferred · not measured · not a design value |
| #7 | 0.1094 in (2.78 mm) | 0.0985 in (2.50 mm) · inferred · not measured · not a design value |
| #8 | 0.1094 in (2.78 mm) | 0.0985 in (2.50 mm) · inferred · not measured · not a design value |
| #9 | 0.125 in (3.17 mm) | 0.1125 in (2.86 mm) · inferred · not measured · not a design value |
| #10 | 0.125 in (3.17 mm) | 0.1125 in (2.86 mm) · inferred · not measured · not a design value |
| #12 | 0.1406 in (3.57 mm) | 0.1265 in (3.21 mm) · inferred · not measured · not a design value |
| #14 | 0.1562 in (3.97 mm) | 0.1406 in (3.57 mm) · inferred · not measured · not a design value |
| #16 | 0.1875 in (4.76 mm) | 0.1688 in (4.29 mm) · inferred · not measured · not a design value |
| #18 | 0.2031 in (5.16 mm) | 0.1828 in (4.64 mm) · inferred · not measured · not a design value |
Assumption behind the hardwood column
The hardwood figures assume that the lead hole in the source table was drilled to the thread root diameter, and then scale it by the Forest Products Laboratory hardwood proportion of 90 per cent. The thread root diameter has no standard value that may be quoted from a public-domain source, so the hardwood column is a rule demonstration only: it is not measured, it is not a design value, and it is not a basis for a quotation.
The load-design rule, for contrast
For a load-carrying joint the lead hole is specified as a proportion of the thread root diameter: about 70 per cent in softwood and about 90 per cent in hardwood. The hardwood proportion is larger because the rule comes from the applicability conditions of the pull-out equation, not from any material preference. The same rule runs back from the Wood Handbook through NBSIR 77-1282 to the 1926 NBS paper, which is where it is first recorded.
Source sentence, quoted in short: “This equation is applicable when screw lead holes have a diameter of about 70% of the root diameter of the threads in softwoods, and about 90% in hardwoods.”
Basis of that rule: the thread root diameter, not the shank diameter and not the body diameter. A different rule in the same chapter applies to lag screws and uses the shank diameter as its basis, so the two must not be mixed.
The rule and the value in the table above come from different places: the table value is a field practice figure for softwood carpentry, the rule is a load-design proportion. In practice they differ by around two fifths for the sizes here, so one must not be converted into the other or averaged with it.
The thread root diameter: two estimates, kept apart
- The first estimate is the industry practice recorded by the National Bureau of Standards in 1977: the root diameter is the body diameter multiplied by 0.67, which is 0.1970 in (5.00 mm) for this screw.
- The second estimate is the starter 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.
These two are alternative estimates with different origins, not two measurements of one quantity, so they are shown side by side and are not averaged or merged into a single root diameter.
Source-table column names in modern terms
Read the table above with this mapping beside it. The reversal in the first two rows is the reason this chart prints the original column name next to every row.
| FM 5-426 column | Modern name |
|---|---|
| SCREW SIZE | Screw gauge (the column header of the source table; that table has no material column) |
| NOMINAL SCREW BODY DIAMETER | Body diameter (today also called the major diameter of the screw) |
| PILOT HOLE DRILL SIZE | Clearance hole: the hole the screw passes through. The source table calls this the pilot hole, the opposite of modern usage |
| PILOT HOLE BIT SIZE | Auger bit number for that clearance hole; the bit number n is n/16 in and is not a diameter |
| STARTER HOLE DRILL SIZE | Lead hole: the hole the threads bite into, which is what is meant by a pilot hole today |
| STARTER HOLE BIT SIZE | Auger bit number for that lead hole, on the same n/16 in basis |
Full PDF
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Other Screw Sizes
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FAQ
What pilot hole should I drill for a #18 wood screw?
Drill 13/64 in, that is 0.2031 in (5.16 mm). The nearest imperial bits are 13/64 at 0.2031 in (delta 0.0000 in) and #6 at 0.2040 in; the nearest metric drills are 5.20 mm and 5.10 mm. The figure comes from the starter hole column of FM 5-426, which is the lead hole in modern terms, and it is a field practice value rather than a standard value. Drill it in the piece that carries the load, to about half to two thirds of the thread length.
What is the difference between a pilot hole and a clearance hole?
They are drilled in different pieces of wood. The clearance hole is drilled in the piece being fixed, and its diameter is at or just over the screw body, 0.2940 in (7.47 mm) for a #18 screw, so this page lists 19/64 in (7.54 mm). The lead hole, which most people mean by a pilot hole, is drilled in the piece that carries the load and is smaller than the body so the threads bite: 13/64 in (0.2031 in) here. The source table names them the other way round, which is why every row above carries the original column name.
Does every screw size have a lead hole value?
No. The source table lists a starter hole for every size from #2 upwards, but leaves it blank for the smallest screw size it covers. Where the cell is blank this chart prints that the source table has no value rather than computing one, because a computed figure would be indistinguishable from a measured one.
Are these figures standard values?
No, and they are not design values either. Every figure on this page is a field practice value or a text rule taken from United States Government works in the public domain. The copyrighted standards for wood screws and drill series are named on this page only so that you know they exist; no table value is reproduced from them, and the thread root diameter that the load-design rule needs is not specified in any public-domain source.
What should I drill for hardwood?
Following the load-design rule rather than the table, a #18 screw takes about 0.1828 in (4.64 mm) in hardwood, which is the hardwood proportion of 90 per cent of the thread root diameter assumed to equal the source-table lead hole of 13/64 in. That is an inference, not a measurement, and this page marks it inferred and keeps it out of the main table for that reason. If the joint carries load, take the value from the timber supplier and from the design requirement rather than from this page.
Which drill bit is closest to the 13/64 in lead hole?
On the imperial series the nearest bits are 13/64 at 0.2031 in (delta 0.0000 in), then #6 at 0.2040 in (delta 0.0009 in) and #7 at 0.2010 in. On the metric series the nearest drills are 5.20 mm (delta 0.0016 in), 5.10 mm (delta 0.0023 in) and 5.25 mm (delta 0.0036 in). Note that the imperial and metric series are two separate sets of drills: only three sizes coincide across the whole range, so a metric drill is usually the closest substitute in name only and not an identical diameter.
How big is a #18 screw?
The nominal body diameter of a #18 wood screw is 19/64 in, that is 0.2940 in or 7.47 mm. The screw gauge is a size number, not a fraction of an inch and not a millimetre diameter, so it has to be looked up rather than measured. Both the lead hole and the clearance hole are quoted against this body diameter.
Can I use a metric drill bit instead of an imperial one?
Often yes, as long as you accept a slightly different hole. For the 13/64 in lead hole the nearest metric drill is 5.20 mm, which is 0.0016 in from the hole; the next nearest is 5.10 mm. Because the metric series steps in small increments across the sizes used here, the difference is usually well under the tolerance of a wood joint, but the two series only coincide at a few sizes, so check the delta rather than the nominal size. If the joint is a load-carrying one, drill a test piece first.
References
Every figure and rule above is taken from a United States Government work in the public domain. The copyrighted standards for wood screws and for drill series are named only so that their existence is on record: no table value is reproduced from them, and the thread root diameter is not specified in any of them.
References
- Drill size constants for the number, letter and fraction series
- FED-STD-H28/2B Unified Inch Screw Threads (1984)
- FF-S-111D Screw, Wood, US Federal Specification (1974)
- FM 5-426 Carpentry, US Army (source table) (1995)
- FPL-GTR-190 Wood Handbook chapter 8, Fastenings, USDA Forest Service (1999)
- TC 9-524 Fundamentals of Machine Tools, US Army (1996)
- NAVEDTRA 14043 Builder 3 and 2, US Navy (1994)
- NBS Technologic Paper 319, Holding Power of Wood Screws (1926)
- NBSIR 77-1282, National Bureau of Standards (1977)
Disclaimer: every figure on this page is an engineering practice value or a text rule taken from public-domain sources. None of them is a standard value, a design value or an acceptance basis. For work that carries load, follow the data from your material supplier and the design requirement, and drill a test piece first. No table value from any copyrighted standard is reproduced here.