M18 screw or bolt dimensions and sizes are heavy-duty fasteners typically found in heavy vehicle hubs, industrial hydraulic systems, large plant machinery, and structural base plates. This size has a 18.00mm nominal major diameter and is available in M18 x 2.5 for the coarse series and M18 x 1.5 for the fine series, where 2.5 and 1.5 represent the thread pitch in millimeters. This reference chart follows respective DIN and ISO standards for each head style and DIN 13-1 (coarse), DIN 13-21 (fine), and ISO 68-1 standards for thread dimensions to ensure global compatibility.
Disclaimer: Actual dimensions and tolerances may vary slightly depending on head style (e.g., Hex Head, Socket Head Cap Screw, Flange Bolt), manufacturer, or material grade — always verify with the official product datasheet before use.
M18 x 2.5 Screw & Bolt – DIN 13-1/ISO 68-1
Dimension
Size (mm)
Min
Max
Major Diameter
17.623
17.958
Pitch Diameter
16.164
16.334
Minor Diameter
14.624
15.252
Thread Pitch
2.5
Major Diameter
17.623
17.958
Pitch Diameter
16.228
16.334
Minor Diameter
14.688
15.252
Thread Pitch
2.5
M18 x 1.5 Screw & Bolt – DIN 13-21/ISO 68-1
Dimension
Size (mm)
Min
Max
Major Diameter
17.732
17.968
Pitch Diameter
16.854
16.994
Minor Diameter
15.93
16.344
Thread Pitch
1.5
Major Diameter
17.732
17.968
Pitch Diameter
16.904
16.994
Minor Diameter
15.98
16.344
Thread Pitch
1.5
M18 x 2.0 Screw & Bolt – DIN 13-21/ISO 68-1
Dimension
Size (mm)
Min
Max
Major Diameter
17.682
17.962
Pitch Diameter
16.503
16.663
Minor Diameter
15.271
15.797
Thread Pitch
2.0
Major Diameter
17.682
17.962
Pitch Diameter
16.563
16.663
Minor Diameter
15.331
15.797
Thread Pitch
2.0
M18 x 1.0 Screw & Bolt – DIN 13-21/ISO 68-1
Dimension
Size (mm)
Min
Max
Major Diameter
17.794
17.974
Pitch Diameter
17.206
17.324
Minor Diameter
16.59
16.891
Thread Pitch
1.0
Major Diameter
17.794
17.974
Pitch Diameter
17.249
17.324
Minor Diameter
16.633
16.891
Thread Pitch
1.0
Major Diameter –
The largest diameter of the external thread, measured crest to crest.
This defines the nominal size (e.g., M12 which is 12 mm of nominal major diameter).
Pitch Diameter –
The diameter of an imaginary cylinder where the thread thickness equals
the space between threads. This is the critical dimension for thread fit
(Tolerance classes 2A/3A for Imperial; 6g/4g6g for Metric).
Minor Diameter –
The smallest diameter of the external thread, measured root to root.
This represents the core strength of the fastener and is the theoretical
maximum diameter of the tap drill.
Thread Pitch –
The distance from a point on one thread to the corresponding point on the next.
What is the tap drill size for M18 screws or bolts?
For metric fasteners, the recommended tap drill size can be calculated using this formula: Major Diameter – Thread Pitch. For an M18 x 2.5 (Coarse), the recommended tap drill is 15.50mm. For an M18 x 1.5 (Fine), the recommended drill size is 16.50mm.
What clearance hole size is needed for an M18 screw or bolt?
For an M18 fastener, a 19.00mm drill bit is utilized for a Close Fit, a 20.00mm bit is used for a Normal Fit, and a 21.00mm bit is recommended for a Loose Fit. A Normal Fit is the industry standard to ensure the bolt shank passes through the assembly without binding while maintaining structural alignment.
What is the difference between a bolt and a screw?
The difference is based on the intent of the fastener’s use; If the primary intent is for the fastener to pass through an unthreaded hole and be assembled with a nut, it is a Bolt. In this application, you usually hold the head and turn the nut to apply tension. But if the primary intent is for the fastener to be installed into a threaded (tapped) hole, it is a Screw. In this application, the connection is tightened by turning the head of the fastener itself.
