EstimatorKitWood, sheet metal and machine screws
Screw size chart — gauge, pilot hole and length
Screw gauge is a diameter written as a code: 0.060 in for a #0 and another 0.013 in for every number after it, which makes a #8 exactly 0.164 in and a #12 exactly 0.216 in. This chart lists all 16 gauges from #0 to #20 with shank and head diameter in both inches and millimeters, threads per inch, driver size, and the two holes each gauge needs — a pilot sized to the wood and a clearance hole sized to the shank. Tell the panel above the table which gauge and how thick the two pieces are, and it names the bits and the screw length rather than making you read across sixteen rows.
- 16 gauges, #0 to #20
- Pilot by wood hardness
- Inches and millimeters
Which bits for this screw, and how long should it be?
Inches unless you say otherwise, so 3/4, 1 1/2, 0.72 and 18 mm all read correctly — nominal 3/4 stock that planed out at 0.71 is worth typing as 0.71.
Pilot bit for a #8
3/32in
- Clearance in the top piece
- 11/64 in
- Countersink to
- 0.332 in
- Buy this length
- 2"
- Bite into the base
- 1 1/4" (63% of the screw)
63% of the screw is in the base piece — a little under the two-thirds ideal, and the normal result whenever the base is only about twice the thickness of what is going onto it. Nothing longer fits: the next stock length is past 2 1/8" and the point would show.
Pilot sizes come from the wood-screw pilot table reproduced in Machinery's Handbook and most trade references: the hardwood column sits close to the thread root diameter, the softwood column at roughly three quarters of it. They move with species and density inside each column, moisture content, and how close the screw lands to an end or an edge, so the hardwood column is the safe starting point in anything dense and the offcut test beats both.
Coarse single-lead thread, unthreaded shank on the traditional pattern, sized by gauge.
| Gauge | Shank | TPI | Flat head | Pilot, softwood | Pilot, hardwood | Clearance | Metric | Driver |
|---|---|---|---|---|---|---|---|---|
| #0 | 0.060 in | 32 | 0.119 in | 1/64 | 1/32 | 1/16 | 1.6 mm | PH0 · — · — |
| #1 | 0.073 in | 28 | 0.146 in | 1/32 | 1/32 | 5/64 | 2 mm | PH0 · — · — |
| #2 | 0.086 in | 26 | 0.172 in | 1/32 | 3/64 | 3/32 | 2 mm | PH1 · — · T6 |
| #3 | 0.099 in | 24 | 0.199 in | 3/64 | 1/16 | 7/64 | 2.5 mm | PH1 · — · T8 |
| #4 | 0.112 in | 22 | 0.225 in | 3/64 | 1/16 | 7/64 | 3 mm | PH1 · R1 · T10 |
| #5 | 0.125 in | 20 | 0.252 in | 1/16 | 5/64 | 1/8 | 3 mm | PH2 · R1 · T15 |
| #6 | 0.138 in | 18 | 0.279 in | 1/16 | 5/64 | 9/64 | 3.5 mm | PH2 · R1 · T15 |
| #7 | 0.151 in | 16 | 0.305 in | 1/16 | 3/32 | 5/32 | 4 mm | PH2 · R2 · T15 |
| #8 | 0.164 in | 15 | 0.332 in | 5/64 | 3/32 | 11/64 | 4 mm | PH2 · R2 · T20 |
| #9 | 0.177 in | 14 | 0.358 in | 5/64 | 7/64 | 3/16 | 4.5 mm | PH2 · R2 · T20 |
| #10 | 0.190 in | 13 | 0.385 in | 3/32 | 7/64 | 3/16 | 5 mm | PH2 · R2 · T25 |
| #12 | 0.216 in | 11 | 0.438 in | 7/64 | 1/8 | 7/32 | 5.5 mm | PH3 · R3 · T25 |
| #14 | 0.242 in | 10 | 0.491 in | 7/64 | 9/64 | 1/4 | 6 mm | PH3 · R3 · T30 |
| #16 | 0.268 in | 9 | 0.545 in | 9/64 | 5/32 | 17/64 | 7 mm | PH3 · R3 · T30 |
| #18 | 0.294 in | 8 | 0.598 in | 9/64 | 3/16 | 19/64 | 7.5 mm | PH4 · — · T40 |
| #20 | 0.320 in | 8 | 0.651 in | 11/64 | 13/64 | 21/64 | 8 mm | PH4 · — · T40 |
Shank and head diameters are the ASME B18.6.1 straight lines — ASME B18.6.1 — basic major diameter is 0.060 + 0.013 × gauge, which is where #8 gets 0.164, and ASME B18.6.1 — flat countersunk head diameter is 0.119 + 0.0266 × gauge, which is why a flat head measures almost exactly twice its own shank. A flat head seats in a countersink cut at 82° included — the standard included angle of a US flat screw head; DIN and ISO heads are 90°, which is the reason imported hardware sits proud in a hole bored for domestic screws. Driver sizes are the driver sizes fastener makers print against each gauge on the box and vary with brand and head style — Torx deck screws in one gauge ship as both T20 and T25. Tapping-screw diameters step up to 0.250 in at #14 rather than following the wood-screw line to 0.242 in, so the two columns part company at the top of the table. Metric equivalents are rounded to a diameter actually stocked, and none of them lands on one exactly — the closest is #6 at 3.51 mm against a 3.5 mm screw.
Screw questions
What diameter is a #8 screw?
0.164 in, or 4.17 mm, measured across the thread crests. The head on a flat #8 is 0.332 in — near enough to twice the shank that you can size a countersink by eye once you know the rule. In metric hardware the closest stocked diameter is 4 mm, which is 0.17 mm slimmer, so a 4 mm screw drops through a hole bored for a #8 and a #8 will not go through one bored for 4 mm.
When can I skip the pilot hole?
In softwood, more than an inch from any end, with a modern screw that has a type-17 auger point and a thinned shank — that combination was designed to make its own hole. Everything else earns a pilot: hardwood of any kind, plywood edges, anything within about two inches of the end of a board, and any brass or stainless screw, because those alloys shear at a fraction of the torque a hardened steel screw survives. The cost of being wrong is a snapped screw buried flush in a finished part.
How long should the screw be?
Long enough that about two thirds of it ends up in the piece doing the holding. Fastening 3/4 in stock to a 1 1/2 in rail, the longest thing that fits is a 2 in screw — 3/4 in through the face and 1 1/4 in of bite, which is 63% and as close to the ideal as that joint gets before the point comes out the back. The rule breaks down at both ends: on 1/4 in stock nothing three times as long is worth buying, and on 2 in stock no rail is thick enough to take a 6 in screw.
Why does the chart give me a clearance hole as well as a pilot?
Because the two holes do opposite jobs and a screw driven through one hole does neither. The pilot in the receiving piece is undersized so the threads bite; the clearance hole in the top piece is oversized so the threads pass straight through and the head alone pulls the joint together. Thread both pieces and the screw jacks them apart at exactly the rate it advances, which is the reason a joint sometimes finishes with a visible gap no amount of extra driving closes.
Is a #8 sheet metal screw the same diameter as a #8 wood screw?
Up to #12, yes — both are 0.164 in at #8 and 0.216 in at #12 — but the threads are different animals and at #14 even the diameters part company, with tapping screws at 0.250 in against 0.242 in for wood. A Type AB tapping screw is threaded to the point with a coarse form meant to cut into a hole in metal; a traditional wood screw carries an unthreaded shank that acts as its own clearance. Putting a sheet metal screw in wood works and holds well in thin material; putting a wood screw in sheet steel does not.
What countersink bit matches these screws?
An 82° included-angle countersink for US flat and oval heads, bored until its rim reaches the head diameter for that gauge — 0.332 in on a #8, 0.385 in on a #10. Metric hardware to DIN and ISO uses 90°, and mixing the two leaves either a ring of daylight around the head or a head that sits proud. Combination bits that cut pilot, clearance and countersink in one plunge are sized by gauge for exactly this reason.
Gauge, pilot holes and the two-thirds rule
The gauge scale is old enough that its arithmetic is the interesting part. ASME B18.6.1 fixes the basic major diameter at 0.060 in plus 0.013 in per gauge, and the flat head at 0.119 in plus 0.0266 in per gauge — two straight lines whose slopes happen to sit at almost exactly two to one. That is why a countersink can be sized by doubling the shank, why the visible head on a #10 is half again the head on a #6 even though the shank grew by a quarter, and why gauges are quoted in even numbers above #10: the odd sizes exist in the standard but nobody stocks a #11 when the step to #12 is thirteen thousandths.
Pilot holes are the part every chart gets half right. The number that matters is not the screw’s outside diameter but its root — the solid core between the threads — because a pilot bored to the root removes the wood the screw cannot compress and leaves the wood the threads have to bite. In dense hardwood that is the whole story: drill the root diameter and the screw goes in. In softwood the fibres crush willingly, so a hole around three quarters of the root gives the thread more to grip without splitting anything. Which is why this page carries two columns rather than one average, and why the gap between them is a single drill size at #8 and three at #18. Two things override both columns: proximity to an end grain edge, where a full-root pilot is the only thing standing between you and a split, and a soft alloy screw, where brass shears long before oak yields.
The length rule most often quoted — two thirds of the screw in the receiving piece — comes from the same place as the pilot table: withdrawal strength is roughly proportional to the depth of thread engaged, so length buys holding power until the point runs out of wood. It is a rule about proportion rather than a code requirement, and where the connection is structural the fastening schedule governs instead, which is the territory the nail size chart covers. A tapped hole in metal is the one case this chart only half covers: the machine-screw view gives the UNC and UNF thread for each gauge, but the tap drill for that thread has its own published table. A screw list is usually written at the same time as the board order, which the linear feet calculator turns into stock lengths.