Bolt Strength Chart: Grade Comparison Table (Free PDF)

So if you’re beginning in either of these systems, read based off of this table. The Metric class include every common metric property class along with the two most used stainless designations.

You’ll see the minimum tensile strength in MPa as well as the same number converted to approximate ksi. Next, it will shows the minimum yield strength in MPa. It will also give you the minimum tensile strength in ksi from the Closest imperial grade / spec column. This give you the name of the SAE J429 grade or ASTM specification that sits in the same strength band. Then you have their minimum tensile and yield in ksi. This way you can easily see them both on the same row and not convert the numbers in your head.

Memorize these two combinations. The first is that SAE Grade 5 equals class 8.8. The second is that SAE Grade 8 equals class 10.9. Both times the min tensile values falls close together by a few percent. With the bigger sizes for 8.8 compared to Grade 5 they are 830 MPa vs 827 MPa. Or with 10.9 compared to Grade 8 they are 1040 MPa vs 1034 MPa.

Because they’re so close, workshops consider them equivalent. That holds true whether it’s going from imperial to metric or vice versa. On the structural side ASTM F3125 Grade A325 shares the Grade 5 level and Grade A490 shares the Grade 8 level. So structurally we has the same two pairings.

This table help you compare metric and imperial fastener strengths side by side so you can find the right equivalent without doing the math in your head.

Where there’s a loose equivalence, it’s flagged by a note in the Match notes column. There’s no such thing as a true imperial twin for classes 5.6, 6.8 and 9.8. Class 6.8 exist between Grades 2 and 5. Class 9.8 exists between Grades 5 and 8 and is manufactured only to M16.

Class 12.9 outstrips all hex-head SAE grades. In the imperial world, it is an ASTM A574 socket head cap screw at 180 ksi. Above 1/2 in., the value is 170 ksi. That’s one reason 12.9 hardware tends to be largely socket-head product.

Keep an eye out for the stainless rows. For example, 18-8 is the same as A2-70 on the basis of corrosion properties (the same alloys). Similarly, 316 is the same as A4-80 based off corrosion properties.

However, the strength is not the same. An A2-70 machine screw will be guaranteed at least 700 MPa minimum tensile (property class 70). Generic 18-8 commercial grade has around 586 MPa, or 85 ksi. This means that replacing an A2-70 with generic 18-8 loses you a fifth of the tensile strength. And you might never know.

There are two traps with this table. The first is that it is an equivalence at the stress level, not the load level. A 3/8 in Grade 5 and a M10 class 8.8 are equivalent materials. But they are vastly different bolts. The metric one has greater cross-section. See the bolt tensile strength chart on this site for the breaking and actual proof loads by size. Note that you should of never convert from this table.

The second trap is that the size splits count. Grade 2 goes to Grade 1 strength above 3/4 in. Grade 5 goes down a step above 1 in. Class 8.8 goes up a bit above M16. Always compare the diameter before saying two bolts are equal.

This applies unless you’re dealing with legacy British grades. You know, the ones that are labeled as BS something or other and have an R S T or a V in them? Those are the ones. Yeah, neither column maps clean up there.

It could be A307 Grade C anchor bolts. It could also be proprietary aerospace standards. Not any of those map into either column. If it says one of those things on a drawing, then do not pick up the closest moddern grade.

When you have a bolt in your hand and want to identify it, use this table. Read down the table to find the marking style family of the markings on the head. The table also shows the exact class or grade identified by the marking, the governing standard that defines it, a brief description of the material, the minimum tensile strength guaranteed by the marking, and the strength band of which it is a member. The bands is expanded in the following table.

There are three marking systems for bolt heads, and they all live together. One is the SAE system of radial lines. These are spaced equally around the head. Count the number of lines and add two to determine the grade. For example, three lines equals a Grade 5 and six lines equals Grade 8.

The ISO metric system use numeric stamps: 8.8 or 10.9. Here, the first number multiplied by 100 is the nominal tensile strength in MPa, while the product of the two numbers multiplied by 10 is the nominal yield strength. So 10.9 decodes to about 1000 MPa tensile and 900 MPa yield.

The ASTM system is just spec stamps, such as A325, 307A or B7, which are the simple names of the governing specification.

The worst mark is no mark at all. An imperial hex bolt with no mark on the head is at best Grade 2. However, it is typically just an ungraded import with no certainty as to its properties. Never allow an unmarked bolt into a joint designed for Class 8.8, Grade 8, or Grade 5.

Stainless is the exception. A lot of the time US 18-8 and 316 hardware has no mark at all to indicate strength, and that’s because what we are looking for is not strength but resistance to rust.

And there’s the almost-but-not-quite stuff: A metric 8.8 ain’t an SAE Grade 8. An 8.8 corresponds to a Grade 5, while a Grade 8 corresponds to a 10.9.

Then there are the boron versions (Grades 5.2 and 8.2). They match their parent grades at room temperature and wear identical radial lines, but they loses some margin in hot service due to their less expensive chemistry. You’ll have to read the paperwork or look for the maker mark to tell them apart.

On the other end of things, ASTM A449 is where the trick lies. It’s equal in strength to Grade 5, but seldom marked except by the manufacturer mark, making it easy to misidentify as ungraded.

Different products has different marks. Socket head cap screws typically has the mark on the side of the head. Hex bolts have it on top of the head. Studs and threaded rod (A193 B7, Grade 4 studs) do not have a head, so they are marked on the end of the rod.

In metric fasteners, the stamp can be either indented or raised up, but both are acceptable per ISO 898-1.

That's the strength grade only. The chart doesn't certify the fastener because head markings can be faked. Critical applications should of always be based off lot certification by the mill with a mill test report, rather than trusting what was read off the parts bin.

Once you have established the grade, consult the bolt tensile strength chart on this site for per-size breaking and proof loads. You can also use the equivalence table above to cross systems.

The table is meant to help you decide what to buy, not to help you identify what you have. So what you want to do is see which band of bolt to grab from the tool box for each job.

The table has rows, one for each strength band. This also indicates the minimum tensile range in ksi and MPa. Then it gives the corresponding imperial grade/specs and metric class. It include typical application and a column for things to avoid.

Many new buyers are surprised at the band's encoding of a rule: stronger is not necessarily better. Tensile refers specifically to tensile strength, and high tensile bolts gets their strength by sacrificing ductility.

That means a 12.9 screw may be too brittle to work in a joint rated for 8.8 preload. It may bend or experience some shock loading, which could cause it to crack. A softer (8.8) bolt would stretch and shrug off the load.

Match the band to what the joint was designed for. Only go up one band if you also recalculate the torque. And make sure your washer and mating threads can handles the increased preload.

The strength ladder is separate from the corrosion resistant band.

Ordinary austenitic stainless bolts has a strength of approximately class 8.8. A4-80 is at 800 MPa. Ordinary US 18-8 is substantially lower than Grade 5.

So: stainless is typically a strength downgrade AND a corrosion upgrade. You have reasonable options for wet environments with high strength numbers. You can use 10.9 hardware with a zinc-flake or geomet coating. Stainless is not a good option here.

There are two cautions regarding band boundaries. The first is hydrogen embrittlement, where galvanizing (hot dip) and electroplating will hydrogen charge the steel above about HRC 33 (1000 MPa). That is where the high band starts. Delayed fracture can occur days later. For that reason A490 structural bolts is never hot-dip galvanized.

Heat: Cold-worked classes and boron-steel variants tends to be more sensitive to loss of strength at high temperatures. These include the cold-worked classes 4.8, 5.8 and 6.8 and the boron variants 5.2 and 8.2. Grade 5 proper (8.8) is better for hot service. Flange work at temperature belong to A193 B7.

These represent a range around the strongest sizes of each grade. Do not use these for calculations. Clamp load and torque specs should of been obtained from the exact diameter and grade. The bolt tensile strength chart on this site has per-size table for you to use.

These are the sources for all those numbers in those other three tables.

This table show the material, heat treatment, metric classes or imperial grades, typical minimum tensile strength, hardness, and what it means in service.

The row contains a Material family with the Heat treatment it was given. The row also contains the Metric classes and Imperial grades made as such. It shows the Min tensile (typical) level the combination attain. It shows the Hardness (typical) range you’d measure on the finished bolt. Finally, it give a plain-language summary of what it means in service.

If you look at those grades you'll see something. Below about 600 MPa, they are just mild or medium carbon steel that has been drawn into shape, and cold working makes them a little stronger.

Class 8.8 and Grade 5 upwards is quenched and tempered. They are hardened by heating and quenching them into a state of hard martensite, followed by tempering them back to a set level of hardness. More carbon allow a harder temper, which makes it harder.

More alloying with elements like chromium, vanadium, boron, nickel and molybdenum will also allow a harder temper and help the bolt harden right through. That differentiates a Grade 5 from a Grade 8. This is the difference between an 8.8 and a 12.9.

Hardness is the quick field proxy for all this; in the field that means

A file will skate across a Grade 8 (HRC 33-39) surface.

The same file will affect a Grade 2 (HRB).

Mixed bins can be sorted reliably with a bench hardness tester.

Each grade has a defined window. On this site you’ll find the bolt tensile strength chart with each class’ hardness windows.

There is a limit here too, a A193 B7 is maxed out at HRC 35. Its B7M variation is lower still. Why? Because a soft bolt resist hydrogen and sulphide cracking in refinery service.

Be careful with the boron-steel rows. Low carbon boron chemistry is used in grades 5.2 and 8.2. A lot of imported 8.8 bolts is made from this chemistry too.

At room temperature, it passes all the tests for the parent grade. It’s when you get to temperature that the difference show up. Hot locations cause boron steels to temper back and become weaker than they were before. Examples of hot locations is exhaust systems and engine bolting. This characteristic leads to limited service temperatures being used in the versions in the standards.

These are the rules of stainless steel.

With austenitic A2 and A4 stainless you can’t even try to harden it by quenching.

Class numbers such as property class 50, 70 and 80 correspond to wire that is annealed, cold drawn and hard drawn. Thick bolts made from stainless (above M24) tend to fall into classes 50 or 70 because they just aren’t cold worked so much in thick bar form.

Class 70 or 80 stainless bolt will become annealed if welded or heated. Then it shifts down towards the floor at 500 MPa.

The table is for common families of steel: plain carbon; alloy; and austenitic stainless. These are the everyday ones.

There are other families, such as martensitic stainless (410), which the table doesn’t cover. Other types include nylon, aluminum, brass, titanium, and silicon bronze fasteners. Those don’t come under this table. They each have their own specs. Choose them out of the relevant standard. Same with those more unusual petrochemical grades that goes above and beyond B7.