Which grade actually carries it
Enter the load and the size. This shows what every grade and property class can hold, whether the nut, washer and finish you plan to use match the bolt, and exactly what capacity is lost if the specification is downgraded. Print the sheet and put the numbers on the table.
Capacity by grade
Selected grade — data sheet
Assembly check
If the specification is downgraded
How bolt grades work
The two numbers on a metric bolt head are its strength written in shorthand. In a property class like 8.8, the first number is the minimum tensile strength in hundreds of megapascals — 8 means 800 MPa — and the two numbers multiplied and taken as a percentage give the yield: 8 × 8 = 64, so yield is 64% of 800, which is 640 MPa. The same logic reads 4.6 as 400 MPa tensile with 240 MPa yield, and 10.9 as 1,040 MPa with 940 MPa yield. The classes come from ISO 898-1, which also fixes the hardness range, the elongation and the test methods behind each number.
American structural grades say the same things in different words: A325M sits at the 8.8 duty and A490M at the 10.9 duty, both under ASTM F3125. Anchor bolts follow ASTM F1554, whose grades 36, 55 and 105 are named after their yield strength in ksi and carry a colour code on the bolt end — blue, yellow and red — so a site engineer can tell them apart after installation.
Proof load, yield and tensile — which one matters
Tensile strength is where the bolt breaks. Yield is where it stretches permanently and never returns. Proof load is the guaranteed no-damage load — a bolt taken to its proof load and released must come back to its original length. Design works from proof or yield, never from tensile: a joint that has yielded is already failed even though nothing has snapped. That is why the calculator above bases its allowable loads on proof load where the standard defines one, and on yield where it does not, and only then divides by your safety factor.
The nut is half the joint
A bolt develops its strength only if the nut can hold it. ISO 898-2 pairs nut classes with bolt classes — class 8 nuts for 8.8 bolts, class 10 for 10.9 — so that the bolt breaks before the thread strips. Fit a cheaper, softer nut and the failure moves into the thread: the assembly now carries roughly the nut’s rating, not the bolt’s, and nothing on the invoice shows it. A stripped thread is also a quiet failure — a broken bolt announces itself, a stripping nut lets go progressively. This substitution is the most common hidden downgrade in fastener supply, which is why the assembly check above tests it first.
Galvanizing has rules
Hot-dip galvanizing protects carbon steel outdoors for decades, but the zinc adds thickness to the thread, so galvanized bolts take nuts tapped oversize to ISO 10684 — bolts and nuts must be ordered together as an assembly. Strength has a ceiling too: class 10.9 needs a qualified process because of hydrogen embrittlement risk, A490M must not be hot-dip galvanized at all, and electroplated high-strength bolts must be baked after plating. If a supplier offers galvanized 12.9 without hesitation, that is a warning sign, not a service.
Read the head before you trust the paperwork
Every rated bolt is marked: the class on a metric head, three radial lines on an A325M, a “B7” on a stud end, a colour on an F1554 anchor bolt. The marking is the field check; the EN 10204 3.1 certificate is the paper one, tying the delivered batch to the heat of steel it came from with its tested yield, tensile and hardness. A grade without a certificate is a claim. At MF every lot carries traceability, and our anchor bolts, studs and plate washers are made in our own Umm Al Quwain plant, with mechanical and chemical testing at independent UAE-accredited laboratories.
Engineers’ questions
What is the difference between 8.8 and 10.9 bolts?
Strength and sensitivity. 10.9 offers roughly 40% more proof load in the same size (830 vs 580–600 MPa proof stress), but it is harder, less ductile, and more sensitive to hydrogen embrittlement — so coating and handling rules tighten. Use 8.8 as the structural default and 10.9 where the design calls for it.
What does 8.8 on a bolt head mean?
Minimum tensile strength 800 MPa, minimum yield 640 MPa (8 × 8 = 64% of 800), to ISO 898-1. Above M16 the class uses slightly higher figures: 830 MPa tensile, 660 MPa yield.
Which nut class goes with an 8.8 bolt?
Class 8 minimum to ISO 898-2 (class 10 is also fine). A lower class nut strips before the bolt reaches its rated load, so the joint performs at the nut’s level however the bolt is marked. Hot-dip galvanized assemblies use oversize-tapped nuts to ISO 10684.
Can high-tensile bolts be hot-dip galvanized?
Up to 8.8, yes, with oversize-tapped nuts. 10.9 only with a qualified process because of hydrogen embrittlement risk. A490M must not be hot-dip galvanized. Electroplated bolts at 10.9 and above must be baked after plating.
What grade are anchor bolts?
Usually ASTM F1554: Grade 36 (weldable, general foundations), Grade 55 (the common choice for steel column bases, weldable with supplement S1) and Grade 105 (heat-treated, heavy plant, not weldable). Metric projects also cast 4.6 and 8.8 anchor bolts. The grade shows as a colour code on the bolt end: blue, yellow or red.
What is proof load?
The load a bolt must carry without any permanent stretch — the guaranteed elastic limit of the finished bolt, tested on the bolt itself rather than a machined specimen. Design and preload calculations start from it.
How much torque for an M20 8.8 bolt?
Indicatively around 400 N·m dry (K = 0.20) to reach 70% of proof load, less when zinc plated, more when hot-dip galvanized. Torque scatters ±25% with lubrication and thread condition — where preload matters, specify the tightening method, not just a number.
Grade chosen? Draw the anchor bolt to your dimensions and send it to our plant as your enquiry.
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