Direct lookup
Bolt Head and Wrench Size Lookup
Choose the fastener system and nominal bolt diameter. “Head size” here means width across opposite flats.
Default example: a standard 1/2 in ASME hex bolt uses a nominal 3/4 in width across flats, so the nominal wrench/socket size is 3/4 in.
Confirm the fastener’s dimensional standard before selecting a tool. Heavy-hex, structural, flange, JIS, legacy DIN, and specialty heads may use different dimensions.
Dimension reference
Hex Bolt Head Width Across Flats
The wrench/socket dimension is measured between opposite parallel faces—not corner to corner.
- Nominal bolt diameter, d
- 1/2 in
- Width across flats, s
- 3/4 in
- Nominal wrench/socket
- 3/4 in
- Dimensional basis
- ASME B18.2.1
Limited common-size reference
Bolt Head Size and Wrench Size Chart
Common standard hex sizes only. This is intentionally not a complete reproduction of ASME or ISO dimensional standards.
Search by bolt designation, wrench size, system, or standard. Use the source system’s nominal units when selecting a tool.
No supported standard hex-bolt size matches the current search. Check the nominal diameter, wrench size, or standard system.
Scroll horizontally to view additional columns and vertically to view additional rows. The bolt size column and column headers remain visible while scrolling.
| System | Dimensional basis | Nominal diameter | Width across flats | Wrench / socket | Alternate / legacy note | Copy | |
|---|---|---|---|---|---|---|---|
| 1/4 in | SAE / ASME Inch | ASME B18.2.1 | 1/4 in | 7/16 in | 7/16 in | — | |
| 5/16 in | SAE / ASME Inch | ASME B18.2.1 | 5/16 in | 1/2 in | 1/2 in | — | |
| 3/8 in | SAE / ASME Inch | ASME B18.2.1 | 3/8 in | 9/16 in | 9/16 in | — | |
| 7/16 in | SAE / ASME Inch | ASME B18.2.1 | 7/16 in | 5/8 in | 5/8 in | — | |
| 1/2 in | SAE / ASME Inch | ASME B18.2.1 | 1/2 in | 3/4 in | 3/4 in | — | |
| 9/16 in | SAE / ASME Inch | ASME B18.2.1 | 9/16 in | 13/16 in | 13/16 in | — | |
| 5/8 in | SAE / ASME Inch | ASME B18.2.1 | 5/8 in | 15/16 in | 15/16 in | — | |
| 3/4 in | SAE / ASME Inch | ASME B18.2.1 | 3/4 in | 1-1/8 in | 1-1/8 in | — | |
| 7/8 in | SAE / ASME Inch | ASME B18.2.1 | 7/8 in | 1-5/16 in | 1-5/16 in | — | |
| 1 in | SAE / ASME Inch | ASME B18.2.1 | 1 in | 1-1/2 in | 1-1/2 in | — | |
| M4 | ISO Metric | ISO 272 + ISO 4014/4017 | 4 mm | 7 mm | 7 mm | — | |
| M5 | ISO Metric | ISO 272 + ISO 4014/4017 | 5 mm | 8 mm | 8 mm | — | |
| M6 | ISO Metric | ISO 272 + ISO 4014/4017 | 6 mm | 10 mm | 10 mm | — | |
| M8 | ISO Metric | ISO 272 + ISO 4014/4017 | 8 mm | 13 mm | 13 mm | — | |
| M10 | ISO Metric | ISO 272 + ISO 4014/4017 | 10 mm | 16 mm | 16 mm | Legacy DIN 933/931 commonly used 17 mm. | |
| M12 | ISO Metric | ISO 272 + ISO 4014/4017 | 12 mm | 18 mm | 18 mm | Legacy DIN 933/931 commonly used 19 mm. | |
| M14 | ISO Metric | ISO 272 + ISO 4014/4017 | 14 mm | 21 mm | 21 mm | Legacy DIN 933/931 commonly used 22 mm. | |
| M16 | ISO Metric | ISO 272 + ISO 4014/4017 | 16 mm | 24 mm | 24 mm | — | |
| M20 | ISO Metric | ISO 272 + ISO 4014/4017 | 20 mm | 30 mm | 30 mm | — | |
| M22 | ISO Metric | ISO 272 + ISO 4014/4017 | 22 mm | 34 mm | 34 mm | Some legacy DIN-family references use 32 mm; verify the fastener standard. | |
| M24 | ISO Metric | ISO 272 + ISO 4014/4017 | 24 mm | 36 mm | 36 mm | — |
Scope: standard hex heads only. Verify the actual fastener standard before use. Values are a limited educational reference, not a substitute for the governing ASME/ISO standard or a dimensional inspection requirement.
How to Measure a Hex Bolt Head
The wrenching dimension is the distance between two opposite, parallel flats. Place caliper jaws squarely against those flats; do not measure corner to corner. If the head is worn, plated heavily, rounded, or corroded, use the nominal thread diameter and fastener standard as the stronger identification clues.
- Identify whether the fastener is inch-series or metric.
- Measure the nominal thread/shank diameter separately from the head.
- Measure across opposite flats and compare that value with the correct standard family.
Field check: A wrench that “almost fits” is not proof of the bolt standard. Close SAE/metric sizes and legacy metric hardware can create misleading near-matches.
SAE Bolt Head Sizes vs Metric Bolt Head Sizes
Keep inch-series and metric hardware in separate dimensional families. The nominal bolt diameter and the wrenching width are specified in the source system; converting one value to the other system is useful for comparison, but it does not create an equivalent standard wrench size.
- Inch example
- A 1/2 in standard hex bolt is paired here with a 3/4 in nominal across-flats dimension.
- Metric example
- An ISO M10 standard hex head is paired here with a 16 mm nominal across-flats dimension.
- Tool choice
- Use the specified wrench/socket system whenever practical rather than a merely close converted size.
Why Some Metric Bolt Heads Use Different Wrench Sizes
Current ISO hex-head dimensions do not always match older DIN-family hardware. The most common surprises are M10, M12, and M14, where current ISO widths across flats are 16, 18, and 21 mm while legacy DIN 933/931 references commonly show 17, 19, and 22 mm. That difference is large enough to matter when choosing a socket.
Do not mix standards silently: If an installed metric bolt does not match the current ISO value, inspect the actual fastener specification or documentation before assuming the chart is wrong.
Bolt Head Size Does Not Determine Bolt Strength
Head geometry tells you which tool fits; it does not establish property class, SAE grade, proof strength, allowable load, or tightening torque. Two fasteners can share the same nominal head width while having different material and strength requirements.
After identifying the fastener size and grade, use a torque reference that matches the thread, strength grade, lubrication condition, and design requirements rather than inferring torque from wrench size.
Common Bolt Head Size Lookup Examples
Example 1: 1/2 in standard hex bolt
Assumption: standard ASME inch-series hex head, not heavy hex.
Lookup: 1/2 in nominal bolt diameter → 3/4 in nominal width across flats → 3/4 in wrench/socket.
Sanity check: If the actual head is materially larger, verify whether the fastener is heavy hex or another head family.
Example 2: ISO M10 hex bolt
Assumption: current ISO 4014/4017-type standard hex head.
Lookup: M10 → 16 mm nominal width across flats → 16 mm wrench/socket.
Limitation: Older DIN-family M10 hardware commonly uses a 17 mm head, so field hardware should be identified by its actual standard.
Standards, Scope, and Source Limitations
Dataset T2E-BOLT-HEAD-COMMON-v1.0 · Source checked 2026-08-16 · Publisher: Turn2Engineering
The page uses a deliberately limited common-size reference. ASME and ISO standards are copyrighted, so this page does not reproduce complete normative dimensional tables, tolerance fields, or a bulk downloadable standards dataset.
- ASME B18.2.1 — Square, Hex, Heavy Hex, and Askew Head Bolts… (Inch Series)Edition B18.2.1-2012 (R2021). Controls the inch-series hex-bolt dimensional family; ASME states the standard remains in effect.
- ISO 272:1982 — Fasteners — Hexagon products — Widths across flatsEdition 2. ISO lists this edition as current but under revision and expected to be replaced by ISO/DIS 272.
- ISO 4017:2022 — Fasteners — Hexagon head screws — Product grades A and BEdition 6, 2022-06. Defines the current metric hex-head product-family scope used for this page.
- Fastenal ISO 4014 metric hex cap screw product specificationPublic ISO 4014 product specification used to cross-check the M10 and M12 width-across-flats values shown on this page.
Dataset and Source-Check Details
- Included geometry
- Standard hex heads; nominal width across flats only
- Excluded geometry
- Heavy hex, structural, flange, JIS, square, socket-head, carriage, and specialty heads
- Primary units
- Inches for ASME rows; millimetres for ISO rows
- Missing values
- Shown as an em dash; never inferred as zero
- Rights status
- Limited transformed educational reference; standards remain copyrighted
- Download decision
- No full CSV or bulk standards export
- Maintenance trigger
- Recheck when ASME B18.2.1 changes or ISO/DIS 272 replaces ISO 272:1982
- Use limitation
- Verify governing fastener specification for design, inspection, procurement, and safety-critical work
Bolt Head Size FAQ
A standard ASME inch-series 1/2 in hex bolt in this limited reference uses a nominal 3/4 in width across flats, so the nominal wrench/socket size is 3/4 in; verify the actual head family.
A current ISO 4014/4017-type M10 standard hex head uses a nominal 16 mm width across flats; older DIN-family M10 hardware commonly uses 17 mm.
No. Bolt diameter identifies the nominal thread/shank size, while head size in this chart means the width across opposite flats used by the wrench or socket.
The installed fastener may follow legacy DIN, JIS, heavy-hex, flange-head, structural, or another dimensional standard rather than the ISO standard-hex family shown here.
A near-equivalent metric tool is not dimensionally identical to the specified SAE size; use the specified system when practical to reduce clearance and head damage.
Use the Standard Before You Trust the Wrench Size
Start with the nominal bolt diameter and fastener standard, then use width across flats to select the wrench or socket. If an installed bolt does not match the expected size, treat that as an identification clue—not a reason to force a close tool size.
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