How to Read Bolt Specifications (Size, Thread, Length, Material)

A bolt specification reads left to right, and the first three numbers carry most of the meaning. In 3/8-16 x 1-1/2, 3/8 is the nominal diameter in inches, 16 is the number of threads per inch (TPI), and 1-1/2 is the length in inches. Because 16 TPI is the coarse thread for a 3/8 inch bolt, that callout is a 3/8-16 UNC bolt, 1-1/2 inches long. If you're holding the bolt and don't have the spec, start with how to measure bolts and screws instead.

Everything after the size describes what the bolt is and how it was made: thread class, head and drive, material, finish and the standard it conforms to. To replace a bolt, the diameter, TPI, length and head style have to match. Material and finish are a choice you make for the environment. A full line from a product page looks like this: 3/8-16 x 1-1/2 hex head cap screw, partially threaded, 18-8 stainless steel (A2-70), plain finish, ASME B18.2.1.

What does each part of a bolt spec mean?

Here is that example line taken apart, field by field. The values come from Fastenio's 3/8-16 partially threaded 18-8 hex head cap bolt page.

The sections below go through the fields that most often cause a wrong order. The size notation itself has its own explainer: what 1/4-20 means.

How do you know if a bolt is coarse or fine thread?

The spec usually doesn't say "UNC" or "UNF". The TPI tells you. Each inch diameter has one coarse and one fine pitch, so a TPI that matches the coarse column is UNC and one that matches the fine column is UNF. These pairs are from the thread pitch chart:

So 1/4-20 is coarse and 1/4-28 is fine. Both are 1/4 inch bolts, and their nuts won't interchange. Coarse is the default for general work. Coarse vs. fine thread covers when fine thread is worth choosing.

What does 2A mean in a thread callout?

The suffix at the end of a full thread callout, as in 3/8-16 UNC-2A, is the thread class. Unified inch threads are toleranced under ASME B1.1 in three fit classes: 1, 2 and 3. A higher number means tighter tolerances. "A" marks an external thread (the bolt) and "B" an internal thread (the nut or tapped hole). Portland Bolt, a bolt manufacturer, notes that most construction fasteners are made to Class 2 tolerances.

Many catalog specs leave the class off: Fastenio's product pages list the thread as "3/8-16 UNC (Coarse)" with no class suffix. Treat a 3A or 3B callout on a drawing as a deliberate requirement, and confirm it with the part's spec before substituting a standard bolt.

Where is bolt length measured from?

Length is measured from the surface the head bears on, so the starting point depends on the head style:

That's why a 1 inch flat head screw and a 1 inch hex bolt don't look the same length on the bench. The rules come from how to measure bolts and screws, which also covers the standard length steps.

Thread length is a separate field. A fully threaded bolt is threaded right up to the head. A partially threaded bolt has a smooth shank and a set length of thread at the tip. ASME B18.2.1 sets that thread length from the diameter and the overall length, and a product page shows the result. For example, the 3/8-16 18-8 partially threaded hex bolts carry 1 inch of thread on lengths from 1-1/2 to 6 inches and 1-1/4 inches on lengths from 6-1/2 to 11 inches. If a nut has to run down against a thick stack, check the thread length, not just the overall length.

Which parts of the spec have to match?

When you replace a bolt or order a second source for one, split the fields into two groups.

Must match:

• Diameter and TPI. A 3/8-16 nut fits only a 3/8-16 bolt. See how to match nuts and bolts.
• Length, measured the right way for the head style.
• Head style. A countersunk hole needs a flat head. A hex bolt and a socket cap screw also take different tools and clearance. The bolt size chart lists head and wrench sizes.
• Thread coverage, if the joint depends on the nut seating on thread rather than on the smooth shank.
• Thread class, if the drawing specifies one other than 2A.

Can vary, as a deliberate choice:

• Material. 18-8 for general indoor and outdoor work. 316 where salt water, pool chemicals or other chlorides reach the joint. The 18-8 vs. other grades guide has the full comparison. On Fastenio, hex head bolts come in 18-8, 316 and silicon bronze. 410 stainless shows up on hardened products such as self-drilling screws.
• Finish. Stainless is usually sold plain, because the base metal already resists corrosion. The catalog also lists chrome-plated, wax-coated and black fluoropolymer versions of some 18-8 hex bolts.

Why don't stainless steel bolts have grade marks?

Grade 5 and Grade 8 marks (the radial lines on a bolt head) come from SAE J429, which covers carbon and alloy steel bolts. Fastenio doesn't sell SAE graded bolts, and a stainless bolt isn't specified that way. In inch sizes, stainless bolts are covered by ASTM F593 for diameters from 1/4 to 1-1/2 inch inclusive, according to Portland Bolt's summary of the standard. F593 sorts stainless into alloy groups: Group 1 includes 304 and 305 (the 18-8 family), Group 2 is 316 and 316L, and Group 5 includes 410. Each group and condition has its own marking code, such as F593C, so a marked stainless head carries an F593 letter code, not grade lines.

Metric stainless uses ISO 3506 names instead. A2 is the 304 (18-8) family and A4 is the 316 family. The number after the dash is the property class, so "A2-70" on a product page means 18-8 stainless in ISO 3506 property class 70. For strength values, read the F593 or ISO 3506 tables for the exact part. Don't assume them from the material name.

How do you read a metric bolt spec?

The order is the same, written with an M: M8-1.25 x 30 is an 8 mm diameter, a 1.25 mm pitch (the distance between threads, not a count per inch), and a 30 mm length. Metric lengths are measured from the same points as inch lengths. An M8 and a 5/16 inch bolt are close in diameter but their threads aren't interchangeable, so the M at the start is the first thing to check on a mixed parts list.

The bottom line

Read a bolt spec left to right: diameter, TPI, length, then head, thread coverage, material, finish and standard. Diameter, TPI, length and head have to match the part you're replacing. Material and finish are the choices you make for where the bolt goes. The technical specs page explains how those fields appear on every Fastenio product page, and the specs table on each product shows the thread, head dimensions, thread length and standard for that exact part.

To buy, start from hex head bolts and filter by size, length and material, then pair them with matching hex nuts.