When it comes to mechanical work, understanding the right spanner size is key to successfully tightening or loosening nuts and bolts. Spanners, often called wrenches in certain regions, are indispensable tools for any toolkit. But the question many people ask is, “What size spanner do I need?
To answer that, we need to consider both the type of thread on your nut or bolt and the spanner size that matches it. Whether you’re dealing with metric or imperial measurements, selecting the correct size is important to avoid damaging the fastener or the tool.
Metric spanners are typically sized from 6mm to 32mm, while imperial spanners might range from 1/4″ to 1″. Knowing the exact spanner size you need isn’t just about looking for a close fit; it’s about ensuring a snug, secure grip to apply the right amount of torque without slipping.
A spanner size chart can be your best friend for those working with various bolt sizes. This chart helps you quickly identify the right spanner for any bolt size, preventing any mishaps during your task. We recommend downloading a bolt and spanner size chart PDF to simplify your job. It’s a handy reference that you can keep on hand for quick access whenever you need it.
Spanner Size Chart for Bolt
| Millimetres | Spanner |
| – | 1/4 AF |
| 7mm | 7mm |
| – | 5/16 AF |
| 8mm | 8mm |
| – | 11/32 AF; 1/8 Wworth |
| 9mm | 9mm |
| – | 3/8 AF |
| 10mm | 10mm |
| 11mm | 11mm |
| – | 7/16 AF |
| – | 3/16 Wworth; 1/4 |
| 12mm | 12mm |
| – | 1/2 AF |
| 13mm | |
| – | 1/4 Wworth; 5/16 BSF |
| 14mm | |
| – | 9/16 AF |
| 15mm | |
| – | 5/16 Wworth; 3/8 BSF |
| – | 5/8 AF |
| 16mm | 16mm |
| 17mm | 17mm |
| – | 11/16 AF |
| 18mm | 18mm |
| – | 3/8 Wworth; 7/16 BSF |
| 19mm | 19mm |
| – | 3/4 AF |
| 20mm | 20mm |
| – | 13/16 AF |
| – | 7/16 Wworth; 1/2 BSF |
| 21mm | 21mm |
| 22mm | 22mm |
| – | 7/8 AF |
| 23mm | 23mm |
| – | 1/2 Wworth; 9/16 BSF |
| – | 15/16 AF |
| 24mm | 24mm |
| 25mm | 25mm |
| – | 1in AF |
| – | 9/16 Wworth; 5/8 BSF |
| 26mm | 26mm |
| 27mm | 1 1/16 AF; 27mm |
| – | 5/8 Wworth; 11/16 BSF |
| – | 1 1/8 AF |
| 30mm | 30mm |
| – | 11/16 Wworth; 3/4 BSF |
| – | 1 1/4 AF |
| 32mm | 32mm |
| – | 3/4 Wworth; 7/8 BSF |
| – | 1 5/16 AF |
| – | 13/16 Wworth; 15/16 BSF |
| 36mm | 36mm |
| – | 1 7/16 AF |
| – | 7/8 Wworth; 1in BSF |
| – | 1 1/2 AF |
| 40mm | 15/16 AF; 40mm exactly |
| 41mm | 41mm |
| – | 1 5/8 AF |
| – | 1 in Wworth; 1 1/8 BSF |
| – | 1 11/16 AF |
| 46mm | 46mm |
| – | 1 13/16 AF |
| – | 1 1/8 Wworth; 1 1/4 BSF |
| – | 1 7/8 AF |
| 50mm | 50mm |
| – | 2in AF |
| – | 1 1/4 Wworth; 1 3/8 BSF |
| 55mm | 55mm |
| 60mm | 60mm |
The spanner sizes for bolts listed in this table are approximate and may vary slightly depending on the manufacturer or the specific bolt type. Checking the exact specifications of the bolt is always recommended to ensure the correct spanner size.
Metric Spanner Size Chart for Bolts
| Bolt Diameter (mm) | Spanner Size (mm) |
| M4 | 7 |
| M5 | 8 |
| M6 | 10 |
| M8 | 13 |
| M10 | 17 |
| M12 | 19 |
| M14 | 22 |
| M16 | 24 |
| M18 | 27 |
| M20 | 30 |
| M22 | 32 |
| M24 | 36 |
| M27 | 41 |
| M30 | 46 |
| M33 | 50 |
| M36 | 55 |
| M39 | 60 |
Imperial Spanner Size Chart
| Bolt Size (Inches) | Spanner Size (Inches) |
| 1/4″ | 7/16″ |
| 5/16″ | 1/2″ |
| 3/8″ | 9/16″ |
| 7/16″ | 5/8″ |
| 1/2″ | 3/4″ |
| 9/16″ | 13/16″ |
| 5/8″ | 15/16″ |
| 3/4″ | 1-1/8″ |
| 7/8″ | 1-5/16″ |
| 1″ | 1-1/2″ |
How To Calculate Spanner Size For A Bolt
To calculate the spanner size for a bolt, follow these simple steps:
- Metric Bolts: Multiply the bolt’s diameter by 1.5. For example, for an M10 bolt, the spanner size is typically 15mm (10mm × 1.5).
- Imperial Bolts: Match the bolt’s head size directly to the spanner size in inches or fractions.
Alternatively, use a standard bolt-to-spanner size chart for quick reference.
What is Spanner Size Calculation Formula For Bolts?
For Metric Bolts:
The spanner size is typically 1.5 times the bolt’s diameter.
- Formula:
Spanner size (mm) = Bolt diameter (mm) × 1.5 - Example: For an M10 bolt (10mm diameter), the spanner size is:
10mm × 1.5 = 15mm.
For Imperial Bolts:
The spanner size corresponds to the across flats measurement of the bolt head, which is usually listed on a size chart.
- Example: A 1/4-inch bolt may require a 7/16-inch spanner.
For the most accurate results, it’s recommended to refer to a bolt-to-spanner size chart, as the formula may vary slightly for different standards (like ISO, DIN, or ANSI).
