What size is a #8 screw?

0.164 in (4.17 mm) across the shank, from Wood Handbook Table 8-9. Machine screws use the same number: an 8-32 is 0.164 in with 32 threads per inch.

What size is a #10 screw?

0.190 in (4.83 mm). As a machine screw it comes as 10-24 coarse or 10-32 fine, from NBS Handbook H28.

What size pilot hole for a #8 screw?

About 0.077 in in softwood, a 5/64 in drill, and 0.098 in in hardwood, a 3/32 in drill, using the Wood Handbook's lead-hole rule of 70% and 90% of a root about two thirds of the shank.

What size pilot hole for a #10 screw?

About 0.089 in in softwood, a 3/32 in drill, and 0.114 in in hardwood, a 7/64 in drill, by the same handbook rule.

What size pilot hole for a 3/8 lag screw?

In Douglas fir or southern pine, 60% to 75% of the shank: 0.225 to 0.281 in, so a 1/4 in drill. In cedar or white pine 0.150 to 0.263 in; in oak 0.244 to 0.319 in.

How are screw sizes measured?

Numbered screws by the shank diameter, 0.060 in plus 0.013 in per number. Lag screws and bolts by their diameter in fractions of an inch. Length is measured separately.

What does 1/4-20 mean?

A 1/4 in diameter bolt with 20 threads per inch, the coarse (UNC) thread for that size. The fine thread is 1/4-28.

Is a #10 screw bigger than a #8?

Yes. A larger number is a thicker screw, 0.013 in per number: #8 is 0.164 in and #10 is 0.190 in.

Home › Framing › Screw Size Chart

Screw Size Chart

USDA Wood Handbook Tables 8-9 and 8-11, NBS Handbook H28 Table 2.7

A #8 screw is 0.164 in across, and so is every other #8: wood screw, machine screw or sheet metal screw. The whole numbered system is one line of arithmetic, 0.060 in plus 0.013 in per number, and it reproduces all 14 wood screw sizes in the Forest Service's Wood Handbook and every numbered size in the federal screw thread standard. The chart below gives the diameters, the pilot holes the handbook's rules produce, the threads per inch for machine screws and bolts, and the lead holes for lag screws.

Look up a screw

Screw type

Size

Wood (wood and lag screws)

Numbered sizes are wood and machine screws; fractional sizes are lag screws and bolts.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

Worked example

A #8 wood screw into pine, then into oak.

Table 8-9. #8 is 0.164 in, and 0.060 + 0.013 × 8 = 0.164.
Root. The handbook says the root averages about two thirds of the shank: 0.109 in.
Pilot. About 70% of the root in softwood is 0.077 in, a 5/64 in drill; about 90% in hardwood is 0.098 in, a 3/32 in drill.

The clearance hole through the top board is the shank size or slightly smaller: 5/32 in for a #8.

The formula

The numbered sizes follow one rule:

diameter (in) = 0.060 + 0.013 × screw number

#0: 0.060 in, the start of the series
each number: adds 0.013 in, so #10 is 0.190 and #12 is 0.216

The rule is not printed in either source as a formula. It is the pattern in their tables: it matches all 14 rows of Wood Handbook Table 8-9 and all ten numbered rows of H28 Table 2.7 exactly.

Where the numbers come from

Wood screw diameters are Wood Handbook Table 8-9 (USDA Forest Products Laboratory, FPL-GTR-282, 2021), screw numbers 4 to 24 in inches and millimeters. The pilot holes are the handbook's own rules applied to them: lead holes of about 70% of the root diameter in softwoods and 90% in hardwoods, with the root averaging about two thirds of the shank. The lag screw lead holes are its percentages of the shank for three wood densities, and the lag diameters are the ones in its Table 8-11.

Threads per inch are NBS Handbook H28, Table 2.7, the federal unified screw thread series (1969). It lists numbered sizes by diameter, 0.060 to 0.216 in, and those diameters are exactly the Wood Handbook's for the same numbers.

Wood screw sizes and pilot holes

Wood Handbook Table 8-9 diameters; pilot holes from the handbook's lead-hole rule, nearest 1/64 in drill
Screw	Shank, in	Shank, mm	Pilot, softwood, in	Pilot, hardwood, in	Shank clearance drill
#4	0.112	2.84	0.052 (3/64 in)	0.067 (1/16 in)	7/64 in
#5	0.125	3.18	0.058 (1/16 in)	0.075 (5/64 in)	1/8 in
#6	0.138	3.51	0.064 (1/16 in)	0.083 (5/64 in)	1/8 in
#7	0.151	3.84	0.070 (5/64 in)	0.091 (3/32 in)	9/64 in
#8	0.164	4.17	0.077 (5/64 in)	0.098 (3/32 in)	5/32 in
#9	0.177	4.50	0.083 (5/64 in)	0.106 (7/64 in)	11/64 in
#10	0.190	4.83	0.089 (3/32 in)	0.114 (7/64 in)	3/16 in
#11	0.203	5.16	0.095 (3/32 in)	0.122 (1/8 in)	3/16 in
#12	0.216	5.49	0.101 (3/32 in)	0.130 (1/8 in)	13/64 in
#14	0.242	6.15	0.113 (7/64 in)	0.145 (9/64 in)	15/64 in
#16	0.268	6.81	0.125 (1/8 in)	0.161 (5/32 in)	17/64 in
#18	0.294	7.47	0.137 (9/64 in)	0.176 (11/64 in)	9/32 in
#20	0.320	8.13	0.149 (5/32 in)	0.192 (3/16 in)	5/16 in
#24	0.372	9.45	0.174 (11/64 in)	0.223 (7/32 in)	23/64 in

The pilot figures are the handbook's rule, not a manufacturer's chart: 70% (softwood) or 90% (hardwood) of a root diameter taken as two thirds of the shank. A screw with an unusually deep or shallow thread needs the hole matched to its actual root.

Which screw lengths go with which numbers

Wood Handbook Table 8-8, screw lengths and the gauge range its withdrawal equation covers
Length	Screw numbers
1/2 in	#1 to #6
3/4 in	#2 to #11
1 in	#3 to #12
1 1/2 in	#5 to #14
2 in	#7 to #16
2 1/2 in	#9 to #18
3 in	#12 to #20

The table exists to bound the handbook's withdrawal equation, but it doubles as a guide to sensible pairings: a 3 in screw thinner than #12 is outside it.

Machine screw threads

Unified threads per inch, numbered sizes, NBS Handbook H28 Table 2.7
Size	Diameter, in	Coarse UNC	Fine UNF	Extra fine UNEF	Common callout
#0	0.060	-	80	-	0-80
#1	0.073	64	72	-	1-64
#2	0.086	56	64	-	2-56
#3	0.099	48	56	-	3-48
#4	0.112	40	48	-	4-40
#5	0.125	40	44	-	5-40
#6	0.138	32	40	-	6-32
#8	0.164	32	36	-	8-32
#10	0.190	24	32	-	10-24
#12	0.216	24	28	32	12-24

#0 has no coarse thread in the series, and #7, #9 and #11 are wood screw sizes only. An 8-32 is a #8 screw with 32 threads per inch; a 10-24 is a #10 with 24.

Bolt thread sizes

Unified threads per inch, 1/4 to 1 in, NBS Handbook H28 Table 2.7
Size	Diameter, in	Coarse UNC	Fine UNF	Extra fine UNEF	Common callout
1/4 in	0.25	20	28	32	1/4-20
5/16 in	0.3125	18	24	32	5/16-18
3/8 in	0.375	16	24	32	3/8-16
7/16 in	0.4375	14	20	28	7/16-14
1/2 in	0.5	13	20	28	1/2-13
9/16 in	0.5625	12	18	24	9/16-12
5/8 in	0.625	11	18	24	5/8-11
3/4 in	0.75	10	16	20	3/4-10
7/8 in	0.875	9	14	20	7/8-9
1 in	1	8	12	20	1-8

Lag screw lead holes

Lead hole for the threaded part, inches, from the Wood Handbook percentages of the shank
Lag screw	Cedar, white pine (40% to 70%)	Douglas fir, southern pine (60% to 75%)	Oak and dense hardwoods (65% to 85%)
3/16 in	0.075 to 0.131	0.113 to 0.141	0.122 to 0.159
1/4 in	0.100 to 0.175	0.150 to 0.188	0.163 to 0.212
5/16 in	0.125 to 0.219	0.188 to 0.234	0.203 to 0.266
3/8 in	0.150 to 0.263	0.225 to 0.281	0.244 to 0.319
7/16 in	0.175 to 0.306	0.263 to 0.328	0.284 to 0.372
1/2 in	0.200 to 0.350	0.300 to 0.375	0.325 to 0.425
5/8 in	0.250 to 0.438	0.375 to 0.469	0.406 to 0.531
3/4 in	0.300 to 0.525	0.450 to 0.562	0.487 to 0.637
7/8 in	0.350 to 0.613	0.525 to 0.656	0.569 to 0.744
1 in	0.400 to 0.700	0.600 to 0.750	0.650 to 0.850

The handbook puts the small end of each range on small lag screws and the large end on large ones, and adds that long screws want holes slightly larger again. The shank's own hole is the full shank diameter. A 3/8 in lag in Douglas fir lands on a 1/4 in drill, inside the 0.225 to 0.281 in range.

When a screw replaces a nail

The IRC has one table that converts nails to screws by diameter, Table R703.3(2), for siding on cold-formed steel framing:

IRC 2021 Table R703.3(2), screw substitution for siding nails
Nail diameter	Minimum screw	Screw diameter
0.099 in	No. 6	0.138 in
0.113 in	No. 7	0.151 in
0.120 in	No. 8	0.164 in

Each screw is 1.34 to 1.39 times the diameter of the nail it replaces. For decks and treated lumber the code's fastener rules are on the deck screw calculator.

Frequently asked questions

What size is a #8 screw?: 0.164 in (4.17 mm) across the shank, from Wood Handbook Table 8-9. Machine screws use the same number: an 8-32 is 0.164 in with 32 threads per inch.
What size is a #10 screw?: 0.190 in (4.83 mm). As a machine screw it comes as 10-24 coarse or 10-32 fine, from NBS Handbook H28.
What size pilot hole for a #8 screw?: About 0.077 in in softwood, a 5/64 in drill, and 0.098 in in hardwood, a 3/32 in drill, using the Wood Handbook's lead-hole rule of 70% and 90% of a root about two thirds of the shank.
What size pilot hole for a #10 screw?: About 0.089 in in softwood, a 3/32 in drill, and 0.114 in in hardwood, a 7/64 in drill, by the same handbook rule.
What size pilot hole for a 3/8 lag screw?: In Douglas fir or southern pine, 60% to 75% of the shank: 0.225 to 0.281 in, so a 1/4 in drill. In cedar or white pine 0.150 to 0.263 in; in oak 0.244 to 0.319 in.
How are screw sizes measured?: Numbered screws by the shank diameter, 0.060 in plus 0.013 in per number. Lag screws and bolts by their diameter in fractions of an inch. Length is measured separately.
What does 1/4-20 mean?: A 1/4 in diameter bolt with 20 threads per inch, the coarse (UNC) thread for that size. The fine thread is 1/4-28.
Is a #10 screw bigger than a #8?: Yes. A larger number is a thicker screw, 0.013 in per number: #8 is 0.164 in and #10 is 0.190 in.

Check these numbers yourself

USDA Forest Products Laboratory, Wood Handbook, Chapter 8 Fastenings (FPL-GTR-282)Wood Handbook: Wood as an Engineering Material, General Technical Report FPL-GTR-282, USDA Forest Service, Forest Products Laboratory, 2021. Chapter 8, pages 8-2 to 8-13. A US government publication in the public domain. Read 2026-10-02.
Screw-Thread Standards for Federal Services, Handbook H28 (1969), Part INational Bureau of Standards, US Department of Commerce, 1969 edition, Part I: Unified, UNJ and unified miniature screw threads. Table 2.7, unified standard screw thread series. A US government publication in the public domain, hosted by NIST. Read 2026-10-02.
Utah Residential Code 2021, Chapter 7 Wall Covering, Table R703.3(2)IRC 2021 as adopted in Utah, read through UpCodes. Table R703.3(2) screw fastener substitution. Read 2026-10-02.

How every figure here is verified: Sources & Method. Who builds this: Lucas Pradella, and the method is on About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-10-02. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Dimensions and drilling rules, not load values. Structural connections follow the IRC tables and the fastener maker's evaluation report; a pilot hole chart does not size a connection.