Quick answer · Hex key size lookup
What Size Allen Wrench Do I Need?
Find common screw-to-key sizes and SAE-to-metric conversions immediately. A screw’s thread diameter is not its wrench size, and different screw head styles may use different keys.
Find the key for your screw
Screw → hex keyMetric M6 socket-head cap screw — ISO 4762 reference.
Selected key size comes from the screw head style, not the thread diameter alone. View screw tables.
SAE ↔ metric size finder
Fraction or decimal0.03125 mm difference; numerically close is not automatically an acceptable replacement.
Enter 5/32, 1/4, 0.15625 or 4. Changing units reinterprets the entered number.
Fit warning: SAE and metric sizes that are numerically close are not automatically interchangeable. Use the specified tool; a loose key can round the socket.
Full reference · 50 listed sizes
Allen Wrench Size Chart — SAE & Metric
Measure the hex key across flats. The chart lists 22 inch sizes and 28 metric size labels with exact nominal unit conversions (rounded for display). “Closest other system” is a numerical comparison, not a safe substitution instruction.
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On mobile, swipe sideways in the table to see nearest sizes.
This table scrolls horizontally. The first column shows the nominal key size.
| Nearest listed other system* | Difference (mm) | |||
|---|---|---|---|---|
| 0.028 in | 0.7112 | 0.028 | 0.7 mm | 0.0112 |
| 0.035 in | 0.889 | 0.035 | 0.9 mm | 0.011 |
| 0.050 in | 1.27 | 0.05 | 1.3 mm | 0.03 |
| 1/16 in | 1.5875 | 0.0625 | 1.5 mm | 0.0875 |
| 5/64 in | 1.984375 | 0.078125 | 2 mm | 0.015625 |
| 3/32 in | 2.38125 | 0.09375 | 2.5 mm | 0.11875 |
| 7/64 in | 2.778125 | 0.109375 | 3 mm | 0.221875 |
| 1/8 in | 3.175 | 0.125 | 3 mm | 0.175 |
| 9/64 in | 3.571875 | 0.140625 | 3.5 mm | 0.071875 |
| 5/32 in | 3.96875 | 0.15625 | 4 mm | 0.03125 |
| 3/16 in | 4.7625 | 0.1875 | 5 mm | 0.2375 |
| 7/32 in | 5.55625 | 0.21875 | 5.5 mm | 0.05625 |
| 1/4 in | 6.35 | 0.25 | 6 mm | 0.35 |
| 5/16 in | 7.9375 | 0.3125 | 8 mm | 0.0625 |
| 3/8 in | 9.525 | 0.375 | 10 mm | 0.475 |
| 7/16 in | 11.1125 | 0.4375 | 11 mm | 0.1125 |
| 1/2 in | 12.7 | 0.5 | 13 mm | 0.3 |
| 9/16 in | 14.2875 | 0.5625 | 14 mm | 0.2875 |
| 5/8 in | 15.875 | 0.625 | 17 mm | 1.125 |
| 3/4 in | 19.05 | 0.75 | 19 mm | 0.05 |
| 7/8 in | 22.225 | 0.875 | 22 mm | 0.225 |
| 1 in | 25.4 | 1 | 24 mm | 1.4 |
| 0.7 mm | 0.7 | 0.027559 | 0.028 in | 0.0112 |
| 0.9 mm | 0.9 | 0.035433 | 0.035 in | 0.011 |
| 1.3 mm | 1.3 | 0.051181 | 0.050 in | 0.03 |
| 1.5 mm | 1.5 | 0.059055 | 1/16 in | 0.0875 |
| 2 mm | 2 | 0.07874 | 5/64 in | 0.015625 |
| 2.5 mm | 2.5 | 0.098425 | 3/32 in | 0.11875 |
| 3 mm | 3 | 0.11811 | 1/8 in | 0.175 |
| 3.5 mm | 3.5 | 0.137795 | 9/64 in | 0.071875 |
| 4 mm | 4 | 0.15748 | 5/32 in | 0.03125 |
| 4.5 mm | 4.5 | 0.177165 | 3/16 in | 0.2625 |
| 5 mm | 5 | 0.19685 | 3/16 in | 0.2375 |
| 5.5 mm | 5.5 | 0.216535 | 7/32 in | 0.05625 |
| 6 mm | 6 | 0.23622 | 1/4 in | 0.35 |
| 7 mm | 7 | 0.275591 | 1/4 in | 0.65 |
| 8 mm | 8 | 0.314961 | 5/16 in | 0.0625 |
| 9 mm | 9 | 0.354331 | 3/8 in | 0.525 |
| 10 mm | 10 | 0.393701 | 3/8 in | 0.475 |
| 11 mm | 11 | 0.433071 | 7/16 in | 0.1125 |
| 12 mm | 12 | 0.472441 | 1/2 in | 0.7 |
| 13 mm | 13 | 0.511811 | 1/2 in | 0.3 |
| 14 mm | 14 | 0.551181 | 9/16 in | 0.2875 |
| 17 mm | 17 | 0.669291 | 5/8 in | 1.125 |
| 19 mm | 19 | 0.748031 | 3/4 in | 0.05 |
| 22 mm | 22 | 0.866142 | 7/8 in | 0.225 |
| 24 mm | 24 | 0.944882 | 1 in | 1.4 |
| 27 mm | 27 | 1.062992 | Outside listed range | — |
| 32 mm | 32 | 1.259843 | Outside listed range | — |
| 36 mm | 36 | 1.417323 | Outside listed range | — |
*Nominal differences show mathematical closeness only. If a size is outside the other system’s listed range, no pairing is shown. For small nominal labels such as 1.3 mm, a manufacturer’s actual specified dimension may be 1.27 mm (0.050 in). Check its drawing and tolerances.
Allen Wrench Size Chart | Turn2Engineering | Nominal dimensions only; verify head style and manufacturer specification.
Find your screw thread & head style
What Size Allen Wrench Fits My Screw?
M6 regular socket-head cap → 5 mm; M6 button head → 4 mm; M6 socket set screw → 3 mm. The key changes with the head style—even when the thread is identical. See the data sources.
Metric socket screws (ISO families)
Reference families: ISO 4762 (regular socket-head cap), ISO 7380 (button head), and ISO 4026–4029 (socket set screws). Sizes below are nominal hex key widths in millimeters.
| Thread | Socket-head cap | Button head | Set screw |
|---|---|---|---|
| M1.6 | 1.5 | — | 0.7 |
| M2 | 1.5 | — | 0.9 |
| M2.5 | 2 | — | 1.3 |
| M3 | 2.5 | 2 | 1.5 |
| M4 | 3 | 2.5 | 2 |
| M5 | 4 | 3 | 2.5 |
| M6 | 5 | 4 | 3 |
| M8 | 6 | 5 | 4 |
| M10 | 8 | 6 | 5 |
| M12 | 10 | 8 | 6 |
| M14 | 12 | — | 6 |
| M16 | 14 | 10 | 8 |
| M20 | 17 | — | 10 |
| M24 | 19 | — | 12 |
| M30 | 22 | — | — |
SAE inch socket-head cap screws
Regular inch socket-head cap screws only, using the Holo-Krome product table based on ASME B18.3. The sizes below are inches; they do not apply automatically to button-head, flat-head, low-head or set screws.
| Screw nominal thread size | Allen key size |
|---|---|
| #0 | 0.050 in |
| #1 | 1/16 in |
| #2 | 5/64 in |
| #3 | 5/64 in |
| #4 | 3/32 in |
| #5 | 3/32 in |
| #6 | 7/64 in |
| #8 | 9/64 in |
| #10 | 5/32 in |
| 1/4 | 3/16 in |
| 5/16 | 1/4 in |
| 3/8 | 5/16 in |
| 7/16 | 3/8 in |
| 1/2 | 3/8 in |
| 5/8 | 1/2 in |
| 3/4 | 5/8 in |
| 7/8 | 3/4 in |
| 1 | 3/4 in |
A dash in the metric table means the cited summary does not supply a mapping, not that the fastener cannot exist. For specialty heads, proprietary screws and critical joints, follow the manufacturer’s fastener drawing.
How to Measure an Allen Wrench
Measure the width between two opposite, parallel flat faces on an undamaged straight section of the tool. Do not measure point-to-point across corners; a hexagon is wider across corners than across flats.
The bit seats fully and has minimal rotational play without forcing.
Check for metric/inch mismatch, rounded socket corners, worn tools, dirt and coating buildup.
Useful for angled access where permitted, but should not be treated as equivalent to full straight-end engagement.
Common SAE–Metric Sizes That Look Alike
These popular pairs are close in nominal dimension, not exact equivalents. Tool and socket manufacturing tolerances, recess wear and applied torque determine whether a mismatched tool slips.
| SAE size | Exact converted width | Nearby metric size | Difference |
|---|---|---|---|
| 5/64 in | 1.984375 mm | 2 mm | 0.015625 mm |
| 5/32 in | 3.96875 mm | 4 mm | 0.03125 mm |
| 5/16 in | 7.9375 mm | 8 mm | 0.0625 mm |
| 1/8 in | 3.175 mm | 3 mm | 0.175 mm |
| 3/16 in | 4.7625 mm | 5 mm | 0.2375 mm |
| 1/4 in | 6.35 mm | 6 mm | 0.35 mm |
Most Common SAE and Metric Allen Key Sizes
Typically include 1.5, 2, 2.5, 3, 4, 5, 6, 8 and 10 mm.
Often include 0.050, 1/16, 5/64, 3/32, 7/64, 1/8, 9/64, 5/32, 3/16, 7/32 and 1/4 in.
Identify the actual screw drive, head style and manufacturer callout before selecting a replacement.
Allen wrench, Allen key, hex key and hex wrench are common names for the same six-flat drive tool. It is not a six-lobe Torx driver, and a 1/4 in bit-holder shank does not mean the tip is a 1/4 in hex key.
Allen Wrench Size FAQ
Is 5/32 inch the same as 4 mm?
No. 5/32 in is exactly 3.96875 mm, or 0.03125 mm smaller than 4 mm. The nominal measurements are close, but the tools are not specified as interchangeable.
What size Allen wrench do I need for an M6 screw?
A regular M6 ISO 4762 socket-head cap screw commonly uses a 5 mm hex key; an M6 ISO 7380 button head uses 4 mm, and an M6 socket set screw in the listed ISO 4026–4029 group uses 3 mm. Match the head style.
What Allen wrench fits a 1/4-20 socket-head cap screw?
A regular 1/4 in socket-head cap screw in the listed Holo-Krome/ASME reference uses a 3/16 in hex key. Do not generalize to 1/4-20 button, flat, low-head or set screws.
How do I tell metric from SAE Allen keys?
Look for markings on the tool or set. If unmarked, measure across opposite flats with calipers, then compare exact nominal dimensions. Similar metric and inch sizes can be close enough to confuse.
What is the metric conversion of a 1/4 inch Allen key?
1/4 in × 25.4 = exactly 6.35 mm. A nominal 6 mm key is 0.35 mm smaller and is not the same size.
Is a 4 mm Allen wrench the same as 5/32 inch?
No. A 4 mm hex key is nominally 0.03125 mm larger than a 5/32 in key. Use the specified drive size rather than relying on the nearest mathematical conversion.
Do all M8 bolts use a 6 mm Allen wrench?
No. A regular M8 socket-head cap screw commonly takes 6 mm; a metric M8 button head often takes 5 mm, and the listed M8 set-screw family takes 4 mm. Other bolt head types may not use a hex key at all.
Can I use a Torx bit instead of an Allen wrench?
No. Torx has a six-lobe profile; an Allen key has six flats. Use the intended drive rather than trying to force a mismatched bit into the recess.
Standards, Data Basis and Limitations
Checked October 2026. Key-size conversions are mathematical nominal conversions using 1 in = exactly 25.4 mm. The size list is selected manufacturer/reference coverage, not a claim that all keys are made in every nominal size. Published standard summaries establish scope; for production inspection, use the applicable purchased standard and actual product drawing.
- ISO 2936:2014 — Hexagon socket screw keys (reviewed/confirmed 2025). Defines hex-key dimensions, marking, and testing.
- ASME B18.3-2026 — Inch socket cap, shoulder and set screws, and hex keys. Scope and current standard listing.
- AFT Fasteners — Allen wrench dimensions. Manufacturer-sold inch/metric size examples, including the small nominal 1.3 mm labeling issue.
- Bossard, Assembly Tools (2025), p. 94. Metric hex-key sizes by thread and head style from the submitted reference.
- Holo-Krome alloy socket-head cap screw technical table. Inch screw-to-key size mapping reproduced as a condensed reference from the submitted table.