Threads per inch, diameter, head size and Phillips driver size for inch wood screws — flat, oval and round heads, slotted or Phillips. ASME B18.6.1.
A wood screw has a sharp (gimlet) point and a coarse thread that cuts its own way into wood. It is sized by a number (gauge): the bigger the number, the fatter the screw — a #8 is about 0.16″ across, a #10 about 0.19″. Each size has one thread, so the number is all you need to match one. Flat heads sink flush into an 82° countersink and are measured overall; oval heads sink the same way but leave a dome showing, and are measured from the rim; round heads sit on top and are measured from under the head.
| Size | Threads per inch | Basic dia | Thread dia max / min |
|---|---|---|---|
| #2 | 26 | .086 | .090 / .079 |
| #3 | 24 | .099 | .103 / .092 |
| #4 | 22 | .112 | .116 / .105 |
| #5 | 20 | .125 | .129 / .118 |
| #6 | 18 | .138 | .142 / .131 |
| #7 | 16 | .151 | .155 / .144 |
| #8 | 15 | .164 | .168 / .157 |
| #9 | 14 | .177 | .181 / .170 |
| #10 | 13 | .190 | .194 / .183 |
| #12 | 11 | .216 | .220 / .209 |
| #14 | 10 | .242 | .246 / .235 |
| #16 | 9 | .268 | .272 / .261 |
| #18 | — | .294 | — |
| Size | Head dia A max / min | Head height H (nominal) | Phillips driver |
|---|---|---|---|
| #2 | .172 / .147 | .051 | — |
| #3 | .199 / — | .059 | — |
| #4 | .225 / .195 | .067 | — |
| #5 | .252 / .220 | .075 | — |
| #6 | .279 / .244 | .083 | — |
| #7 | .305 / .268 | .091 | — |
| #8 | .332 / .292 | .100 | — |
| #9 | .358 / .316 | .108 | — |
| #10 | .385 / .340 | .116 | — |
| #12 | .438 / .389 | .132 | — |
| #14 | .507 / .452 | .153 | — |
| #16 | .544 / .485 | .164 | — |
| Size | Head dia A max / min | Side height H (nominal) | Overall head height max / min | Phillips driver |
|---|---|---|---|---|
| #4 | .225 / .195 | .067 | .104 / .084 | #1 Phillips |
| #5 | .252 / .220 | .075 | .116 / .095 | #2 Phillips |
| #6 | .279 / .244 | .083 | .128 / .105 | #2 Phillips |
| #8 | .332 / .292 | .100 | .152 / .126 | #2 Phillips |
| #9 | .358 / .316 | .108 | .164 / .137 | #2 Phillips |
| #10 | .385 / .340 | .116 | .176 / .148 | — |
| #12 | .438 / .389 | .132 | .200 / .169 | #3 Phillips |
| Size | Head dia A max / min | Head height H max / min | Phillips driver |
|---|---|---|---|
| #2 | .162 / .146 | .069 / .059 | — |
| #4 | .211 / .193 | .086 / .075 | — |
| #5 | .236 / .217 | .095 / .083 | — |
| #6 | .260 / .240 | .103 / .091 | — |
| #7 | .285 / .264 | .111 / .099 | — |
| #8 | .309 / .287 | .120 / .107 | — |
| #9 | .334 / .311 | .128 / .115 | — |
| #10 | .359 / .334 | .137 / .123 | — |
| #12 | .408 / .382 | .153 / .139 | — |
| #14 | .457 / .429 | .170 / .155 | — |
Good to know
Thread length. A cut-thread wood screw is threaded about two-thirds of its length. A rolled-thread screw is threaded at least 4 times its basic diameter or two-thirds of its length, whichever is more; a screw too short for that is threaded as close to the head as practical.
Pilot holes depend on the wood, so they aren’t on this chart — the tables we checked don’t agree. Test in a scrap of the same wood, and use a bigger pilot in hardwood than in softwood. Brass screws are soft: drive a steel screw of the same size first, then the brass one.
Deck, cabinet, particle-board and other specialty screws are not made to this standard and aren’t on this chart. How to measure a flush head is on How to Measure a Flat Head; heads that sit on top on How to Measure a Bolt.
Quick answers
- How many threads per inch on a wood screw?
- #4 – 22, #6 – 18, #8 – 15, #10 – 13, #12 – 11, #14 – 10 (ASME B18.6.1).
- What is the diameter of a #10 wood screw?
- Its basic size is .190″; the thread measures .183″ to .194″ across (ASME B18.6.1).
- What Phillips bit fits an oval head wood screw?
- #4 screw – #1 bit, #5 screw – #2 bit, #6 screw – #2 bit, #8 screw – #2 bit, #9 screw – #2 bit, #12 screw – #3 bit (ASME B18.6.1).
- How big is the head on a #10 flat head wood screw?
- Up to .385″ across, .116″ tall (nominal), for an 82° countersink.
- How big is the head on a #10 round head wood screw?
- Up to .359″ across and .137″ tall.