Screw Size Chart: Gauge Numbers, Diameters, Threads and Pilot Holes
How to read screw sizes such as #8 × 1 1/4, 10-32 and M6 × 1.0, with exact diameters, pilot and clearance holes, Allen key and wrench sizes, and the right screw length for common jobs.
A screw's gauge number sets its outside thread diameter: #6 = 0.138 in (3.5 mm), #8 = 0.164 in (4.2 mm) and #10 = 0.190 in (4.8 mm). Each step up adds 0.013 in, so diameter = 0.060 + 0.013 × gauge. Metric screws state the diameter directly: an M6 screw is 6 mm.
- #6 screw
- 0.138 in (3.5 mm)
- #8 screw
- 0.164 in (4.2 mm)
- #10 screw
- 0.190 in (4.8 mm)
- Formula
- 0.060 + 0.013 × N in
On this page
Screw gauge chart: diameter in inches and mm
US screw sizes are written as gauge × length, for example #8 × 1 1/4. The gauge is the outside diameter of the thread. The chart applies to wood, sheet metal and machine screws, which share the same gauge diameters.
| Gauge | Diameter (in) | Nearest fraction | Diameter (mm) | Typical uses |
|---|---|---|---|---|
| #0 | 0.060 | 1/16 | 1.52 | Eyeglasses, models |
| #2 | 0.086 | 3/32 | 2.18 | Small hinges, electronics |
| #4 | 0.112 | 7/64 | 2.84 | Jewelry boxes, light hardware |
| #6 | 0.138 | 9/64 | 3.51 | Drywall screws, cabinet hinges, light trim |
| #8 | 0.164 | 5/32+ | 4.17 | General woodwork, deck screws, door hinges |
| #9 | 0.177 | 11/64 | 4.50 | Heavier hinges and framing screws |
| #10 | 0.190 | 3/16 | 4.83 | Cabinets to studs, decks, electrical plates (10-32) |
| #12 | 0.216 | 7/32 | 5.49 | Heavy hinges, ledgers, metal framing |
| #14 | 0.242 | 1/4- | 6.15 | Heavy brackets, roofing screws |
Gauge diameters follow ASME B18.6.1 and B18.6.3: 0.060 in plus 0.013 in per gauge step. Odd gauges (#3, #5, #7) exist but are hard to find in stores.
Pilot and clearance hole sizes for wood screws
A pilot hole stops wood splitting and is sized to the screw's core (root) diameter. A clearance hole in the top board lets the screw pull the boards tight instead of jacking them apart. Shop charts differ by about 1/64 in, so test in scrap first.
| Screw | Pilot, softwood | Pilot, hardwood | Clearance hole |
|---|---|---|---|
| #4 | 1/16 in | 5/64 in | 1/8 in |
| #6 | 5/64 in | 3/32 in | 5/32 in |
| #8 | 3/32 in | 7/64 in | 11/64 in |
| #10 | 7/64 in | 1/8 in | 13/64 in |
| #12 | 1/8 in | 9/64 in | 15/64 in |
| #14 | 9/64 in | 5/32 in | 17/64 in |
Self-drilling construction screws (most deck, cabinet and structural screws) do not need a pilot in softwood. Near the end of a board, in hardwoods such as oak and maple, and in anything thinner than about 3/4 in, drill anyway. See the drill bit size chart for decimal and metric equivalents.
Machine screw sizes and threads (UNC and UNF)
A machine screw size such as 10-32 gives gauge (or fractional diameter) and threads per inch. Coarse (UNC) is the default; fine (UNF) holds better in thin metal and resists vibration.
| Size | Diameter | UNC (coarse) | UNF (fine) | Tap drill (coarse) |
|---|---|---|---|---|
| #4 | 0.112 in (2.84 mm) | 4-40 | 4-48 | #43 |
| #6 | 0.138 in (3.51 mm) | 6-32 | 6-40 | #36 |
| #8 | 0.164 in (4.17 mm) | 8-32 | 8-36 | #29 |
| #10 | 0.190 in (4.83 mm) | 10-24 | 10-32 | #25 |
| #12 | 0.216 in (5.49 mm) | 12-24 | 12-28 | #16 |
| 1/4 in | 0.250 in (6.35 mm) | 1/4-20 | 1/4-28 | #7 |
| 5/16 in | 0.3125 in (7.94 mm) | 5/16-18 | 5/16-24 | F |
| 3/8 in | 0.375 in (9.53 mm) | 3/8-16 | 3/8-24 | 5/16 in |
| 1/2 in | 0.500 in (12.7 mm) | 1/2-13 | 1/2-20 | 27/64 in |
Everyday examples: wall plates and outlets use 6-32 screws; electrical boxes use 8-32 or 10-32 ground screws; rack-mount equipment uses 10-32 or 12-24; camera tripods use 1/4-20. Full tap-drill tables for number and letter drills are on the drill bit page.
Metric screw sizes (M3 to M12)
Metric sizes are written as M, diameter in mm, × pitch (the distance between threads) × length: M6 × 1.0 × 20 is a 6 mm screw with 1 mm thread pitch, 20 mm long. If pitch is omitted, it is the standard coarse thread.
| Size | Coarse pitch | Tap drill | Hex key (socket cap) | Wrench (hex head, ISO) | Closest inch size |
|---|---|---|---|---|---|
| M3 | 0.5 mm | 2.5 mm | 2.5 mm | 5.5 mm | #4 (2.84 mm) |
| M4 | 0.7 mm | 3.3 mm | 3 mm | 7 mm | #8 (4.17 mm) |
| M5 | 0.8 mm | 4.2 mm | 4 mm | 8 mm | #10 (4.83 mm) |
| M6 | 1.0 mm | 5.0 mm | 5 mm | 10 mm | 1/4 in (6.35 mm) |
| M8 | 1.25 mm | 6.8 mm | 6 mm | 13 mm | 5/16 in (7.94 mm) |
| M10 | 1.5 mm | 8.5 mm | 8 mm | 16 mm (17 mm DIN) | 3/8 in (9.53 mm) |
| M12 | 1.75 mm | 10.2 mm | 10 mm | 18 mm (19 mm DIN) | 1/2 in is larger (12.7 mm) |
Inch and metric threads never interchange, even where the diameters are close: an M5 nut on a 10-24 screw will start and then bind. If a screw spins freely and wobbles or jams after two turns, check the thread system before forcing it.
European wood screws are sold by diameter × length in mm, typically 3.0, 3.5, 4.0, 4.5, 5.0 and 6.0 mm. Rough US equivalents: 3.5 mm ≈ #6, 4.0 mm ≈ #8, 4.5 mm ≈ #9, 5.0 mm ≈ #10, 6.0 mm ≈ #12 to #14.
Allen key and wrench sizes for inch screws
| Screw | Hex key, socket head cap screw | Wrench, hex bolt |
|---|---|---|
| #4 | 3/32 in | varies by head style |
| #6 | 7/64 in | varies by head style |
| #8 | 9/64 in | varies by head style |
| #10 | 5/32 in | 5/16 in on most hex washer-head self-drillers |
| 1/4 in | 3/16 in | 7/16 in |
| 5/16 in | 1/4 in | 1/2 in |
| 3/8 in | 5/16 in | 9/16 in |
| 1/2 in | 3/8 in | 3/4 in |
Button-head and flat-head socket screws use a smaller key than socket cap screws of the same diameter (for example 5/32 in for a 1/4 in button head, against 3/16 in for a socket cap screw). Small hex-head screws come in several head sizes, so check with a nut driver set..
Screw head drive sizes
- Phillips: PH1 for #3-#4 screws, PH2 for #5-#10 (the most common bit), PH3 for #12 and up.
- Square (Robertson): #1 green for #5-#7, #2 red for #8-#10, #3 black for #12-#14. Square drive cams out less, which is why deck and cabinet screws favor it.
- Torx (star): T15 for #6-#7, T20 or T25 for #8-#10, T30 and up for structural screws. Brands differ, so use the bit packed in the box.
- Slotted: blade width should match the head; mostly found on hardware and old work.
Which screw length to use
A wood screw should pass through the top piece and bury at least half its length, and ideally two-thirds, in the base. In framing, aim for at least 1 in of thread in the stud.
| Job | Typical screw | Why |
|---|---|---|
| 1/2 in drywall on wood studs | #6 × 1 1/4 in coarse thread | About 3/4 in into the stud |
| 5/8 in drywall on wood | #6 × 1 5/8 in coarse thread | About 1 in into the stud |
| Drywall on steel studs | #6 × 1 1/4 in fine thread | Fine threads grip thin steel |
| 5/4 decking (1 in thick) | #8 or #10 × 2 1/2 in | 1 1/2 in into the joist |
| 2x decking (1 1/2 in thick) | #10 × 3 in | 1 1/2 in into the joist |
| Wall cabinets to studs | #10 × 2 1/2 to 3 in cabinet screw | Through 3/4 in rail and drywall into stud |
| Door hinge to jamb | #9 or #10 × 3 in (one per hinge) | Reaches the framing behind the jamb |
| Pocket-hole joints in 3/4 in stock | #8 × 1 1/4 in | Sized for the jig's angle |
For hanging kitchen cabinets, use screws rated for the job, never drywall screws, which are brittle and can snap under shear. Drywall thickness and screw spacing are covered in the drywall size guide; deck framing in deck dimensions. Screws hold better than nails in withdrawal, but structural connections such as ledgers need the specific screw the manufacturer or code lists.
Frequently asked questions
What is the diameter of a #10 screw?
0.190 in (4.83 mm), just over 3/16 in (0.1875 in). The same gauge applies to #10 wood, sheet metal and machine screws.
What gauge is a 1/4 inch screw?
No gauge is exactly 1/4 in. The closest is #14 at 0.242 in (6.1 mm). Above #14, screws are usually sold by fractional size (1/4, 5/16, 3/8 in) instead of by gauge, especially lag and machine screws.
What does 8-32 mean on a screw?
It is a machine screw thread: #8 gauge (0.164 in diameter) with 32 threads per inch. A 10-24 is #10 with 24 threads per inch, and 1/4-20 is a 1/4 in screw with 20 threads per inch.
What are standard screw sizes in mm?
Common metric machine screws are M3, M4, M5, M6, M8, M10 and M12, where the number is the diameter in mm. Wood screws in Europe come in 3.0, 3.5, 4.0, 4.5, 5.0 and 6.0 mm diameters. A 4.0 mm wood screw is close to a US #8.
What Allen key fits an M6 or 1/4 inch screw?
A standard M6 socket head cap screw takes a 5 mm hex key, and a 1/4 in socket head cap screw takes a 3/16 in key. Button and flat head socket screws take one size smaller.
Is screw gauge the same as sheet metal gauge?
No. Screw gauge is the screw's diameter number (#8, #10). Sheet metal gauge is the thickness of the metal being joined. A #8 sheet metal screw can be used in 20-gauge or 26-gauge steel; the two numbers are unrelated.