Bolt Size Chart

Standard Bolt Sizes (US) 

Our chart provides detailed specifications for standard US hex bolts, helping you select the right bolt size for your project. Bolt dimensions as per ASME B18.2.1.

[Download our printable standard bolt size chart pdf.]

Disclaimer: The above is a compilation of data, from various industry standards. Fastener Depot has taken every effort to present the data accurately. However Fastener Depot cannot be held liable for any claim traceable to any errors typographical or otherwise contained herein.

[Download our US bolt size chart.]

How To Read The Bolt Size Chart

Nominal Size or Basic Product Diameter

This column indicates the common reference size for bolts, which is typically related to the outer diameter of the bolt's threads.

Body Diameter

The body diameter refers to the actual diameter range of the bolt shank. It indicates the largest and smallest permissible diameters of the unthreaded portion of the shank.

Width Across Flats

The distance across the flat sides of the hex head, which is critical as it ensures proper wrench fit during tightening.

Width Across Corners

This measures the diagonal distance across the hex head and is important for determining clearance in restricted spaces.

Head Height

The height of the bolt head from the base to the top. This dimension is essential in ensuring there is enough clearance for bolt installation and socket depth.

Thread Length

Listed for screws 6 inches and shorter, and for those over 6 inches. These reference dimensions indicate standard practice but might vary slightly based on specific application requirements or manufacturing practices.

Max & Min

Max and Min are values that describe the range of measurements that a bolt must fall under to meet industry specifications. These are used to ensure that bolts can be used with standard tools, have uniform fits, and can be safely swapped out with other standard bolts. 

Ref

Ref is short form for "reference." A reference dimension is provided for informational purposes only and is not subject to the same strict tolerances as other critical dimensions. It can be used for guidance or to provide additional context to the engineering or manufacturing process. 

Bolt Size FAQs

How to Measure Hex Bolt Diameter?

To measure a hex bolt’s diameter, you can use calipers, a bolt gauge, or a ruler to measure across the shank (unthreaded portion) of a bolt. Calipers or standard bolt gauges are the most accurate, but a ruler or wrench can be used in a pinch. However, accuracy is crucial when fastening metal, so be sure to use calipers or a bolt gauge for those applications.

Hex Cap Screws Vs Hex Bolts

Hex cap screws and hex bolts both feature hexagonal heads, but they differ in terms of their design and application. Hex cap screws are typically manufactured with tighter tolerances on the pitch diameter and are often specified when a precise fit is needed; they frequently feature a washer face underneath the head for a more consistent load distribution. 

Hex bolts, on the other hand, might have looser tolerances and may not include a washer face, making them more suitable for general-purpose fastening where precision is less critical. Both serve essential roles, but the choice between them depends on the specific requirements of your application.

Are Hex Cap Screws Measured The Same As Hex Bolts?

Yes, the same measurements can generally be applied to both hex cap screws and hex head bolts since they share similar dimensions such as diameter, head size, and thread length. Both types of fasteners typically conform to standards like ASME B18.2.1, which define the dimensional requirements for inch series fasteners. However, there are a few distinctions to consider:

1. Tolerances: Hex cap screws usually have tighter manufacturing tolerances, especially concerning the thread pitch and head dimensions, to ensure a more precise fit. This distinction might not always be apparent in a general size chart, but it's a critical consideration for applications requiring precision.

2. Features: Hex cap screws often include a washer face under the head and are produced to higher quality control standards, which might not be explicitly detailed in basic size charts but can be an important factor in their selection for specific applications.

3. Application: While size charts apply to both, the intended application can influence whether a hex bolt or a hex cap screw is more appropriate. When precision and shear load distribution are key, a cap screw may be preferred.

What Size Bolts To Use?

Unfortunately, there is no one-size-fits-all bolt to use across your projects, as bolts vary greatly in their load bearing capacity, material compatibility, fit, and more. If you’re not sure what size bolt to use, it’s best to discuss your needs with a professional.

Are Standard Bolt Sizes Measured In mm?

No, standard bolt sizes are measured in inches in compliance with ASME (American Society of Mechanical Engineers) standards. Outside of the US, most bolts are manufactured and measured in accordance with ISO, the international standard, which measures bolts in millimeters.

About The Fastener Depot

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