What size wrench or spanner fits M18?

M18 bolt uses a 27 mm wrench or spanner, measured across the flats of the hex head.

What is the torque for M18?

A typical tightening torque for M18 bolt in 8.8 on dry threads is about 260 Nm (191.8 ft·lb). Always confirm against the manufacturer spec for important joints.

What thread pitch and tap drill does M18 use?

M18 bolt has a thread pitch of 2.5 mm and uses a 15.5 mm tap drill, with a 20 mm clearance hole.

What is M18 used for?

M18 is commonly used in heavy structural, machinery, automotive. It is one of the most popular sizes in its category.

What is the closest imperial size to M18?

The closest imperial size to M18 is approximately ~3/4 inch, though they are not exactly interchangeable.

Home › Metric Bolts › M18

M18 bolt size

27 mm wrench

across-flats spanner / socket size for M18

M18 bolt — Heavy structural, machinery, automotive.

Full specification

System	Metric coarse
Nominal diameter	18 mm
Thread pitch	2.5 mm
Wrench / spanner (across flats)	27 mm
Tap drill	15.5 mm
Clearance hole	20 mm
Common grade / class	8.8
Typical torque	260 Nm (191.8 ft·lb)
Closest other-system size	~3/4 inch

🔧 Torque unit converter

Newton-metres (Nm) Foot-pounds (ft·lb) Inch-pounds (in·lb)

Pre-filled with the typical torque for M18 (8.8, dry). Edit any field to convert.

⚠️ Torque values here are general references for 8.8 on clean, dry threads. Lubrication, coatings, and the joint material change the figure significantly. For structural, automotive, or safety-critical work, always use the torque specified by the manufacturer or engineer.

About M18

Heavy structural, machinery, automotive.

A 15.5 mm tap drill is correct for M18 coarse thread. The standard clearance hole is 20 mm.

M18 is part of the ISO metric coarse thread series, defined in ISO 261. The wrench size 27 mm is measured across the flats of the hex head.

Common applications

M18 bolts are widely used in:

Heavy structural, machinery, automotive
General machinery and equipment assembly
Automotive components and repairs
Structural connections (when specified by engineer)
Furniture and fixture assembly

Material and grade information

The typical grade for M18 is 8.8. This indicates:

Property class 8.8: Medium carbon steel, quenched and tempered
Tensile strength: 800 MPa minimum
Yield strength: 640 MPa minimum (0.2% proof stress)
Common coatings: Zinc plated, hot-dip galvanized, or plain

Frequently asked questions

What size wrench or spanner fits M18?: M18 bolt uses a 27 mm wrench or spanner, measured across the flats of the hex head.
What is the torque for M18?: A typical tightening torque for M18 bolt in 8.8 on dry threads is about 260 Nm (191.8 ft·lb). Always confirm against the manufacturer spec for important joints.
What thread pitch and tap drill does M18 use?: M18 bolt has a thread pitch of 2.5 mm and uses a 15.5 mm tap drill, with a 20 mm clearance hole.
What is M18 used for?: M18 is commonly used in heavy structural, machinery, automotive. It is one of the most popular sizes in its category.
What is the closest imperial size to M18?: The closest imperial size to M18 is approximately ~3/4 inch, though they are not exactly interchangeable.