Inch Fasteners - Socket Screw Hex Key and Nut Wrench Size
Ideally, identify a screw by its thread diameter + thread pattern (UNC/UNF). When all you have is a tool size, use these charts to estimate the fastener size. To reduce mistakes, we’ve added metric (mm) equivalents right in the tables.
- Tool fit: The key/bit should seat fully with no wobble. If you need to tap it in, stop and re-check size (see Near-Miss table below).
- Don’t mix systems: A “close” mm key may fit an inch socket (and vice versa) but will round out under torque.
- Stainless & galling: Use anti-seize, clean threads, and moderate speed. Learn more about thread galling.
- Torque: Use the correct series/material spec. (See our socket screw torque info.)
Hex Key (Allen) Size by Socket Screw Type
Infer a likely screw size from the internal hex that fits. Values show inch size with metric equivalent in parentheses.
| Screw Size | Socket Cap | Low Head Socket | Button Head | Flat Head | Set Screw |
|---|---|---|---|---|---|
| #0 | 0.050″ (1.27 mm) | – | 0.035″ (0.89 mm) | 0.035″ (0.89 mm) | 0.028″ (0.71 mm) |
| #1 | 1/16″ (1.59 mm) | – | 0.050″ (1.27 mm) | 0.050″ (1.27 mm) | 0.035″ (0.89 mm) |
| #2 | 5/64″ (1.98 mm) | – | 0.050″ (1.27 mm) | 0.050″ (1.27 mm) | 0.035″ (0.89 mm) |
| #3 | 5/64″ (1.98 mm) | – | 1/16″ (1.59 mm) | 1/16″ (1.59 mm) | 0.050″ (1.27 mm) |
| #4 | 3/32″ (2.38 mm) | 0.050″ (1.27 mm) | 1/16″ (1.59 mm) | 1/16″ (1.59 mm) | 0.050″ (1.27 mm) |
| #5 | 3/32″ (2.38 mm) | 1/16″ (1.59 mm) | 5/64″ (1.98 mm) | 5/64″ (1.98 mm) | 1/16″ (1.59 mm) |
| #6 | 7/64″ (2.78 mm) | 1/16″ (1.59 mm) | 5/64″ (1.98 mm) | 5/64″ (1.98 mm) | 1/16″ (1.59 mm) |
| #8 | 9/64″ (3.57 mm) | 5/64″ (1.98 mm) | 3/32″ (2.38 mm) | 3/32″ (2.38 mm) | 5/64″ (1.98 mm) |
| #10 | 5/32″ (3.97 mm) | 3/32″ (2.38 mm) | 1/8″ (3.18 mm) | 1/8″ (3.18 mm) | 3/32″ (2.38 mm) |
| 1/4″ | 3/16″ (4.76 mm) | 1/8″ (3.18 mm) | 5/32″ (3.97 mm) | 5/32″ (3.97 mm) | 1/8″ (3.18 mm) |
| 5/16″ | 1/4″ (6.35 mm) | 5/32″ (3.97 mm) | 3/16″ (4.76 mm) | 3/16″ (4.76 mm) | 5/32″ (3.97 mm) |
| 3/8″ | 5/16″ (7.94 mm) | 3/16″ (4.76 mm) | 7/32″ (5.56 mm) | 7/32″ (5.56 mm) | 3/16″ (4.76 mm) |
| 1/2″ | 3/8″ (9.53 mm) | 1/4″ (6.35 mm) | 5/16″ (7.94 mm) | 5/16″ (7.94 mm) | 1/4″ (6.35 mm) |
| 5/8″ | 1/2″ (12.70 mm) | 5/16″ (7.94 mm) | 3/8″ (9.53 mm) | 3/8″ (9.53 mm) | 5/16″ (7.94 mm) |
Nut Wrench Size by Screw (Bolt) Size
Estimate screw/bolt size from the external hex that fits your nut. Values show inch size with metric equivalent in parentheses.
| Screw Size | Standard Nuts Wrench Size * |
Heavy Hex Wrench Size ** |
Alt Size *** |
|---|---|---|---|
| #0 | 5/32″ (3.97 mm) | – | – |
| #1 | 5/32″ (3.97 mm) | – | – |
| #2 | 3/16″ (4.76 mm) | – | – |
| #3 | 3/16″ (4.76 mm) | – | – |
| #4 | 1/4″ (6.35 mm) | – | – |
| #5 | 5/16″ (7.94 mm) | – | – |
| #6 | 5/16″ (7.94 mm) | – | – |
| #8 | 11/32″ (8.73 mm) | – | – |
| #10 | 3/8″ (9.53 mm) | – | – |
| 1/4″ | 7/16″ (11.11 mm) | 1/2″ (12.70 mm) | – |
| 5/16″ | 1/2″ (12.70 mm) | 9/16″ (14.29 mm) | – |
| 3/8″ | 9/16″ (14.29 mm) | 11/16″ (17.46 mm) | – |
| 1/2″ | 3/4″ (19.05 mm) | 7/8″ (22.23 mm) | – |
| 5/8″ | 15/16″ (23.81 mm) | 1-1/16″ (26.99 mm) | – |
* Standard Nuts include: Hex Nut, Hex Jam, Square, Flange, Nylon Insert (regular & jam), Acorn, Stover equivalents.
** Heavy Hex examples: A194, A563 (structural), Heavy Hex Jam.
*** Alt Size: uncommon sizes seen on specialty nuts; always verify before torque-critical assemblies.
Near-Miss Sizes (Look-alikes to avoid)
Common inch ↔ mm tool sizes that seem to fit but can damage heads or nuts under load:
| Inch tool | mm (approx) | Notes |
|---|---|---|
| 5/32″ | ≈ 4.00 mm (actual 3.97) | Tempting swap; can round sockets. |
| 3/16″ | ≈ 5.00 mm (actual 4.76) | Feels close; avoid. |
| 7/32″ | ≈ 5.5 mm (actual 5.56) | Common mis-match on buttons. |
| 1/4″ | ≈ 6 mm (actual 6.35) | Looks right—don’t do it. |
| 5/16″ | ≈ 8 mm (actual 7.94) | Very common confusion. |
| 3/8″ | ≈ 10 mm (actual 9.53) | Short-term “fits” fail later. |
| 7/16″ | ≈ 11 mm (actual 11.11) | Rounding risk. |
| 1/2″ | ≈ 13 mm (actual 12.70) | Classic mismatch. |
| 9/16″ | ≈ 14 mm (actual 14.29) | Easy to mix up. |
| 11/16″ | ≈ 17 mm (actual 17.46) | Verify before torque. |
| 3/4″ | ≈ 19 mm (actual 19.05) | Looks perfect—don’t use. |
| 7/8″ | ≈ 22 mm (actual 22.23) | Check spec. |
| 15/16″ | ≈ 24 mm (actual 23.81) | High rounding risk. |
- Measure major diameter with calipers or our printable ruler.
- Identify thread pattern: US TPI Table or Metric Pitch Table.
- Check length rules by head type: Measuring Fastener Length & Diameter.