Hex Bolt vs Carriage Bolt vs Lag Screw: Key Differences and Uses

Three Bolt Types, Three Different Jobs

Hex bolts, carriage bolts, and lag screws all hold things together — but they differ in how they install, what materials they suit, and how strong the finished joint is. Picking the wrong one means either a joint that works loose, or paying for more bolt than the job needs.

Hex Bolts (DIN 933 / DIN 931)

The hex bolt is the general-purpose machine fastener: six-sided head, driven by a wrench, installed with a nut (or into a tapped hole). It offers the highest strength of the three — property classes up to 12.9 — and full torque control, which is why structural steel, machinery, and flanged connections all use hex bolts. Both fully threaded (DIN 933) and partially threaded (DIN 931) versions are stocked, in zinc, hot-dip galvanized and stainless finishes.

Best for: metal-to-metal joints, anything needing high and controlled clamping force, joints that may need disassembly.

Carriage Bolts (DIN 603)

A carriage bolt (coach bolt) has a smooth, domed head and a square neck under the head. You push it into a hole from the outside: the square neck locks into the wood (or a square hole in steel) so the bolt cannot rotate while you tighten the nut from the back. The finished side shows only a neat dome — no drive slots, which also makes it marginally tamper-resistant.

Best for: wood-to-metal and wood-to-wood joints where the head side must look clean or be hard to unscrew: gates, fencing, docks, playground equipment, furniture.

Lag Screws (DIN 571)

A lag screw (lag bolt) is a big woodscrew with a hex head: a gimlet point, coarse thread, and no nut at all. You drill a pilot hole (about 60–75% of the shank diameter) and drive it straight into the wood with a socket wrench. Thread length should be at least twice the diameter in the holding member.

Best for: heavy timber connections, ledger boards, attaching hardware to beams and posts — anywhere you can only access one side.

Side-by-Side Comparison

Feature	Hex Bolt	Carriage Bolt	Lag Screw
Standard	DIN 933 / DIN 931	DIN 603	DIN 571
Head	Hex, wrench-driven	Dome + square neck	Hex, wrench-driven
Installation	Nut or tapped hole	Nut; square neck locks rotation	Threads into pilot hole, no nut
Base material	Metal (general purpose)	Wood-to-metal, wood-to-wood	Solid wood / timber
Strength	Up to class 12.9 (highest)	Typically 4.8 / 8.8	Set by wood thread holding
Pilot hole	Equal to bolt diameter	Equal to shank	60–75% of diameter
Corrosion options	Zinc, HDG, A2/A4 stainless, black oxide	Zinc, HDG, stainless	Zinc, HDG, stainless

Which Fastener for Your Application?

Both sides accessible, metal parts, high load: hex bolt + nut + flat washer.
Wood construction, clean or exposed head side: carriage bolt, with a flat washer under the nut.
Only one side accessible, timber: lag screw.
Wet or outdoor service: specify hot-dip galvanized or stainless in all three families.
Custom dimensions or non-standard heads? Our custom CNC fastener service produces one-offs to production runs.

Frequently Asked Questions

Which is the strongest — hex bolt, carriage bolt, or lag screw?

The hex bolt. A class 8.8 or 10.9 hex bolt with a matched nut carries far more load than a lag screw (limited by the wood threads) or a carriage bolt (typically class 4.8–8.8). If joint strength drives the design, use a hex bolt.

Can carriage bolts be used in steel?

Yes, if the steel has a square hole (or the neck is welded) to stop rotation. In round holes they will simply spin. They are designed primarily for wood.

Do lag screws need a nut or washer?

No nut — they thread into the wood. A flat washer under the head is recommended when clamping softwood, to stop the head sinking into the grain.