US Bolt Head/Wrench Size Bolt Diameter Head and Wrench Size Hex Bolt – Lag Bolt – Square Bolt Heavy Hex Bolt 9/16″ 13/16″ – 5/8″ 15/16″ 1-1/16″ 3/4″ 1-1/8″ 1-1/4″ Click to see full answer. Hereof, what size wrench fits a 20mm bolt? Metric Bolt Head/Wrench Size Bolt Diameter (mm) Head/Wrench Size (mm) ANSI/ISO DIN/ISO Heavy Hex 16 24 27 18 – – 20 30 34/32* Also Know, what size bolt takes a 15mm wrench? The standard wrenches needed are: 11,13,15,17,19,22,24,27,30,32,36mm. I believe that the bold that goes with the 15mm wrench is normally an M8. M8s usually use 12 or 13mm heads. People also ask, what size nut goes on a 3/8 bolt? US Nut Size Table Size Diameter* Height Hex Nut Hex Nut 1/4″ 7/16″ 7/32″ 5/16″ 1/2″ 17/64″ 3/8″ 9/16″ 21/64″ What size hole do I need for an m12 bolt?When using the M12 Bolt, you will need to drill a 12mm diameter hole into the substrate and there will need to be a 14mm diameter clearance hole in the item to be fixed in position.