5/16” Screw & Bolt Dimensions: Diameter & Head Size

5/16″ screw dimensions and bolt sizes are essential for heavy automotive applications, industrial machinery, and structural assemblies. This size have a 0.313″ nominal major diameter and is available in 5/16″-18 coarse (UNC), 5/16″-24 fine (UNF), or 5/16″-32 extra-fine (UNEF) thread pitches, representing 18, 24, and 32 threads per inch (TPI) respectively. This reference chart follows ANSI/ASME B18.6.3 and B18.3 standards for the head size and ANSI/ASME B1.1 standards for the thread dimensions to ensure compatibility across imperial fastening applications.

Disclaimer: Dimensions and tolerances may vary slightly depending on head style, manufacturer, or material — always verify with the official product datasheet before use.

Thread diagram fot bolt and screw depicting the major diameter, minor diameter, pitch diameter, and threads per inch
Hex head diagram
Cap head diagram
Low-cap head diagram
Button head diagram
Flat head diagram
Pan head diagram
5/16-18 UNC Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 0.2982 0.3113
Pitch Diameter 0.2691 0.2752
Minor Diameter Not specified 0.2451
Threads Per Inch (TPI) 18
Major Diameter 0.3026 0.3113
Pitch Diameter 0.2712 0.2752
Minor Diameter Not specified 0.2451
Threads Per Inch (TPI) 18
Major Diameter 0.3038 0.3125
Pitch Diameter 0.2734 0.2764
Minor Diameter Not specified 0.2463
Threads Per Inch (TPI) 18
5/16-20 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 0.3032 0.3113
Pitch Diameter 0.2747 0.2788
Minor Diameter Not specified 0.2518
Threads Per Inch (TPI) 20
Major Diameter 0.3044 0.3125
Pitch Diameter 0.277 0.28
Minor Diameter Not specified 0.253
Threads Per Inch (TPI) 20
5/16-24 UNF Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 0.3006 0.3114
Pitch Diameter 0.2788 0.2843
Minor Diameter Not specified 0.2618
Threads Per Inch (TPI) 24
Major Diameter 0.3042 0.3114
Pitch Diameter 0.2806 0.2843
Minor Diameter Not specified 0.2618
Threads Per Inch (TPI) 24
Major Diameter 0.3053 0.3125
Pitch Diameter 0.2827 0.2854
Minor Diameter Not specified 0.2629
Threads Per Inch (TPI) 24
5/16-28 UN Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 0.305 0.3115
Pitch Diameter 0.2848 0.2883
Minor Diameter Not specified 0.269
Threads Per Inch (TPI) 28
Major Diameter 0.306 0.3125
Pitch Diameter 0.2867 0.2893
Minor Diameter Not specified 0.27
Threads Per Inch (TPI) 28
5/16-32 UNEF Screw & Bolt – ANSI/ASME B1.1
Dimension Size (in)
Min Max
Major Diameter 0.3055 0.3115
Pitch Diameter 0.2879 0.2912
Minor Diameter Not specified 0.2743
Threads Per Inch (TPI) 32
Major Diameter 0.3065 0.3125
Pitch Diameter 0.2897 0.2922
Minor Diameter Not specified 0.2753
Threads Per Inch (TPI) 32
  • Major Diameter – The largest diameter of the external thread, measured crest to crest. This defines the nominal size of the screw or bolt.
  • Pitch Diameter – The diameter of an imaginary cylinder where the thread thickness equals the space between threads. This dimension primarily controls thread fit and tolerance class of 2A and 3A.
  • Minor Diameter – The smallest diameter of the external thread, measured root to root. This affects thread strength and clearance.
  • Threads Per Inch (TPI) – The number of complete threads within one inch of thread length. Higher TPI values indicate a finer thread pitch.
Hex Head – ANSI/ASME B18.2.1
Dimension Size (in)
MinMax
Width Across Flats 0.484 0.5
Width Across Corners 0.552 0.577
Head Height 0.195 0.235
Hex Socket Cap Head – ANSI/ASME B18.3
Dimension Size (in)
MinMax
Head Diameter 0.446 0.469
Head Height 0.306 0.312
Hex Socket Size 1/4 (0.25 in)
Hex Socket Low Cap Head – ANSI/ASME B18.3
Dimension Size (in)
MinMax
Head Diameter 0.431 0.437
Head Height 0.152 0.158
Hex Socket Size 5/32 (0.156 in)
Hex Socket Button Head – ANSI/ASME B18.3
Dimension Size (in)
MinMax
Head Diameter 0.527 0.547
Head Height 0.152 0.166
Hex Socket Size 3/16 (0.188 in)
Hex Socket Flat Head – ANSI/ASME B18.3
Dimension Size (in)
MinMax
Head Diameter 0.6 0.656
Head Height 0.198 Not specified
Hex Socket Size 3/16 (0.188 in)
Slotted Flat Head – ANSI/ASME B18.6.3
Dimension Size (in)
MinMax
Head Diameter 0.556 0.597
Head Height 0.191 Not specified
Slot Width 0.072 0.084
Slot Depth 0.058 0.088
Phillips Flat Head – ANSI/ASME B18.6.3
Dimension Size (in)
MinMax
Head Diameter 0.556 0.597
Head Height 0.191 Not specified
Drive Size 4
Phillips Pan Head – ANSI/ASME B18.6.3
Dimension Size (in)
MinMax
Head Diameter 0.594 0.615
Head Height 0.203 0.218
Drive Size 4
Inch to Metric Converter
Enter inches to convert to mm
Previous & Next Size Chart
← 1/4
→ 3/8

← Choose Another Size

Frequently Asked Questions

What is the tap drill size for 5/16″-18, 5/16″-24, and 5/16″-32 screws?

The appropriate tap drill depends on the material and required thread engagement. For aluminum, brass, or plastic (75% thread), an F (0.257″) drill bit is specified for 5/16″-18, an I (0.272″) for 5/16″-24, and a 9/32″ (0.281″) for 5/16″-32. For steel, stainless, or iron (50% thread), a J (0.277″) drill bit is recommended for 5/16″-18, a 9/32″ (0.281″) for 5/16″-24, and an L (0.290″) for 5/16″-32.

What clearance hole size is needed for a 5/16″ screw?

For a Close Fit, a P (0.323″) drill bit is utilized. For a Normal/Free Fit (standard clearance), a Q (0.332″) drill bit is specified to allow the screw body to pass through the material without binding.

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 holes 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.