Fastening Know-How
Wrench Size Chart: Bolt, Nut, Allen Key and Washer Sizes
Wrench and socket sizes for inch and metric bolts and nuts, Allen key sizes for socket head cap screws, and flat washer dimensions.
How to Use These Charts
Find your bolt size in the left column and read across for the wrench or socket, the Allen key, or the matching washer. Inch and metric bolts use different tables. A 1/2 inch bolt head takes a 3/4 inch wrench; an M10 bolt head takes a 16 mm wrench under ISO but 17 mm under DIN.
Wrench Size Chart for Inch Bolts and Nuts
Standard hex cap screws and nuts share a wrench size except at 7/16 inch (cap screw 5/8, nut 11/16) and 9/16 inch (cap screw 13/16, nut 7/8). Heavy hex structural bolts and nuts are one size larger.
| Bolt size | Hex cap screw head (wrench) | Finished hex nut (wrench) | Heavy hex bolt and nut, structural |
|---|---|---|---|
| 1/4" | 7/16" | 7/16" | - |
| 5/16" | 1/2" | 1/2" | - |
| 3/8" | 9/16" | 9/16" | - |
| 7/16" | 5/8" | 11/16" | - |
| 1/2" | 3/4" | 3/4" | 7/8" |
| 9/16" | 13/16" | 7/8" | 15/16" (nut only) |
| 5/8" | 15/16" | 15/16" | 1-1/16" |
| 3/4" | 1-1/8" | 1-1/8" | 1-1/4" |
| 7/8" | 1-5/16" | 1-5/16" | 1-7/16" |
| 1" | 1-1/2" | 1-1/2" | 1-5/8" |
| 1-1/8" | 1-11/16" | 1-11/16" | 1-13/16" |
| 1-1/4" | 1-7/8" | 1-7/8" | 2" |
| 1-3/8" | 2-1/16" | 2-1/16" | 2-3/16" |
| 1-1/2" | 2-1/4" | 2-1/4" | 2-3/8" |
Hex cap screws ASME B18.2.1, hex nuts ASME B18.2.2, heavy hex structural bolts ASME B18.2.6 (ASTM F3125). Note 7/16": the bolt head takes a 5/8" wrench but the nut an 11/16" wrench. Heavy hex sizes are 1/8" larger than standard.
Metric Wrench Size Chart (ISO and DIN)
Metric hex bolts and nuts to ISO 4017/4032 or DIN 933/934.
| Bolt size | ISO 4017 / ISO 4032 (wrench) | DIN 933 / DIN 934 (wrench) |
|---|---|---|
| M3 | 5.5 mm | 5.5 mm |
| M4 | 7 mm | 7 mm |
| M5 | 8 mm | 8 mm |
| M6 | 10 mm | 10 mm |
| M8 | 13 mm | 13 mm |
| M10 | 16 mm | 17 mm |
| M12 | 18 mm | 19 mm |
| M14 | 21 mm | 22 mm |
| M16 | 24 mm | 24 mm |
| M18 | 27 mm | 27 mm |
| M20 | 30 mm | 30 mm |
| M22 | 34 mm | 32 mm |
| M24 | 36 mm | 36 mm |
| M27 | 41 mm | 41 mm |
| M30 | 46 mm | 46 mm |
| M33 | 50 mm | 50 mm |
| M36 | 55 mm | 55 mm |
The two standards differ at M10, M12, M14 and M22 (bold). Check which standard the drawing calls out before choosing a socket.
Allen Key Size Chart
Hex key sizes for socket head cap screws.
| Screw size (inch) | Hex key | Decimal inch |
|---|---|---|
| #0 | 0.050" | 0.0500 |
| #1 | 1/16" | 0.0625 |
| #2 | 5/64" | 0.0781 |
| #3 | 5/64" | 0.0781 |
| #4 | 3/32" | 0.0938 |
| #5 | 3/32" | 0.0938 |
| #6 | 7/64" | 0.1094 |
| #8 | 9/64" | 0.1406 |
| #10 | 5/32" | 0.1563 |
| 1/4" | 3/16" | 0.1875 |
| 5/16" | 1/4" | 0.2500 |
| 3/8" | 5/16" | 0.3125 |
| 7/16" | 3/8" | 0.3750 |
| 1/2" | 3/8" | 0.3750 |
| 5/8" | 1/2" | 0.5000 |
| 3/4" | 5/8" | 0.6250 |
| 7/8" | 3/4" | 0.7500 |
| 1" | 3/4" | 0.7500 |
Socket head cap screws, ASME B18.3.
| Screw size (metric) | Hex key |
|---|---|
| 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 |
| M18 | 14 mm |
| M20 | 17 mm |
| M22 | 17 mm |
| M24 | 19 mm |
Socket head cap screws, ISO 4762 / DIN 912.
Flat Washer Size Chart
Inside and outside diameters for SAE, USS and ISO 7089 flat washers.
| Bolt size | SAE ID | SAE OD | USS ID | USS OD |
|---|---|---|---|---|
| 1/4" | 0.281" | 0.625" | 0.312" | 0.734" / 0.750"* |
| 5/16" | 0.344" | 0.688" | 0.375" | 0.875" |
| 3/8" | 0.406" | 0.812" | 0.438" | 1.000" |
| 7/16" | 0.469" | 0.922" | 0.500" | 1.250" |
| 1/2" | 0.531" | 1.062" | 0.562" | 1.375" |
| 9/16" | 0.594" | 1.156" | 0.625" | 1.469" / 1.500"* |
| 5/8" | 0.656" | 1.312" | 0.688" / 0.656"* | 1.750" |
| 3/4" | 0.812" | 1.469" | 0.812" | 2.000" |
| 7/8" | 0.938" | 1.750" | 0.938" | 2.250" |
| 1" | 1.062" | 2.000" | 1.062" | 2.500" |
ASME B18.22.1 Type A: SAE is the narrow series, USS the wide series. Thickness varies within the standard tolerance; see ASME B18.22.1. *Where two published tables differ, both values are shown.
| Bolt size | ID (mm) | OD (mm) | Thickness (mm) |
|---|---|---|---|
| M3 | 3.2 | 7 | 0.5 |
| M4 | 4.3 | 9 | 0.8 |
| M5 | 5.3 | 10 | 1 |
| M6 | 6.4 | 12 | 1.6 |
| M8 | 8.4 | 16 | 1.6 |
| M10 | 10.5 | 20 | 2 |
| M12 | 13 | 24 | 2.5 |
| M14 | 15 | 28 | 2.5 |
| M16 | 17 | 30 | 3 |
| M18 | 19 | 34 | 3 |
| M20 | 21 | 37 | 3 |
| M22 | 23 | 39 | 3 |
| M24 | 25 | 44 | 4 |
| M27 | 28 | 50 | 4 |
| M30 | 31 | 56 | 4 |
| M33 | 34 | 60 | 5 |
| M36 | 37 | 66 | 5 |
ISO 7089 normal series plain washers.
Frequently Asked Questions
What size wrench for a 1/2 inch bolt?
3/4 inch for a standard hex cap screw or nut, and 7/8 inch for a heavy hex structural bolt.
What size wrench for an M10 bolt?
16 mm under ISO 4017 and 17 mm under DIN 933. Many older and European parts are DIN, so check the drawing.
What size Allen key for an M8 socket head cap screw?
6 mm. A 1/4 inch socket head cap screw takes a 3/16 inch key.
What is the difference between SAE and USS washers?
Both fit the same bolt, but USS washers have a larger outside diameter to spread the load on softer materials, while SAE washers are narrower for machinery and tighter spaces.
Related Charts and Calculators
See thread and head dimensions in the bolt size chart, strength in the bolt grade chart, tightening values with the Bolt Torque and Preload Calculator, or browse washers and nuts.
Specifying a Safety-Critical Joint?
These charts are reference values drawn from published standards. Send us your drawing or specification and our team will review the fastener choice with you.