Wood Screw Size Converter
Wood Screw Size Converter
Screw gauge number to shank diameter in inches and millimetres and back, with the metric equivalent — and the pilot hole and clearance hole sizes for softwood and hardwood, which is what the reader actually came for. The gauge-to-diameter rule is a published straight line, not a table.
Wood screw gauges
a #8 screw into softwood
One straight line, and three holes
- 0.013 in
- one gauge step, 0.330 mm. It is the same step as the American machine screw gauge, which is why a #8 wood screw and a #8 machine screw share a diameter
- 0.060 in
- the diameter of a #0, 1.52 mm — the intercept of the line
- root
- the core diameter between the threads, about 70% of the shank on a traditional wood screw, which is what the two published pilot tables’ ratios come out at
- clearance
- the hole in the piece being fixed. It takes the SHANK, so the thread never bites in the top piece and the joint can pull tight
Worked example
a #8 screw into softwood
The gauge rule gives 0.060 + 0.013 × 8 = 0.164 in, which is 4.17 mm — the shank diameter
The root is about 70% of that, 2.92 mm, and a softwood pilot is 85% of the root: 2.48 mm, so a 2.5 mm drill
In hardwood the pilot would be the root itself, 2.92 mm, so a 3.0 mm drill
The clearance hole in the top piece is the shank: 4.17 mm, taken up to a 4.2 mm drill
The two published pilot tables give 2.4 mm and 2.8 mm for a #8 in softwood — a whole drill size apart, which is the honest spread on this figure
Every gauge, with its holes
| Gauge | Shank (in) | Shank (mm) | Nearest metric sold | Root (mm) | Softwood pilot (mm) | Softwood pilot (/64 in) | Hardwood pilot (/64 in) | Clearance drill (mm) |
|---|---|---|---|---|---|---|---|---|
| #0 | 0.0600 | 1.52 | 2.0 | 1.07 | 0.91 | 2.3 | 2.7 | 1.6 |
| #1 | 0.0730 | 1.85 | 2.0 | 1.30 | 1.10 | 2.8 | 3.3 | 1.9 |
| #2 | 0.0860 | 2.18 | 2.0 | 1.53 | 1.30 | 3.3 | 3.9 | 2.2 |
| #3 | 0.0990 | 2.51 | 2.5 | 1.76 | 1.50 | 3.8 | 4.4 | 2.6 |
| #4 | 0.1120 | 2.84 | 3.0 | 1.99 | 1.69 | 4.3 | 5.0 | 2.9 |
| #5 | 0.1250 | 3.17 | 3.0 | 2.22 | 1.89 | 4.8 | 5.6 | 3.2 |
| #6 | 0.1380 | 3.51 | 3.5 | 2.45 | 2.09 | 5.3 | 6.2 | 3.6 |
| #7 | 0.1510 | 3.84 | 4.0 | 2.68 | 2.28 | 5.8 | 6.8 | 3.9 |
| #8 | 0.1640 | 4.17 | 4.0 | 2.92 | 2.48 | 6.2 | 7.3 | 4.2 |
| #9 | 0.1770 | 4.50 | 4.5 | 3.15 | 2.68 | 6.7 | 7.9 | 4.5 |
| #10 | 0.1900 | 4.83 | 5.0 | 3.38 | 2.87 | 7.2 | 8.5 | 4.9 |
| #11 | 0.2030 | 5.16 | 5.0 | 3.61 | 3.07 | 7.7 | 9.1 | 5.2 |
| #12 | 0.2160 | 5.49 | 5.5 | 3.84 | 3.26 | 8.2 | 9.7 | 5.5 |
| #14 | 0.2420 | 6.15 | 6.0 | 4.30 | 3.66 | 9.2 | 10.8 | 6.2 |
| #16 | 0.2680 | 6.81 | 7.0 | 4.77 | 4.05 | 10.2 | 12.0 | 6.9 |
| #18 | 0.2940 | 7.47 | 7.0 | 5.23 | 4.44 | 11.2 | 13.2 | 7.5 |
| #20 | 0.3200 | 8.13 | 8.0 | 5.69 | 4.84 | 12.2 | 14.3 | 8.2 |
| #24 | 0.3720 | 9.45 | 8.0 | 6.61 | 5.62 | 14.2 | 16.7 | 9.5 |
The two published pilot tables, and where they part
| Gauge | Hardwood, woodwiki (mm) | Hardwood, BuildToolHQ (mm) | Softwood, woodwiki (mm) | Softwood, BuildToolHQ (mm) | Hard ÷ shank (ww) | Hard ÷ shank (BTH) | Soft ÷ shank (ww) | Soft ÷ shank (BTH) |
|---|---|---|---|---|---|---|---|---|
| #4 | 1.98 | 1.59 | 1.59 | 1.19 | 0.698 | 0.558 | 0.558 | 0.419 |
| #5 | 2.38 | — | 1.98 | — | 0.750 | — | 0.625 | — |
| #6 | 2.38 | 2.38 | 1.98 | 1.98 | 0.679 | 0.679 | 0.566 | 0.566 |
| #7 | 2.78 | — | 2.38 | — | 0.724 | — | 0.621 | — |
| #8 | 3.17 | 3.17 | 2.38 | 2.78 | 0.762 | 0.762 | 0.572 | 0.667 |
| #9 | 3.57 | — | 2.78 | — | 0.794 | — | 0.618 | — |
| #10 | 3.57 | 3.57 | 2.78 | 3.17 | 0.740 | 0.740 | 0.576 | 0.658 |
| #12 | 3.97 | 4.37 | 3.17 | 3.97 | 0.723 | 0.796 | 0.579 | 0.723 |
| #14 | — | 4.76 | — | 4.37 | — | 0.775 | — | 0.710 |
Gauge numbers and metric sizes
| Gauge | Shank (mm) | UK chart says | Nearest size sold | Do they agree? |
|---|---|---|---|---|
| #3 | 2.51 | 2.5 | 2.5 | yes |
| #4 | 2.84 | 3.0 | 3.0 | yes |
| #6 | 3.51 | 3.5 | 3.5 | yes |
| #8 | 4.17 | 4.0 | 4.0 | yes |
| #10 | 4.83 | 5.0 | 5.0 | yes |
| #12 | 5.49 | 5.5 | 5.5 | yes |
| #14 | 6.15 | 6.5 | 6.0 | one step out |
Lengths, and the gauges sold with them
| Length | Gauges commonly stocked | What it is for |
|---|---|---|
| 12 mm (1/2 in) | #4 to #6 | Thin panels, hinges on 12 mm stock |
| 16 mm (5/8 in) | #4 to #8 | 18 mm sheet goods edge-on is marginal here |
| 25 mm (1 in) | #6 to #8 | Fixing 12 mm material to a frame |
| 30 mm | #6 to #10 | The general-purpose cabinet screw |
| 40 mm | #8 to #10 | 18 mm sheet into softwood framing |
| 50 mm (2 in) | #8 to #12 | Framing, battens, 25 mm boards |
| 65 mm | #10 to #12 | Heavier framing and stair work |
| 75 mm (3 in) | #10 to #14 | Structural timber, decking |
| 100 mm (4 in) | #12 to #14 | Deep structural fixings |
The gauge is the shank, and the shank is a straight line
A wood screw gauge is not a mystery table. The shank diameter in inches is 0.060 + 0.013 × gauge, a published straight line with a step of thirteen thousandths of an inch — 0.330 mm — per gauge number. A #6 is 0.138 in, a #8 is 0.164 in, a #10 is 0.190 in. That is the same rule and the same step as the American machine screw gauge, which is why a #8 wood screw and a #8 machine screw have the same nominal diameter and why the numbering feels familiar if you have tapped a hole.
The gauge describes the shank, and that matters. The thread on a traditional wood screw is larger than the shank and the root between the threads is smaller — about seventy per cent of it. So there are three different diameters in play for one screw, and three different holes: the clearance hole in the piece being fixed, which takes the shank so the thread never bites there and the joint can pull tight; the pilot hole in the piece being fixed to; and the countersink for the head. Measuring a screw across its threads instead of its shank puts you a gauge or two high, which is the commonest way to get this wrong.
Pilot holes, honestly. The rule the published tables are trying to express is: in hardwood, drill the root diameter, so the thread cuts into solid wood and nothing is being pushed apart; in softwood, drill about 85 per cent of the root, because softwood compresses rather than splits and the extra grip is worth having. That works out at roughly 0.70 and 0.60 of the shank. This page computes those and also prints two real published tables — and they disagree by up to a whole drill size. For a #8 in softwood one says 2.4 mm and the other 2.8 mm. Neither is wrong; they are different trade-offs between grip and splitting, and the spread is the honest answer to “what size pilot hole”. Near an end grain or an edge, drill the hardwood size whatever the species.
UK and US gauge numbering is the same scale. British merchants now sell screws in millimetres, and the standard UK gauge-to-metric chart — gauge 4 to 3 mm, 6 to 3.5, 8 to 4, 10 to 5, 12 to 5.5 — is nothing more than the shank diameter of the American rule rounded to the nearest size a UK merchant stocks. It matches that calculation at six of its seven rows, drifting only at #14, where 6.15 mm could reasonably be called either 6 or 6.5. What is not shared is everything above the shank: head diameters, countersink angles and drive types differ between the two traditions and between eras, so a #8 from one century will not necessarily take the driver or sit in the countersink of another.
Length, since screws are sold by gauge and length together. An “8 × 40” is a gauge 8, forty millimetres long. Length is measured to the tip, and for a countersunk head it includes the head, because the whole screw goes into the wood; for a round or pan head it is measured from under the head, because the head sits on the surface. The published guide for how long is that the threaded part should reach at least two thirds of the way into the base piece, which this page works out for the two thicknesses you enter. For the drills these holes need, in every naming system, see the drill bit size converter; for the sheet the screws go through, the sheet metal gauge converter. This page does the arithmetic. Where the answer carries a consequence — a dose, a structural load, a legal area, a medical measure — confirm it against the authority that governs it before relying on it.
Frequently asked questions
What size is a #8 wood screw in millimetres?
4.17 mm on the shank, from 0.060 + 0.013 × 8 = 0.164 in. British merchants sell it as a 4 mm screw, which is the same figure rounded to a stocked size.
What size pilot hole for a #8 screw?
About 2.5 mm in softwood and 3.0 mm in hardwood. The rule is the thread’s root diameter for hardwood — roughly 70% of the shank, 2.92 mm here — and about 85% of that for softwood. Two published tables give 2.4 and 2.8 mm for the softwood case, so treat anything in that range as reasonable and drill the bigger hole near an edge.
Is the pilot hole the same as the clearance hole?
No, and using one for both is why joints do not pull tight. The clearance hole goes in the piece being fixed and is the full shank diameter, so the thread cannot grip there. The pilot hole goes in the piece being fixed to and is about the root diameter, so the thread can.
Are UK and US wood screw gauges the same?
The numbering is, yes — both follow the 0.060 + 0.013 n rule, and the British gauge-to-millimetre chart is just that rule rounded to stocked sizes. Head diameters, countersink angles and drive types are not shared, so the same gauge number does not guarantee the same head or the same driver.
How long should a screw be?
Long enough that the threaded part reaches at least two thirds of the way into the base piece, which is the published rule this page applies. For 18 mm sheet into softwood framing that lands around 40 to 50 mm. Longer is not automatically better: a long screw in a fine gauge bends and wanders off line.
What does 8 × 40 mean?
Gauge 8, forty millimetres long. Screws are sold by the pair because they are not independent choices. Length is measured to the tip and includes a countersunk head but not a raised or domed one.
What is the metric equivalent of a #10 screw?
4.83 mm on the shank, sold as a 5 mm screw. The gauge ladder steps by 0.330 mm and the metric sizes merchants stock step by 0.5 mm, so the two scales do not map one to one and the nearest metric size can be a fifth of a millimetre away.
Related calculators
References
- woodwiki.org, Picking the right wood screws for your project (read 26 September 2026), which states the gauge rule outright — diameter in inches = gauge × 0.013 + 0.060 — and tabulates shank diameter, hardwood and softwood pilot holes and clearance holes for gauges 4 to 14.
- BuildToolHQ, Pilot Hole Size Chart: Every Gauge, Every Material (read 26 September 2026) — the second pilot hole table, which disagrees with the first by up to one drill size. It also states the rule both are aiming at: a softwood pilot is about 85 per cent of the thread’s root diameter and a hardwood pilot is the root diameter itself.
- finepowertools.com, Wood Screw Sizes (read 26 September 2026), which tabulates gauges #0 to #24 with their major diameters in fractions and decimal inches — the check that the 0.060 + 0.013 n rule holds across the whole commercial range and not just the middle of it.
- DIY Doctor, Screw Sizes Conversion Chart, diydoctor.org.uk (read 26 September 2026): the UK gauge-to-millimetre chart — gauge 3 to 2.5 mm, 4 to 3, 6 to 3.5, 8 to 4, 10 to 5, 12 to 5.5, 14 to 6.5 — which is the evidence that UK and US gauge numbering is one scale, since the chart is the shank diameter rounded to the nearest size a UK merchant sells.
- ASME B1.1, Unified Inch Screw Threads (UN, UNR and UNJ Thread Form), which defines the UNC and UNF series. Copyrighted; cited by number. The threads per inch used here are part of the thread’s own designation, and the number-size major diameter follows the published American screw gauge rule, 0.060 in plus 0.013 in per gauge number.