Socket Screw Torque Chart & Hex Key Size Reference
Socket screws are driven by an internal hex key, which means torque is applied through a relatively small contact area inside the head. Using the right torque value—and the right hex key size—is essential for proper clamping force without stripping the socket or yielding the fastener. This page provides reference torque values and hex key sizes for alloy steel and stainless socket head cap screws, button head screws, flat head screws, and set screws in both inch and metric sizes.
Eugene Fastener stocks socket screws in alloy steel (ASTM A574), 18-8 stainless, and 316 stainless in a broad range of inch and metric sizes from our warehouse in Eugene, Oregon.
In This Reference:
- Hex Key Sizes — Inch & Metric
- SHCS Torque — Inch Alloy
- SHCS Torque — Metric
- Button Head & Flat Head Notes
- Set Screw Hex Key Sizes
- Stainless Steel Torque Factor
- How to Apply Torque Correctly
- Frequently Asked Questions
Hex Key Sizes for Socket Screws
Each socket screw thread size uses a standardized internal hex drive size. The tables below list the correct hex key (Allen wrench) for socket head cap screws in inch and metric series. Note that button head socket screws use a smaller hex key than a socket head cap screw of the same thread size—see the button head section below.
Inch Series — Socket Head Cap Screws (ASME B18.3)
| Thread Size | Hex Key (in) | Hex Key (decimal) |
|---|---|---|
| #2-56 | 5/64” | 0.0781” |
| #4-40 | 3/32” | 0.0938” |
| #6-32 | 7/64” | 0.1094” |
| #8-32 | 9/64” | 0.1406” |
| #10-24 / #10-32 | 5/32” | 0.1563” |
| 1/4-20 / 1/4-28 | 3/16” | 0.1875” |
| 5/16-18 / 5/16-24 | 1/4” | 0.2500” |
| 3/8-16 / 3/8-24 | 5/16” | 0.3125” |
| 7/16-14 / 7/16-20 | 3/8” | 0.3750” |
| 1/2-13 / 1/2-20 | 3/8” | 0.3750” |
| 5/8-11 / 5/8-18 | 1/2” | 0.5000” |
| 3/4-10 / 3/4-16 | 5/8” | 0.6250” |
| 7/8-9 / 7/8-14 | 3/4” | 0.7500” |
| 1”-8 / 1”-12 | 3/4” | 0.7500” |
Metric Series — Socket Head Cap Screws (ISO 4762 / DIN 912)
| Thread Size | Hex Key (mm) |
|---|---|
| M2 | 1.5 mm |
| M2.5 | 2 mm |
| M3 | 2.5 mm |
| M4 | 3 mm |
| M5 | 4 mm |
| M6 | 5 mm |
| M8 | 6 mm |
| M10 | 8 mm |
| M12 | 10 mm |
| M14 | 12 mm |
| M16 | 14 mm |
| M20 | 17 mm |
| M24 | 19 mm |
Socket Head Cap Screw Torque — Inch Alloy Steel
Torque values below are for alloy steel socket head cap screws (ASTM A574), dry installation (no lubricant), at approximately 65% of proof load. These are reference values; always verify against your application’s joint requirements and the fastener manufacturer’s specification. If a lubricant or thread locking compound is used, reduce torque by 20–25%.
| Thread | Hex Key | Torque (in·lb) | Torque (ft·lb) |
|---|---|---|---|
| #2-56 | 5/64” | 2.2 | — |
| #4-40 | 3/32” | 5.3 | — |
| #6-32 | 7/64” | 9.5 | — |
| #8-32 | 9/64” | 19 | — |
| #10-24 | 5/32” | 29 | — |
| #10-32 | 5/32” | 32 | — |
| 1/4-20 | 3/16” | 87 | 7.3 |
| 1/4-28 | 3/16” | 96 | 8.0 |
| 5/16-18 | 1/4” | 170 | 14.2 |
| 5/16-24 | 1/4” | 195 | 16.3 |
| 3/8-16 | 5/16” | 300 | 25.0 |
| 3/8-24 | 5/16” | 350 | 29.2 |
| 7/16-14 | 3/8” | 475 | 39.6 |
| 1/2-13 | 3/8” | 710 | 59.2 |
| 1/2-20 | 3/8” | 800 | 66.7 |
| 5/8-11 | 1/2” | 1,380 | 115 |
| 3/4-10 | 5/8” | 2,450 | 204 |
Socket Head Cap Screw Torque — Metric
Torque values below are for metric socket head cap screws, property class 12.9 (ISO 4762 / DIN 912), dry installation. For class 10.9, multiply values by 0.83. For stainless steel (A2-70), see the stainless factor section below.
| Thread | Hex Key | Torque — Class 12.9 (N·m) | Torque — Class 10.9 (N·m) |
|---|---|---|---|
| M3 | 2.5 mm | 1.9 | 1.6 |
| M4 | 3 mm | 4.4 | 3.7 |
| M5 | 4 mm | 8.7 | 7.2 |
| M6 | 5 mm | 15 | 12.5 |
| M8 | 6 mm | 37 | 31 |
| M10 | 8 mm | 73 | 61 |
| M12 | 10 mm | 127 | 105 |
| M14 | 12 mm | 200 | 166 |
| M16 | 14 mm | 310 | 257 |
| M20 | 17 mm | 610 | 506 |
| M24 | 19 mm | 1,050 | 872 |
Button Head & Flat Head Socket Screw Hex Keys
Button head socket screws use a smaller hex key than a socket head cap screw of the same thread size. This is because the low-profile dome head has less metal to accommodate the socket, so the drive cavity is shallower and narrower. Using a standard SHCS hex key in a button head socket will cause rounding and drive failure.
| Thread | SHCS Hex Key | Button Head Hex Key |
|---|---|---|
| #4-40 | 3/32” | 1/16” |
| #6-32 | 7/64” | 5/64” |
| #8-32 | 9/64” | 3/32” |
| #10-32 | 5/32” | 1/8” |
| 1/4-20 | 3/16” | 5/32” |
| 5/16-18 | 1/4” | 3/16” |
| 3/8-16 | 5/16” | 1/4” |
| 1/2-13 | 3/8” | 5/16” |
Flat head socket screws use the same hex key size as socket head cap screws of the same thread. The countersunk flat head has sufficient depth to accommodate the standard hex socket.
Set Screw Hex Key Sizes
Set screws are fully threaded with no head—the hex socket occupies the full face of the fastener. Inch set screws follow ASME B18.3 hex socket sizes.
| Thread | Hex Key (in) |
|---|---|
| #4-40 | 1/16” |
| #6-32 | 1/16” |
| #8-32 | 5/64” |
| #10-32 | 3/32” |
| 1/4-20 | 1/8” |
| 5/16-18 | 5/32” |
| 3/8-16 | 3/16” |
| 1/2-13 | 1/4” |
| 5/8-11 | 5/16” |
| 3/4-10 | 3/8” |
Stainless Steel Torque Values
Stainless steel socket screws (18-8 / A2, 316 / A4) have lower tensile and yield strength than alloy steel ASTM A574. Apply a 0.65 factor to the alloy steel torque values above for 18-8 stainless dry installation. For example, a 1/4-20 alloy SHCS targets 87 in·lb; the 18-8 stainless equivalent is approximately 57 in·lb.
Additionally, stainless fasteners are susceptible to galling (cold welding) during installation. Using an anti-seize compound reduces galling risk—but also reduces friction, so reduce the target torque by an additional 20% when anti-seize is applied.
How to Apply Torque Correctly to Socket Screws
Socket screws require full hex key engagement to distribute torque across all six flats of the drive cavity. Partial engagement—where the key does not bottom out in the socket—concentrates load on fewer flats and causes rounding. Key tips for correct installation:
- Seat the hex key fully before applying torque. The key should contact the bottom of the socket, not float in the opening.
- Use quality L-keys or T-handles. Worn, undersized, or metric/inch-mixed keys are the leading cause of socket rounding.
- Do not substitute metric keys for inch sockets or vice versa. A 4mm key in a 5/32" socket (0.156") appears to fit but is 0.006" undersize and will round under load.
- Apply lubricant consistently or not at all. Partial lubrication produces inconsistent clamping force across a joint pattern.
- Use a torque wrench or torque screwdriver for critical joints. Torque-by-feel is unreliable, particularly for small screws below 1/4".
Frequently Asked Questions
What hex key size does a 1/4-20 socket head cap screw use?
A 1/4-20 socket head cap screw uses a 3/16" hex key (Allen wrench). This applies to both coarse (1/4-20) and fine (1/4-28) thread variants in the SHCS head style.
What is the correct torque for a 3/8-16 socket head cap screw?
For an alloy steel (ASTM A574) 3/8-16 SHCS, dry installation, the reference torque is 300 in·lb (25.0 ft·lb). For stainless steel, apply a 0.65 factor: approximately 195 in·lb (16.3 ft·lb). Always verify against your specific joint analysis.
Do button head socket screws use the same hex key as socket head cap screws?
No. Button head socket screws use a smaller hex key than a socket head cap screw of the same thread size. For example, a 1/4-20 SHCS uses 3/16" while a 1/4-20 button head uses 5/32". Using the wrong size key will round the socket.
How much does lubricant affect socket screw torque?
Lubricant reduces the friction component of torque, meaning the same torque value produces more clamping force than dry installation. If you switch from dry to lubricated installation, reduce torque by 20–25% to achieve equivalent bolt load. Never mix lubricated and unlubricated fasteners in the same joint pattern without recalculating.
Can I use a metric hex key in an inch socket screw?
Not reliably. While some sizes are close (e.g., 5mm ≈ 3/16"), the tolerances are different and an undersized key will round the socket under torque. Always match metric keys to metric screws and inch keys to inch screws.
Socket Screw Material & Grade Guide • Set Screw Point Style Guide • Shoulder Bolt Selection Guide