Spanner size is the across-flats (AF) measurement of a fastener head — not the bolt thread diameter. An M8 bolt has a 13mm hex head, so it takes a 13mm spanner, never an 8mm one. The compact reference below covers the most-used metric sizes; full metric, imperial (AF) and BSP charts are further down.
Quick answer — bolt to spanner
M6 = 10mm · M8 = 13mm · M10 = 17mm · M12 = 19mm · M14 = 22mm · M16 = 24mm · M18 = 27mm · M20 = 30mm · M22 = 32mm · M24 = 36mm
Metric → Imperial: 10mm ≈ 3/8" · 13mm ≈ 1/2" · 14mm ≈ 9/16" · 17mm ≈ 11/16" · 19mm ≈ 3/4" · 22mm ≈ 7/8" · 24mm ≈ 15/16"
AF = Across Flats. The distance between two parallel faces of a hex fastener. All spanners (metric and imperial) measure across the flats — bolt diameter (M-number) is a different measurement.
Spanner Size Chart — Quick Reference
The most common metric bolt heads and their matching spanner sizes:
| Bolt Size | Spanner (AF) | Bolt Size | Spanner (AF) |
| M5 | 8mm | M14 | 22mm |
| M6 | 10mm | M16 | 24mm |
| M8 | 13mm | M18 | 27mm |
| M10 | 17mm | M20 | 30mm |
| M12 | 19mm | M24 | 36mm |
How Spanner Sizes Work
Spanner size is measured across the flats (AF) of the fastener head — the distance between two parallel faces of the hex. A 19mm spanner fits any fastener that measures 19mm across the flats, regardless of whether the fastener thread is metric or imperial.
Bolt thread diameter (M8, M12 etc.) and spanner size are different measurements. The tables below show the relationship between bolt size and spanner size. An M8 bolt has a 13mm hex head — so you need a 13mm spanner, not an 8mm one.
Open-end spanners engage two flats and are faster to use. Ring spanners (12-point) engage all six flats and are preferred for high-torque work, as they're less likely to round a fastener head. Combination spanners give you both in one tool — open end for speed, ring end for torque.
Metric Spanner Size Chart — Bolt Thread to Spanner Size
This table shows the spanner size required for each metric bolt thread size. Sizes follow ISO standard hex dimensions. Always confirm against the actual fastener if in doubt — some manufacturers use non-standard hex sizes.
| Bolt Size | Spanner Size (AF) | Common Application |
| M4 | 7mm | Small fasteners, electronics, thin sheet |
| M5 | 8mm | Small fasteners, covers, guards |
| M6 | 10mm | Most common — engines, brackets, interior panels |
| M7 | 11mm | Less common metric size |
| M8 | 13mm | General engineering, structural fasteners |
| M10 | 17mm | General engineering, machinery |
| M12 | 19mm | Automotive, structural, machinery |
| M14 | 22mm | Suspension components, driveline |
| M16 | 24mm | Heavy structural fasteners |
| M18 | 27mm | Heavy fasteners, industrial equipment |
| M20 | 30mm | Large structural and machinery fasteners |
| M22 | 32mm | Heavy machinery, plant equipment |
| M24 | 36mm | Large bolts, plant and structural |
| M27 | 41mm | Heavy plant and infrastructure |
| M30 | 46mm | Large plant, civil infrastructure |
| M33 | 50mm | Very large structural fasteners |
| M36 | 55mm | Heavy infrastructure, mining |
Metric Spanner Size Chart — Full Range
The table below covers the full common metric spanner range from 6mm to 50mm, showing typical fastener applications for each size. Useful when you know which spanner you have and need to identify what it fits.
| Spanner Size (mm) | Typical Bolt / Fastener | Notes |
| 6 | M3.5 bolt head | Uncommon — small precision fasteners |
| 7 | M4 bolt head | Electronics, small assemblies |
| 8 | M5 bolt head | Light fasteners, covers |
| 9 | General use | Less common in metric sets |
| 10 | M6 bolt head | Most common metric spanner size |
| 11 | M7 bolt head | Less common metric size |
| 12 | General use | Some fittings and M7 fine thread |
| 13 | M8 bolt head | Common workshop size |
| 14 | 1/8" BSP fittings | Hydraulic and pneumatic fittings |
| 15 | General use | Some M9 fasteners, brake fittings |
| 16 | General use | Some M10 fine thread |
| 17 | M10 bolt head (standard) | Common automotive and machinery size |
| 18 | General use | Some hydraulic fittings |
| 19 | M12 bolt head | Also close to 3/4" AF (19.05mm) |
| 21 | General use | Some wheel nuts and couplings |
| 22 | M14 bolt head / 3/8" BSP | Common fitting and fastener size |
| 24 | M16 bolt head | Heavy structural applications |
| 26 | 1/2" BSP fittings | Most common BSP fitting size |
| 27 | M18 bolt head | Industrial and heavy equipment |
| 30 | M20 bolt head | Large structural fasteners |
| 32 | M22 bolt head / 3/4" BSP | Heavy machinery and plant |
| 36 | M24 bolt head | Large bolts, plant equipment |
| 41 | M27 bolt head / 1" BSP | Heavy plant and large fittings |
| 46 | M30 bolt head | Large plant and infrastructure |
| 50 | M33 bolt head / 1-1/4" BSP | Very large structural and fittings |
Imperial (AF) Spanner Size Chart
Imperial spanners are sized in fractions of an inch and are common on American-manufactured vehicles and equipment, agricultural machinery, and older plant. The sizing follows the across-flats (AF) convention — the same measurement system as metric, just in inches.
| Spanner Size (inch) | Decimal (inch) | Metric Equivalent (mm) | Typical Use |
| 1/4" | 0.250" | 6.35 | Very small fasteners |
| 5/16" | 0.313" | 7.94 | Small fasteners |
| 3/8" | 0.375" | 9.53 | Light fasteners |
| 7/16" | 0.438" | 11.11 | General use |
| 1/2" | 0.500" | 12.70 | General use |
| 9/16" | 0.563" | 14.29 | General use |
| 5/8" | 0.625" | 15.88 | General use |
| 11/16" | 0.688" | 17.46 | General use |
| 3/4" | 0.750" | 19.05 | Common — close to 19mm metric |
| 13/16" | 0.813" | 20.64 | General use |
| 7/8" | 0.875" | 22.23 | Common — close to 22mm metric |
| 15/16" | 0.938" | 23.81 | General use |
| 1" | 1.000" | 25.40 | General use |
| 1-1/16" | 1.063" | 26.99 | Close to 27mm metric |
| 1-1/8" | 1.125" | 28.58 | General use |
| 1-3/16" | 1.188" | 30.16 | Close to 30mm metric |
| 1-1/4" | 1.250" | 31.75 | General use |
| 1-5/16" | 1.313" | 33.34 | General use |
| 1-3/8" | 1.375" | 34.93 | General use |
| 1-7/16" | 1.438" | 36.51 | Close to 36mm metric |
| 1-1/2" | 1.500" | 38.10 | General use |
Metric to Imperial Spanner Conversion Chart
No exact metric-to-imperial match exists for most sizes — the measurement systems are independent. The table below shows the closest imperial spanner to each common metric size. Where the difference is large, the fit will be too loose for torqued fasteners. Always use the correct size where precision matters.
| Metric Size (mm) | Closest Imperial | Imperial in mm | Difference |
| 7 | 9/32" | 7.14 | +0.14mm |
| 8 | 5/16" | 7.94 | -0.06mm (tight) |
| 10 | 3/8" | 9.53 | -0.47mm (won't fit) |
| 11 | 7/16" | 11.11 | +0.11mm |
| 13 | 1/2" | 12.70 | -0.30mm (won't fit) |
| 14 | 9/16" | 14.29 | +0.29mm |
| 17 | 11/16" | 17.46 | +0.46mm |
| 19 | 3/4" | 19.05 | +0.05mm ✓ |
| 22 | 7/8" | 22.23 | +0.23mm |
| 24 | 15/16" | 23.81 | -0.19mm (tight) |
| 27 | 1-1/16" | 26.99 | -0.01mm ✓ |
| 30 | 1-3/16" | 30.16 | +0.16mm |
| 32 | 1-1/4" | 31.75 | -0.25mm (tight) |
| 36 | 1-7/16" | 36.51 | +0.51mm |
| 41 | 1-5/8" | 41.28 | +0.28mm |
| 46 | 1-13/16" | 46.04 | +0.04mm ✓ |
BSP Fitting Spanner Sizes
BSP (British Standard Pipe) sizes are nominal pipe bore sizes — not the actual across-flats measurement of the fitting. This catches people out: a 1/2" BSP fitting requires a 26mm spanner, not a 1/2" (12.7mm) one. The table below shows the spanner size needed for common BSP male threaded fittings. Sizes may vary slightly between fitting types and manufacturers.
| BSP Size | Spanner Size (AF) | Common Application |
| 1/8" BSP | 14mm | Small fittings, gauges, bleed nipples |
| 1/4" BSP | 19mm | Air fittings, small hydraulic connectors |
| 3/8" BSP | 22mm | General plumbing, pneumatic lines |
| 1/2" BSP | 26mm | Most common BSP size — hydraulic and pneumatic fittings |
| 3/4" BSP | 32mm | General industrial plumbing |
| 1" BSP | 41mm | Larger hydraulic and plumbing fittings |
| 1-1/4" BSP | 50mm | Large pipe and industrial fittings |
| 1-1/2" BSP | 55mm | Large pipe fittings |
| 2" BSP | 65mm | Very large industrial fittings |
Open-End, Ring and Combination Spanners
Choosing the right type of spanner matters as much as choosing the right size. Each type suits different situations.
| Spanner Type | How It Grips | Use When | Limitation |
| Open-end | 2 flats | Access is tight, fastener is in good condition, speed matters | More likely to round worn fasteners |
| Ring (box-end) | All 6 flats (12-point) | High torque, corroded or tight fasteners, precision work | Must be dropped over the fastener — needs clearance above |
| Combination | Open one end, ring other end | General use — ring to break loose or torque, open to run down | Both ends are the same size |
| Flare nut (crow's foot) | 5 flats — slotted ring | Brake and fuel lines — allows the spanner to pass over the line | Lower torque rating than a solid ring spanner |
| Ratchet spanner | Ring with ratchet mechanism | Tight spaces where a full swing arc isn't possible | Not suited to very high torque |
Frequently Asked Questions
What does AF mean in spanner size?
AF stands for Across Flats — the distance between two parallel faces of a hex fastener. Both metric and imperial spanners are sized by this measurement. When you see a spanner marked "3/4 AF", it means the jaw opens to 3/4 inch across the flats. Metric spanners don't usually carry the AF label but are sized the same way.
Is M6 a 10mm spanner?
Yes. An M6 bolt has a 10mm hex head, so it takes a 10mm spanner. The M6 refers to the thread diameter (6mm), not the spanner size — the head is always larger than the thread.
What spanner size for M8 bolt?
An M8 bolt has a 13mm hex head, so you need a 13mm spanner. The bolt diameter (8mm) and the spanner size (13mm) are different measurements — the spanner fits the hex head, not the thread shank.
What spanner size for M10 bolt?
An M10 bolt has a 17mm hex head in the AS 1112 standard (most common in Australia). Some older or imported M10 bolts use a 16mm head — measure across the flats if uncertain.
What spanner size for M12 bolt?
An M12 bolt has a 19mm hex head. M12 fasteners are extremely common on automotive suspension, structural brackets and heavy machinery — 19mm is a workshop staple.
What spanner size for M16 bolt?
An M16 bolt has a 24mm hex head. M16 is used for heavy structural connections, lifting points and large machinery — pair with a long-handle ring spanner for adequate leverage.
What is 13mm spanner in imperial?
A 13mm spanner is approximately 1/2 inch (12.7mm). The difference is 0.3mm — close enough for occasional use but always use the matching size on torqued fasteners to avoid rounding.
What is 17mm spanner in imperial?
A 17mm spanner is approximately 11/16 inch (17.46mm). 17mm fits M10 hex heads (the most common automotive bolt size) — 11/16" will fit loosely but isn't recommended for torqued work.
What is 19mm spanner in imperial?
A 19mm spanner is equivalent to 3/4 inch (19.05mm) — only 0.05mm difference. This is the closest metric/imperial pair and either spanner can usually be used interchangeably on a light fit.
What is 22mm spanner in imperial?
A 22mm spanner is approximately 7/8 inch (22.23mm). 22mm fits M14 hex bolts and is a common spark plug spanner size.
What is the most common spanner size?
In metric, 10mm is the most frequently used spanner size — it fits M6 bolt heads, which appear on engines, brackets and interior components across virtually every vehicle and machine. In imperial, 1/2" and 9/16" are among the most common AF sizes.
What is the difference between AF and metric spanners?
Both metric and AF (across flats) spanners measure the jaw opening the same way — across the flats of the fastener. The difference is the unit: metric spanners are sized in millimetres, AF spanners in fractions of an inch. A metric spanner is labelled in whole millimetres (19mm, 22mm), an AF spanner in fractions (3/4", 7/8").
Can I use a metric spanner on an imperial fastener?
In some cases yes — where the metric size is very close to the imperial size. The best match is 19mm and 3/4" (19.05mm), where the difference is only 0.05mm. For torqued fasteners always use the correct spanner to avoid rounding. The conversion chart above shows the closest matches and their differences.
What spanner do I need for BSP fittings?
BSP fittings require a larger spanner than the pipe size suggests. The most common — 1/2" BSP — requires a 26mm spanner, not a 1/2" (12.7mm) one. Always refer to the BSP fitting spanner chart above, as the pipe bore and the fitting hex are completely different measurements.
What is the difference between a ring spanner and an open-end spanner?
A ring spanner has a closed circular end that fits over the fastener and engages all six flats. This reduces rounding and allows more torque safely. An open-end spanner has a U-shaped jaw that engages only two flats — it can be inserted sideways for tight spaces but is more likely to slip or round a worn fastener. For any high-torque application, use the ring end.
What is a flare nut spanner used for?
A flare nut spanner (also called a crow's foot spanner) has a ring end with a slot cut into it, allowing it to pass over a brake line or fuel line before engaging the fitting nut. It grips five of the six flats rather than two — giving much better purchase than an open-end while still letting you slide onto a fitting with a line attached. Essential for brake and fuel line work.
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