The complete US screw gauge reference. Diameter in inches, decimal, metric, plus the right pilot hole size for hardwood and softwood. Standard wood screws, machine screws, and sheet metal screws all use the same gauge system.
US screw gauge to diameter
| Gauge | Major diameter (in) | Decimal | Metric (mm) | Pilot — hardwood | Pilot — softwood |
|---|---|---|---|---|---|
| #0 | 1/16" | 0.060 | 1.52 | 1/16" | 3/64" |
| #1 | 5/64" | 0.073 | 1.85 | 5/64" | 1/16" |
| #2 | 3/32" | 0.086 | 2.18 | 3/32" | 5/64" |
| #3 | 7/64" | 0.099 | 2.51 | 7/64" | 3/32" |
| #4 | 7/64" | 0.112 | 2.84 | 7/64" | 3/32" |
| #5 | 1/8" | 0.125 | 3.18 | 1/8" | 7/64" |
| #6 | 9/64" | 0.138 | 3.51 | 9/64" | 1/8" |
| #7 | 5/32" | 0.151 | 3.84 | 5/32" | 9/64" |
| #8 | 11/64" | 0.164 | 4.17 | 11/64" | 5/32" |
| #9 | 11/64" | 0.177 | 4.50 | 11/64" | 5/32" |
| #10 | 3/16" | 0.190 | 4.83 | 3/16" | 11/64" |
| #11 | 13/64" | 0.203 | 5.16 | 13/64" | 3/16" |
| #12 | 7/32" | 0.216 | 5.49 | 7/32" | 13/64" |
| #14 | 1/4" | 0.242 | 6.15 | 1/4" | 7/32" |
| #16 | 17/64" | 0.268 | 6.81 | 17/64" | 1/4" |
| #18 | 19/64" | 0.294 | 7.47 | 19/64" | 17/64" |
| #20 | 5/16" | 0.320 | 8.13 | 5/16" | 19/64" |
| #24 | 23/64" | 0.372 | 9.45 | 23/64" | 11/32" |
The two most common construction screw sizes — #8 and #10 — are highlighted. Most cordless drill bit kits include the right pilot drills for both.
Quick selection guide
| Application | Screw size | Length |
|---|---|---|
| Drywall to stud | #6 coarse | 1-1/4" to 1-5/8" |
| Cabinet to wall | #8 | 2-1/2" to 3" |
| 2×4 framing | #8 or #10 | 2-1/2" to 3-1/2" |
| Deck board to joist | #8 or #10 deck | 2-1/2" to 3" |
| Subfloor to joist | #8 or #10 | 2-1/2" |
| Hinge to door | #8 or #10 | 1" to 1-1/2" |
| Furniture assembly | #6 or #8 | 1" to 1-1/2" |
| Light fixture mounting | #6 or #8 | 1" to 2" |
Length conventions
Screw length is measured from the underside of the head to the tip for flat-head and bugle-head screws (most wood screws). For pan-head and round-head screws, length is measured from the bottom of the head to the tip.
Standard lengths in 1/4" increments from 3/8" to 4", then 1/2" increments to 6", then 1" increments above 6". Construction screws (GRK, SPAX) come in lengths up to 12".
Coarse vs fine thread
| Material | Use |
|---|---|
| Softwood (pine, fir, cedar) | Coarse thread |
| Hardwood (oak, maple, walnut) | Fine thread |
| Drywall to wood stud | Coarse thread |
| Drywall to metal stud | Fine thread |
| Particleboard / MDF | Coarse thread |
| Sheet metal | Sheet metal screws (different thread design) |
| Machine assembly | Machine screws (matched to tapped hole) |
Head type quick reference
- Phillips: Most common. Cams out under high torque (a feature, not a bug — prevents over-driving).
- Square (Robertson): Better grip than Phillips. Common on cabinet screws and Canadian framing.
- Torx (star): Best grip, no cam-out. Standard on most construction screws (GRK, SPAX, structural).
- Hex: Lag bolts and structural screws over 1/4" diameter. Needs a hex bit or socket.
- Slotted: Largely obsolete for construction. Found on antique restoration and decorative hardware.
For construction work, Torx is the right choice if you have a choice — it grips better and lasts longer than Phillips. For everything else, Phillips is fine.
Frequently asked questions
What does the screw gauge number mean?
The gauge number (e.g. #6, #8, #10) refers to the major diameter of the screw shank, not the head. Higher gauge = larger diameter. The relationship is: diameter (inches) = (gauge × 0.013) + 0.060. So #6 = 0.138", #8 = 0.164", #10 = 0.190".
What size screw for 2x4 framing?
#8 × 2-1/2" or #8 × 3" wood screws for general 2×4 framing. For deck building or load-bearing connections, use #10 × 3" structural screws (e.g. GRK RSS, SPAX HCR). For framing in pressure-treated lumber, always use coated/stainless screws to prevent corrosion.
What's the difference between coarse and fine thread screws?
Coarse thread: Fewer threads per inch — bites into soft materials (wood, drywall, soft plastic) quickly and holds well. Fine thread: More threads per inch — better for hardwoods, metal-to-metal, and applications where you need precise depth control. Coarse threads strip out faster in soft material but are forgiving.
Do I need to pre-drill for screws?
Pilot holes prevent splitting in hardwoods, near board edges, and for any screw over #8. For softwoods (pine, fir) with screws under #8, you can usually skip pilot holes for speed. The pilot drill should be slightly smaller than the screw shank — see the recommended pilot column in the chart.
What's the head type difference?
Phillips (cross): Most common, but cams out under torque. Square / Robertson: Better grip, harder to find. Star / Torx: Best grip, common on construction screws. Slotted: Largely obsolete for construction. Hex: Common on lag bolts and structural screws — needs a hex bit/socket.