When tightening or loosening bolts, it’s essential to use the correct spanner (wrench) size to avoid rounding off the bolt head or damaging the tool.
Spanner size is directly related to the bolt diameter (M-size) and can be estimated using a simple formula.
Following formula are used to calculate spanner size for metric bolts-
✅ For bolt size upto M14
Spanner Size = ( Bolt Size * 1.5 ) + 1
Example for M12 bolt , Spanner Size = ( Bolt Size * 1.5 ) + 1 = ( 12 * 1.5 ) + 1 = 18 + 1 = 19
✅ Above from M16 bolt
Spanner size = Bolt Size * 1.5
Example for M16 bolt , Spanner Size = Bolt size * 1.5 = 16 * 1.5 = 24
Why Formula Differs for Smaller Bolts
For smaller bolts (M5 to M14), the head thickness and tolerance are proportionally higher, so the formula includes a +1 correction factor to ensure the wrench fits snugly.
Above M16, the bolt head size follows a standardized proportion (1.5× the nominal diameter), hence no correction is needed.
Common Types of Spanners/Wrenches
| Spanner Type | Description & Use |
|---|---|
| Open-End Spanner | Has U-shaped openings; used where rotation is limited. |
| Ring Spanner | Encloses the bolt head completely; better grip and torque. |
| Combination Spanner | One open-end + one ring-end; most commonly used. |
| Adjustable Spanner (Crescent Wrench) | Adjustable jaw fits various sizes. |
| Socket Wrench / Ratchet | Used for fast repetitive tightening or loosening. |
| Box Spanner / Tube Spanner | Used for deep or recessed bolts. |
| Allen Key / Hex Key | Used for socket head cap screws. |
Quick Reference Examples
| Bolt Size | Formula Used | Spanner Size |
|---|---|---|
| M10 | (10 × 1.5) + 1 | 16 mm ≈ 17 mm standard |
| M12 | (12 × 1.5) + 1 | 19 mm |
| M14 | (14 × 1.5) + 1 | 22 mm |
| M16 | (16 × 1.5) | 24 mm |
| M20 | (20 × 1.5) | 30 mm |
| M24 | (24 × 1.5) | 36 mm |