ecomfasteners.com/pages/wrench-size-chart · Ecom Fasteners
Wrench & Socket Size Chart for SAE & Metric Bolts
Use this wrench size chart and socket size chart to find the right tool for any hex bolt, nut or cap screw. It lists SAE and metric wrench sizes for hex bolts and cap screws, heavy hex and structural bolts, and hex nuts. Socket cap screw rows give the Allen key size. A conversion table shows which SAE and metric wrenches can stand in for each other. Metric rows show both ISO and DIN head sizes, since some M10, M12 and M14 bolts use a larger DIN head.
Bolt size ≠ wrench size. A "1/2 inch bolt" refers to the thread diameter, not the head size. A 1/2"-13 hex bolt actually requires a 3/4" wrench. Shop Grade 5 bolts
SAE (inch) hex bolt & cap screw wrench sizes
Hex bolt head, heavy hex head and socket cap (Allen) key size by thread diameter.
| Bolt Diameter | Hex Bolt Head | Heavy Hex Head | Socket Cap (Allen) | |
|---|---|---|---|---|
| #6 (0.138") | 5/16" | — | 7/64" | |
| #8 (0.164") | 11/32" | — | 9/64" | |
| #10 (0.190") | 3/8" | — | 5/32" | |
| 1/4" (0.250") | 7/16" | — | 3/16" | |
| 5/16" (0.312") | 1/2" | — | 1/4" | |
| 3/8" (0.375") | 9/16" | — | 5/16" | |
| 7/16" (0.437") | 5/8" | — | 3/8" | |
| 1/2" (0.500") | 3/4" | 7/8" | 3/8" | |
| 9/16" (0.562") | 13/16" | — | 7/16" | |
| 5/8" (0.625") | 15/16" | 1-1/16" | 1/2" | |
| 3/4" (0.750") | 1-1/8" | 1-1/4" | 5/8" | |
| 7/8" (0.875") | 1-5/16" | 1-7/16" | 3/4" | |
| 1" (1.000") | 1-1/2" | 1-5/8" | 3/4" | |
| 1-1/8" (1.125") | 1-11/16" | 1-13/16" | 7/8" | |
| 1-1/4" (1.250") | 1-7/8" | 2" | 1" |
Metric hex bolt & cap screw wrench sizes
Hex head (ISO 4017/4014 and DIN 933/931) and socket cap (Allen) key in mm, with the closest SAE wrench.
| Bolt Diameter | ISO | DIN | Socket Cap (Allen) | Closest SAE | |
|---|---|---|---|---|---|
| M3 (3mm) | 5.5mm | 5.5mm | 2.5mm | — | |
| M4 (4mm) | 7mm | 7mm | 3mm | 9/32" | |
| M5 (5mm) | 8mm | 8mm | 4mm | 5/16" | |
| M6 (6mm) | 10mm | 10mm | 5mm | 3/8" | |
| M8 (8mm) | 13mm | 13mm | 6mm | 1/2" | |
| M10 (10mm) | 16mm · DIN 17 | 17mm | 8mm | 11/16" | |
| M12 (12mm) | 18mm · DIN 19 | 19mm | 10mm | 3/4" | |
| M14 (14mm) | 21mm · DIN 22 | 22mm | 12mm | 7/8" | |
| M16 (16mm) | 24mm | 24mm | 14mm | 15/16" | |
| M18 (18mm) | 27mm | 27mm | 14mm | 1-1/16" | |
| M20 (20mm) | 30mm | 30mm | 17mm | 1-3/16" | |
| M24 (24mm) | 36mm | 36mm | 19mm | 1-7/16" |
Structural bolt wrench sizes A325 / A490 / F3125
Heavy hex head, heavy hex nut and TC bolt spline.
| Bolt Diameter | Heavy Hex Head | Heavy Hex Nut | TC Bolt Spline | |
|---|---|---|---|---|
| 1/2" | 7/8" | 7/8" | 7/16" | |
| 5/8" | 1-1/16" | 1-1/16" | 9/16" | |
| 3/4" | 1-1/4" | 1-1/4" | 11/16" | |
| 7/8" | 1-7/16" | 1-7/16" | 13/16" | |
| 1" | 1-5/8" | 1-5/8" | 15/16" | |
| 1-1/8" | 1-13/16" | 1-13/16" | 1-1/16" | |
| 1-1/4" | 2" | 2" | 1-3/16" |
Hex nut wrench sizes
Nut thread size → wrench size.
SAE hex nuts
- #6
- 5/16"
- #8
- 11/32"
- #10
- 3/8"
- 1/4"
- 7/16"
- 5/16"
- 1/2"
- 3/8"
- 9/16"
- 1/2"
- 3/4"
- 5/8"
- 15/16"
- 3/4"
- 1-1/8"
- 1"
- 1-1/2"
Metric hex nuts ISO 4032 / DIN 934
- Nut
- ISO
- DIN
- M3
- 5.5mm
- 5.5mm
- M4
- 7mm
- 7mm
- M5
- 8mm
- 8mm
- M6
- 10mm
- 10mm
- M8
- 13mm
- 13mm
- M10
- 16mm · DIN 17
- 17mm
- M12
- 18mm · DIN 19
- 19mm
- M14
- 21mm · DIN 22
- 22mm
- M16
- 24mm
- 24mm
- M20
- 30mm
- 30mm
SAE ↔ metric wrench swaps
In a pinch, these SAE and metric wrenches are close enough to substitute (within 0.5mm). Bold pairs differ by 0.15mm or less.
| SAE Wrench | Metric Wrench | Difference | |
|---|---|---|---|
| 1/4" (6.35mm) | 6mm | 0.35mm | |
| 5/16" (7.94mm) | 8mm | 0.06mm | |
| 3/8" (9.52mm) | 10mm | 0.48mm | |
| 7/16" (11.11mm) | 11mm | 0.11mm | |
| 1/2" (12.70mm) | 13mm | 0.30mm | |
| 9/16" (14.29mm) | 14mm | 0.29mm | |
| 5/8" (15.87mm) | 16mm | 0.13mm | |
| 11/16" (17.46mm) | 17mm | 0.46mm | |
| 3/4" (19.05mm) | 19mm | 0.05mm | |
| 15/16" (23.81mm) | 24mm | 0.19mm |
General reference. Source: ecomfasteners.com/pages/wrench-size-chart
Frequently asked questions
Why is the wrench size different from the bolt size?
The bolt size (1/4", 3/8", 1/2", etc.) refers to the thread diameter — the part that screws into the hole. The wrench size is the distance across the flats of the hex head, which must be larger than the thread to provide enough material strength. A general rule: wrench size ≈ 1.5× bolt diameter.
Can I use a metric wrench on SAE bolts (or vice versa)?
In emergencies, yes — if the sizes are close (see conversion table above). For example, a 13mm wrench works on 1/2" hex heads. However, using the wrong size risks rounding off the bolt head corners. For regular use, always use the correct wrench.
What's the difference between hex bolts and heavy hex bolts?
Heavy hex bolts have a larger head (and require a larger wrench) than standard hex bolts of the same diameter. Heavy hex is standard for structural applications (A325, A490) because the larger head provides more bearing surface and can handle higher torque values. Shop heavy hex bolts
What size Allen key do I need for socket cap screws?
Socket cap screws use an internal hex (Allen) drive. The key size is typically about 1/2 the bolt diameter for inch sizes (e.g., 1/4" bolt uses 3/16" Allen) and follows a standard progression for metric. See the Socket Cap column in the tables above. Shop socket head cap screws
Why do some bolts have the same wrench size but different thread sizes?
Head dimensions are standardized by grade and application, not just thread size. For example, a 1/2" structural heavy hex takes a 7/8" wrench. This is why it's important to know the bolt specification, not just match wrenches by feel.
Need hex bolts, nuts or socket cap screws?
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