WTFastenersHex Bolt Grades Explained: 4.8 vs 8.8 vs 10.9
The numbers on a bolt head are its ISO property class — the key to tensile strength. This guide explains grades 4.8, 8.8 and 10.9, which nut to pair with each, and where each grade is used.
What “4.8” actually means
Hex bolt heads are stamped with a property class such as 4.8, 8.8 or 10.9, defined by ISO 898-1. The number is not a size and not a brand — it is a promise about mechanical strength. The figure before the point gives the nominal tensile strength in hundreds of megapascals: a 4.8 bolt is rated at 400 MPa, an 8.8 at 800 MPa and a 10.9 at 1,000 MPa. The figure after the point shows the yield-to-tensile ratio: in 8.8, the yield strength is 80% of the tensile strength. Read the stamp and you know exactly how much load the bolt is designed to carry — which is why the grade belongs in every RFQ, right next to the size.
Grade 4.8: general-purpose fastening
Grade 4.8 is the everyday commercial bolt — low-carbon steel, economical to produce and easy to stock by the carton. It covers machinery guards, light brackets, furniture, agricultural equipment and any joint where the loads are modest and well understood. Cataloged to DIN 931 (partial thread) and DIN 933 (full thread), it is the grade distributors stock deepest because it fits the widest range of non-critical applications. Finishes are usually bright zinc or galvanized (G.I.).
Grade 8.8: the structural workhorse
Grade 8.8 is medium-carbon steel, quenched and tempered, and it is the default choice for structural steelwork, machinery and automotive applications. It carries roughly twice the tensile strength of 4.8, which means smaller diameters can do the same job — a real saving in weight and steel on framed structures. In the export market it is the grade most often specified for construction projects: hex bolts to DIN 931/933, flange bolts to DIN 6921, and heavy hex structural bolts in the ASTM A325 pattern. Hot-dip galvanizing is a frequent finish for outdoor 8.8 assemblies.
Grade 10.9: high-strength connections
Grade 10.9 steps into high-strength territory: alloy steel, quenched and tempered, rated at 1,000 MPa tensile. It is specified for preloaded and friction-grip connections in steel structures, heavy equipment and anywhere an 8.8 bolt would need an impractically large diameter. Because the material is harder and less forgiving, 10.9 assemblies demand correct tightening and matched components — this is not a grade to substitute casually. Socket head cap screws to DIN 912 / ISO 4762 are frequently produced in 10.9 for machine building, where a compact, high-strength head is needed.
Match the nut and washer to the grade
A bolt is only as strong as its weakest partner. Pair 8.8 bolts with grade 8 nuts to DIN 934 and 10.9 bolts with grade 10 nuts, or the nut threads will strip before the bolt reaches its rated load. Where vibration is a concern, nylon-insert lock nuts to DIN 985 keep the joint from backing off. Washers to DIN 125, DIN 127 (spring) or DIN 9021 spread the load and protect the clamped surface. And the finish should suit the exposure: zinc and G.I. indoors, Dacromet or Ruspert for corrosion duty, black oxide for a clean shop appearance.
Put the grade in your RFQ
A dimension string like “DIN 933, M16 × 70, grade 8.8, hot-dip galvanized” is all a factory needs to price the exact item. WTFasteners — Hebei W.T Fasteners Co., Ltd. — is a source factory in Yongnian, Handan, China: browse the bolts & nuts range for the cataloged grades and finishes, or request a quotation — WhatsApp +86 138 0310 6448, postmaster@wtfastener.com — with your grade, size and finish.
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