Screw gauge to mm converter
Imperial wood-screw gauges to millimetres, the metric size sold in UK shops, the pilot hole to drill, and the wall plug to use. Type a gauge or read the table.
Convert a gauge
Screw gauge table
The gauge number is the old imperial size of the shank. The exact diameter follows a formula (0.060" + 0.013" × gauge); what the shop sells is the nearest metric size, so a No. 8 is a 4.0 mm screw and a No. 10 is a 5.0 mm screw. Pilot and clearance drills are the usual sizes; go one down in soft timber if the screw isn't biting.
| Gauge | Exact mm | Inches | Sold as | Pilot, softwood | Pilot, hardwood | Clearance | Wall plug |
|---|---|---|---|---|---|---|---|
| No. 2 | 2.18 | 0.086" | 2.0 mm | 1.0 | 1.5 | 2.5 | — |
| No. 3 | 2.51 | 0.099" | 2.5 mm | 1.5 | 1.5 | 3.0 | — |
| No. 4 | 2.84 | 0.112" | 3.0 mm | 1.5 | 2.0 | 3.5 | Yellow (5 mm) |
| No. 5 | 3.17 | 0.125" | 3.0 mm | 2.0 | 2.0 | 3.5 | Yellow (5 mm) |
| No. 6 | 3.51 | 0.138" | 3.5 mm | 2.0 | 2.5 | 4.0 | Yellow (5 mm) / Red (6 mm) |
| No. 7 | 3.84 | 0.151" | 4.0 mm | 2.5 | 3.0 | 4.5 | Red (6 mm) |
| No. 8 | 4.17 | 0.164" | 4.0 mm | 2.5 | 3.0 | 4.5 | Red (6 mm) |
| No. 9 | 4.50 | 0.177" | 4.5 mm | 3.0 | 3.5 | 5.0 | Red (6 mm) |
| No. 10 | 4.83 | 0.190" | 5.0 mm | 3.0 | 3.5 | 5.5 | Red (6 mm) / Brown (7 mm) |
| No. 12 | 5.49 | 0.216" | 5.5 mm | 3.5 | 4.0 | 6.0 | Brown (7 mm) |
| No. 14 | 6.15 | 0.242" | 6.0 mm | 4.0 | 4.5 | 6.5 | Brown (7 mm) / Blue (10 mm) |
| No. 16 | 6.81 | 0.268" | 7.0 mm | 4.5 | 5.0 | 7.5 | Blue (10 mm) |
| No. 18 | 7.47 | 0.294" | 8.0 mm | 5.0 | 5.5 | 8.5 | Blue (10 mm) |
| No. 20 | 8.13 | 0.320" | 8.0 mm | 5.5 | 6.0 | 9.0 | Blue (10 mm) |
Drill sizes in mm. Plug colours follow the common UK convention (yellow 5 mm hole, red 6 mm, brown 7 mm, blue 10 mm); check the pack, since makers differ by half a millimetre.
Screw lengths, imperial to metric
Lengths are also rounded on the shelf: a 1½" screw is sold as 40 mm, a 2" as 50 mm, a 3" as 75 mm. Put the two tables together and No. 8 × 1½" is a 4.0 × 40 mm screw.
| Length | Exact mm | Sold as |
|---|---|---|
| ½" | 12.7 | 12 mm |
| ⅝" | 15.9 | 16 mm |
| ¾" | 19.1 | 20 mm |
| 1" | 25.4 | 25 mm |
| 1¼" | 31.8 | 30 mm |
| 1½" | 38.1 | 40 mm |
| 1¾" | 44.5 | 45 mm |
| 2" | 50.8 | 50 mm |
| 2½" | 63.5 | 60 mm |
| 3" | 76.2 | 75 mm |
| 3½" | 88.9 | 90 mm |
| 4" | 101.6 | 100 mm |
| 5" | 127.0 | 125 mm |
| 6" | 152.4 | 150 mm |
Common conversions
- No. 6 × 1" = 3.5 × 25 mm
- No. 8 × 1¼" = 4.0 × 30 mm
- No. 8 × 1½" = 4.0 × 40 mm
- No. 8 × 2" = 4.0 × 50 mm
- No. 10 × 2" = 5.0 × 50 mm
- No. 10 × 3" = 5.0 × 75 mm
- No. 12 × 2½" = 5.5 × 60 mm
- No. 14 × 3" = 6.0 × 75 mm
Machine screws and bolts use a different gauge series (M-sizes and UNC/UNF); this page is for wood screws, chipboard screws and the like.
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Common questions
A No. 8 wood screw has a shank of about 4.2 mm (0.164"). UK shops sell it as a 4.0 mm screw. A No. 8 × 1½" is a 4.0 × 40 mm screw.
A No. 10 wood screw measures about 4.8 mm (0.190") across the shank and is sold as a 5.0 mm screw. No. 10 × 2" is 5.0 × 50 mm.
About 3.5 mm (0.138"). It is sold as a 3.5 mm screw; No. 6 × 1" is 3.5 × 25 mm.
For wood screws the shank diameter in inches is 0.060 + 0.013 × gauge. The calculator on this page uses that formula; the 'sold as' column is the rounded metric size you'll find on the shelf.
A pilot hole of roughly 60–70% of the shank diameter in softwood and 70–80% in hardwood. The table gives the usual drill sizes; go a size smaller in soft timber if the screw is not biting.
Yellow plugs take 3–4 mm screws (gauges 4–8) in a 5 mm hole, red take 3.5–5 mm (6–10) in a 6 mm hole, brown take 5–6 mm (10–14) in a 7 mm hole, blue take 6–8 mm (14–18) in a 10 mm hole. Check the pack: manufacturers differ slightly.