Chart · Drilling and fasteners

Wood Screw Size Chart: Gauge, Diameter, Threads and Head

A wood screw's number is its gauge, and the gauge fixes its shank diameter, its threads and its head. The sheet above gives those dimensions for inch wood screws from #0 to #24, as the ASME standard sets them, with the Wood Handbook's shank diameters beside them. Below: how the numbers relate, where one maker's head sizes differ from the standard's, and what the gauge leaves out — length, root diameter and modern screws.

About this guide

Written by: BenchRoot Editorial
Published: 15 October 2026
Reading time: 2 min
Level: Beginner

Wood screws

The Gauge Is a Diameter.

Answers: How thick is a #8 screw, and how wide is its head?
Units: gauge · in · mm

A wood screw's number is its gauge, and each gauge stands for one basic shank diameter. The table gives the inch-series wood screw dimensions of ASME B18.6.1 as published in engineering references, with the Wood Handbook's own shank diameters beside them. Length is chosen separately; it is not part of the gauge.

A flat-head wood screw, not to scale. The gauge fixes the shank (basic) diameter and the head; the root diameter, where the threads start, is smaller — the Wood Handbook puts it at about two-thirds of the shank on average.

Inch wood screws by gaugeASME B18.6.1 values as reproduced by Engineers Edge; flat head diameter as maximum to minimum. The last column is the Wood Handbook's Table 8–9, which lists #4 to #24 only. Millimetres are computed.
Gauge	Threads per inch	Basic (shank) diameter	Flat head diameter	Wood Handbook shank
#0	32	0.060 in (1.52 mm)	0.119–0.099 in	Not listed
#1	28	0.073 in (1.85 mm)	0.146–0.123 in	Not listed
#2	26	0.086 in (2.18 mm)	0.172–0.147 in	Not listed
#3	24	0.099 in (2.51 mm)	0.199–0.171 in	Not listed
#4	22	0.112 in (2.84 mm)	0.225–0.195 in	0.112 in (2.84 mm)
#5	20	0.125 in (3.17 mm)	0.252–0.220 in	0.125 in (3.18 mm)
#6	18	0.138 in (3.51 mm)	0.279–0.244 in	0.138 in (3.51 mm)
#7	16	0.151 in (3.84 mm)	0.305–0.268 in	0.151 in (3.84 mm)
#8	15	0.164 in (4.17 mm)	0.332–0.292 in	0.164 in (4.17 mm)
#9	14	0.177 in (4.50 mm)	0.358–0.316 in	0.177 in (4.50 mm)
#10	13	0.190 in (4.83 mm)	0.385–0.340 in	0.190 in (4.83 mm)
#12	11	0.216 in (5.49 mm)	0.438–0.389 in	0.216 in (5.49 mm)
#14	10	0.242 in (6.15 mm)	0.507–0.452 in	0.242 in (6.15 mm)
#16	9	0.268 in (6.81 mm)	0.544–0.485 in	0.268 in (6.81 mm)
#18	8	0.294 in (7.47 mm)	0.635–0.568 in	0.294 in (7.47 mm)
#20	8	0.320 in (8.13 mm)	0.650–0.582 in	0.320 in (8.13 mm)
#24	7	0.372 in (9.45 mm)	0.762–0.685 in	0.372 in (9.45 mm)

These are traditional inch wood screws. Construction, structural, cabinet and pocket-hole screws are made to their makers' own specifications; read the box, not this table.
Metric screws are sized in millimetres and are not tabulated here. A metric screw close to a gauge's diameter is not thereby a substitute for it.
Number sizes also name machine screws, which are made to a different standard. This sheet is for wood screws only.

Reading a gauge

The gauge names the shank: a #8 screw has a basic diameter of 0.164 in (4.17 mm) whoever makes it. The Wood Handbook's Table 8–9 lists the same shank diameters as the standard for every gauge from #4 to #24, and every value in both follows one pattern — 0.060 in plus 0.013 in per gauge number. The standard's tables, not the pattern, are what govern. Threads per inch fall as the gauge rises, from 32 on a #0 to 7 on a #24.

Head sizes: standard and one maker

Flat head diameter by gauge, two sources
Gauge	ASME B18.6.1, max–min	W. L. Fuller's actual head size
#6	0.279–0.244 in	9/32 in (0.281)
#8	0.332–0.292 in	11/32 in (0.344)
#10	0.385–0.340 in	25/64 in (0.391)
#12	0.438–0.389 in	7/16 in (0.438)

Fuller gives each head as a fraction, beside the minimum counterbore that clears it. Across its chart every head is at least the standard's minimum; most are at or above its maximum, but #2 (by 0.0001 in), #5, #12, #14 and #18 fall below it. Both are shown, not reconciled. Fuller's counterbores by gauge are on the pilot hole chart.

What the gauge does not tell you

Length. Gauge and length are chosen separately. Fuller's rule of thumb is about a third of the length in the piece being attached and two-thirds in the piece it is attached to.
Root diameter. The core the threads sit on is smaller than the shank — about two-thirds of it on average, the Wood Handbook says. It sets the pilot hole, and is not tabulated here because no source reviewed gives it by gauge.
Head shape. Flat heads sit flush in a countersink; the Handbook notes oval and round heads for appearance, and round heads where countersinking is objectionable.
Modern screws. Construction, structural, cabinet and pocket-hole screws, and the tapping and self-drilling screws the Handbook mentions, are made to their makers' own specifications; this chart is for traditional wood screws.
Metric screws. They are named in millimetres and are not listed. A metric screw near a gauge's diameter is not thereby an equivalent of it.

To drill for a screw, take its gauge to the pilot hole chart; to find a bit for a decimal size, use the drill bit size chart. The other drilling pages are mapped in the guide to wood drill bits.

Before choosing a screw

4 checks

Gauge read from the box, not guessed from the head
Length chosen for the two pieces it joins
Head shape chosen for flush, raised or plugged
Maker's own data used for construction or pocket-hole screws

Sources

ASME B18.6.1 itself is sold and was not reviewed; its values are as two engineering references reproduce them, and they agree with the Wood Handbook's Table 8–9 wherever both list a gauge. Millimetres are computed from inches. Root diameters, lengths and metric screws are not tabulated, because no source reviewed gives them.

1.
ANSI Wood Screws per ANSI B18.6.1 (flat, pan and oval head size table). Engineers Edge.
Used for: ASME B18.6.1 inch wood screw dimensions as reproduced: threads per inch, basic screw diameter and flat head diameter (maximum and minimum) for gauges #0 to #24.
2.
Wood Screws: Dimensions and Mechanical Specs (ASME B18.6.1 flat head table). AFT Fasteners.
Used for: A second reproduction of the standard's flat head table, agreeing on head diameters (for example #2 at 0.172–0.147 in and #16 at 0.544–0.485 in).
3.
Chapter 8: Fastenings. USDA Forest Service, Forest Products Laboratory — Wood Handbook, FPL-GTR-282, 2021.
Used for: Table 8–9, screw shank diameters for gauges #4 to #24 in millimetres and inches; flat, oval and round heads; a root diameter averaging about two-thirds of the shank; tapping and self-drilling screws.
4.
Countersinks, Counterbores, Tapered Drills, Plug Cutters catalog (PDF dated 2006), page 4: Recommendations for Wood Screw Predrill Sizes and Lengths. W. L. Fuller, Inc., 2006.
Used for: A countersink maker's actual head sizes and counterbore diameters by gauge (#2 to #18), and a screw-length rule of about a third in the piece attached and two-thirds in the piece it is attached to.

About this chart

Written by BenchRoot Editorial. How BenchRoot sources, checks and corrects its pages is set out in the Editorial Policy. If something here is wrong or unclear, the contact page explains how to tell us.