DIN and ISO Numbers Decoded: A Metric Stainless Fastener Reference
Every week I get at least one email that looks like this:
"Need 40x DIN 912 M8x30 A4, 20x DIN 985 M8, 40x DIN 125 M8. Thanks."
No pleasantries, no explanation, just a string of numbers. And honestly, I love these emails. The person sending it knows exactly what they want, and if you speak the language, it's the clearest possible way to order a fastener.
The trouble is that most people don't speak the language. Maybe you've got a European built sailboat, a German engine, or an Italian windlass, and the parts manual lists everything by DIN number. Maybe you pulled a bolt out of your boat and it had "A4-70" stamped on the head and you're wondering what that means. Either way, you're staring at a bunch of four digit codes that mean nothing to you yet.
This guide is meant to fix that. I'll walk through how metric fastener standards work, what the common DIN and ISO numbers actually are, and where you'll run into them on a boat. There are reference charts at the bottom for thread pitch, wrench and hex key sizes, and washer dimensions. Bookmark it. I use charts like these every day.
If you're still deciding whether metric is even the right choice for your project, start with our article on Metric vs. Imperial Stainless Steel Fasteners. This one assumes you've already landed in metric territory.
What DIN and ISO Actually Mean
DIN stands for Deutsches Institut für Normung, the German national standards body. For most of the 20th century, German industrial standards were the backbone of European manufacturing, so a huge number of metric fasteners got defined by a DIN number. DIN 912 is a socket head cap screw. DIN 934 is a hex nut. DIN 125 is a flat washer. Each standard spells out every dimension of that part: head height, head diameter, thread length, tolerances, the works.
ISO is the International Organization for Standardization. Over the years, ISO took most of the DIN fastener standards and turned them into international standards with new numbers. DIN 912 became ISO 4762. DIN 934 became ISO 4032. Germany officially withdrew a lot of the old DIN fastener standards in favor of the ISO versions.
Here's the funny part. Nobody in the hardware world really listened. Walk into any fastener supplier in Europe or the US and ask for an ISO 4762 and you might get a blank stare. Ask for a DIN 912 and they'll hand you a box. Manufacturers, distributors, and boat builders still use the DIN numbers because that's what everyone learned, and you'll see both numbers printed on labels, catalogs, and spec sheets.
So you need to know both, and you need to know which ones are truly identical and which ones aren't.
Are DIN and ISO Parts Interchangeable?
Mostly, yes. In many cases the ISO version is dimensionally identical to the DIN it replaced, or close enough that you'd never notice.
The big exception is hex heads and hex nuts. When ISO adopted the hex bolt and nut standards, they changed the width across the flats on four sizes:
| Thread size | DIN across flats | ISO across flats |
|---|---|---|
| M10 | 17 mm | 16 mm |
| M12 | 19 mm | 18 mm |
| M14 | 22 mm | 21 mm |
| M22 | 32 mm | 34 mm |
This catches people all the time. You've got a 17mm wrench in your hand because every M10 bolt on your boat has taken a 17mm wrench for 30 years, and then you install a new M10 ISO bolt and your wrench spins right off it. Nothing is wrong with the bolt. It's just the ISO spec.
The threads are the same, so a DIN nut will thread onto an ISO bolt with no problem. It just means you might need two different wrenches for the same connection. If you're refitting a whole system, it's worth keeping everything on one standard so your toolkit stays simple.
How to Read a Metric Fastener Callout
A typical metric fastener description looks like this:
DIN 912 M8 x 30 A4-70
Here's what each piece tells you.
DIN 912 is the standard, so it's a socket head cap screw.
M8 is the nominal thread diameter in millimeters. An M8 bolt is 8mm across the outside of the threads. The "M" just means ISO metric thread.
x 30 is the length in millimeters. How that length is measured depends on the head style, and I'll get to that in a minute because it trips people up.
A4-70 is the material and strength grade. A4 is the metric designation for the 316 family of stainless steels, the marine grade alloy we use for everything we sell in stainless. The 70 is the property class, meaning a minimum tensile strength of 700 megapascals. You'll also see A4-80, which is stronger at 800 MPa. You'll see A2 as well, which is 304 stainless. For anything that lives near salt water, you want A4. We covered the reasons in The Difference Between 304 and 316 Stainless Steel.
What about thread pitch?
You'll notice the example above doesn't list a pitch. That's because metric bolts default to coarse thread. If nobody mentions pitch, assume coarse. An M8 bolt is M8 x 1.25 unless someone tells you otherwise.
When a fine thread is used, it gets called out, usually like M8 x 1.0 x 30 or M8x1 - 30. Fine threads show up on things like engine parts, some deck hardware, and precision mechanical assemblies. For general marine fastening, coarse is the norm.
The catch is that the formatting isn't always consistent. "M10 x 1.25" could be a fine thread M10 with no length listed, or someone could have meant a 1.25 inch long bolt and mixed up their systems. When in doubt, ask, or measure what you've got with a thread pitch gauge.
How Metric Screw Length Is Measured
This is the single most common ordering mistake I see, so it gets its own section.
For most head styles, length is measured from under the head to the tip. That applies to socket head cap screws (DIN 912), hex heads (DIN 933 and 931), button heads (ISO 7380), pan heads (DIN 7985), and carriage bolts (DIN 603).
For countersunk or flat head screws, length is measured overall, including the head. That applies to DIN 7991 flat socket screws, DIN 965 countersunk Phillips, and DIN 963 countersunk slotted. The logic is that the head sits flush with the surface, so the whole screw is buried in the material.
Raised countersunk (oval) heads like DIN 966 are measured from the widest point of the head, where it meets the surface, down to the tip. The dome sticks up, and it isn't counted.
So if you pull a 30mm flat head out of a hinge and replace it with a 30mm socket cap, your new screw is going to stick out a few extra millimeters past where the old one did. Measure the old one the right way and you'll get it right.
The Common DIN and ISO Numbers
This isn't every metric fastener standard in existence. There are hundreds. These are the ones you'll actually run into on boats, in marine parts manuals, and in our catalog.
Screws and Bolts
| DIN | ISO | What it is | Where you'll see it |
|---|---|---|---|
| DIN 912 | ISO 4762 | Socket head cap screw | Engine mounts, machinery, deck hardware, anything needing high clamping force in a tight space |
| (DIN 7380) | ISO 7380 | Button head socket screw | Trim, cleats, panels, spots where you want a low, smooth head |
| DIN 7991 | ISO 10642 | Flat (countersunk) socket screw, 90° | Hinges, track, hatches, anywhere the head must sit flush |
| DIN 933 | ISO 4017 | Hex head cap screw, fully threaded | General purpose bolting, brackets, mounts |
| DIN 931 | ISO 4014 | Hex bolt, partially threaded | Structural joints where the smooth shank carries shear load |
| DIN 7985 | ISO 7045 | Pan head Phillips machine screw | Electrical panels, instrument mounts, light fixtures |
| DIN 965 | ISO 7046 | Countersunk Phillips machine screw | Flush mounted fittings, cover plates |
| DIN 966 | ISO 7047 | Raised countersunk (oval) Phillips machine screw | Trim and fittings where a finished look matters |
| DIN 84 | ISO 1207 | Cheese head slotted machine screw | Older European equipment, terminal blocks |
| DIN 963 | ISO 2009 | Countersunk slotted machine screw | Traditional hardware, restorations |
| DIN 603 | (ISO 8677) | Carriage bolt, mushroom head | Through bolting wood: rub rails, benches, docks |
| DIN 571 | none | Hex head lag screw (wood screw) | Heavy hardware into wood, dock and deck framing |
| DIN 7981 | ISO 7049 | Pan head Phillips self tapping screw | Fastening into fiberglass or sheet metal |
| DIN 7982 | ISO 7050 | Countersunk Phillips self tapping screw | Flush fastening into fiberglass or sheet metal |
| DIN 916 | ISO 4029 | Socket set screw, cup point | Collars, pulleys, shaft fittings |
| DIN 976 | none | Threaded rod | Custom bolts, long through bolts, backing plate studs |
A note on DIN 7380: the button head has been an ISO standard for decades, ISO 7380, and Germany publishes it as DIN EN ISO 7380. People drop the "EN ISO" part out of habit, so you'll see "DIN 7380" printed all over the place. It's the same screw either way.
DIN 603 and ISO 8677 carriage bolts are close but not identical. The ISO version has a slightly different head and neck. If you're replacing carriage bolts into existing square holes, check the neck size.
One more flat head gotcha: DIN 7991 and ISO 10642 flat socket screws are close, but the ISO heads are a bit larger in diameter. If you're dropping a new screw into an existing countersink, check that the head sits flush before you commit to a whole box.
Nuts
| DIN | ISO | What it is | Where you'll see it |
|---|---|---|---|
| DIN 934 | ISO 4032 | Standard hex nut | Everywhere |
| DIN 985 | ISO 10511 | Nylon insert lock nut (nyloc), thin | Anything that vibrates: engines, rigging hardware, fittings |
| DIN 982 | ISO 7040 | Nylon insert lock nut, regular height | Same as above, when you want more thread engagement |
| DIN 439 | ISO 4035 | Thin hex jam nut | Locking a second nut, tight clearances, turnbuckle adjustments |
| DIN 1587 | none | Acorn (dome cap) nut | Exposed bolt ends where you want a finished look and no snag points |
| DIN 315 | none | Wing nut | Hatches, covers, anything you remove by hand |
| DIN 6923 | ISO 4161 | Hex flange nut | Spreads load without a separate washer |
| DIN 557 | none | Square nut | Captured in slots and channels, traditional joinery |
| DIN 582 | none | Lifting eye nut | Lifting engines and heavy equipment |
DIN 985 vs DIN 982 comes up a lot. Both are nylon insert lock nuts, but the 985 is the thinner one. It's by far the more common of the two, so if someone just says "M8 nyloc," they almost always mean DIN 985.
And yes, the same across flats difference applies here. A DIN 934 M10 nut takes a 17mm wrench and an ISO 4032 M10 nut takes a 16mm.
Washers
| DIN | ISO | What it is | Where you'll see it |
|---|---|---|---|
| DIN 125 | ISO 7089 / 7090 | Standard flat washer | Under nuts and bolt heads everywhere |
| DIN 9021 | ISO 7093 | Large OD fender washer (roughly 3x the bolt diameter) | Spreading load on fiberglass, soft wood, thin panels |
| DIN 440 R | ISO 7094 | Extra thick, extra wide washer for timber | Through bolting wood structure: docks, beams, backing blocks (ISO sizes differ slightly) |
| DIN 127 | none | Split lock washer | Older equipment, electrical connections |
| DIN 6798 A | none | External tooth lock washer | Electrical grounding, bonding connections |
| DIN 6798 J | none | Internal tooth lock washer | Small screws where the teeth need to stay hidden |
DIN 125 comes in two flavors. Type A has no chamfer, and Type B has a chamfered outer edge. For most uses they're interchangeable.
I'll say it plainly: on a fiberglass boat, DIN 9021 fender washers are your friend. A standard DIN 125 washer on an M6 bolt is only 12mm across. Crank that down on a fiberglass deck and you can crush the laminate. A DIN 9021 on the same M6 bolt is 18mm across, which spreads the load over more than twice the area. For anything structural, use a proper backing plate as well.
Pins and Everything Else
| DIN | ISO | What it is |
|---|---|---|
| DIN 94 | ISO 1234 | Split cotter pin |
| DIN 1444 | ISO 2341 | Clevis pin with head |
| DIN 444 | none | Eye bolt (non lifting) |
| DIN 580 | none | Lifting eye bolt, shouldered, load rated |
A word on eye bolts: DIN 444 and DIN 580 look similar but they're very different animals. DIN 580 is made and load rated for lifting. DIN 444 is not. Never pick up an engine with a DIN 444.
Reference Charts
Thread Pitch, Tap Drill, and Clearance Holes (Coarse Thread)
| Size | Coarse pitch | Tap drill | Clearance hole (normal fit) |
|---|---|---|---|
| M3 | 0.5 mm | 2.5 mm | 3.4 mm |
| M4 | 0.7 mm | 3.3 mm | 4.5 mm |
| M5 | 0.8 mm | 4.2 mm | 5.5 mm |
| M6 | 1.0 mm | 5.0 mm | 6.6 mm |
| M8 | 1.25 mm | 6.8 mm | 9.0 mm |
| M10 | 1.5 mm | 8.5 mm | 11.0 mm |
| M12 | 1.75 mm | 10.2 mm | 13.5 mm |
| M14 | 2.0 mm | 12.0 mm | 15.5 mm |
| M16 | 2.0 mm | 14.0 mm | 17.5 mm |
| M20 | 2.5 mm | 17.5 mm | 22.0 mm |
| M24 | 3.0 mm | 21.0 mm | 26.0 mm |
Here's a handy trick for coarse metric threads: tap drill size is the thread diameter minus the pitch. An M10 x 1.5 needs a 10 minus 1.5, or 8.5mm drill. It works for every size in the chart, rounded to the nearest standard drill.
When you're tapping 316 stainless, go slow, use plenty of cutting fluid, and back the tap out often to break the chip. Stainless work hardens fast, and a broken tap in a stainless part will ruin your afternoon.
Hex Key Sizes for Socket Screws
| Size | DIN 912 socket cap (head dia / key) | ISO 7380 button head (head dia / key) | DIN 7991 flat socket (head dia / key) |
|---|---|---|---|
| M3 | 5.5 mm / 2.5 mm | 5.7 mm / 2 mm | 6 mm / 2 mm |
| M4 | 7 mm / 3 mm | 7.6 mm / 2.5 mm | 8 mm / 2.5 mm |
| M5 | 8.5 mm / 4 mm | 9.5 mm / 3 mm | 10 mm / 3 mm |
| M6 | 10 mm / 5 mm | 10.5 mm / 4 mm | 12 mm / 4 mm |
| M8 | 13 mm / 6 mm | 14 mm / 5 mm | 16 mm / 5 mm |
| M10 | 16 mm / 8 mm | 17.5 mm / 6 mm | 20 mm / 6 mm |
| M12 | 18 mm / 10 mm | 21 mm / 8 mm | 24 mm / 8 mm |
| M16 | 24 mm / 14 mm | 28 mm / 10 mm | 30 mm / 10 mm |
| M20 | 30 mm / 17 mm | none | 36 mm / 12 mm |
Flat head diameters shown are DIN 7991 figures. ISO 10642 heads run slightly larger.
Notice that button heads and flat heads take a smaller hex key than a socket cap of the same thread size. That's worth knowing, because the smaller socket means less torque before you strip it out. With stainless on stainless, strip the socket and you're drilling it out. Use a good quality, full size hex key, keep it seated all the way, and put some anti-seize on the threads. That helps with galling too, which we wrote about in Fastener Galling: What it is and how to prevent it.
Wrench Sizes for Hex Heads and Nuts
| Size | DIN 933 / 931 / 934 | ISO 4017 / 4014 / 4032 |
|---|---|---|
| M3 | 5.5 mm | 5.5 mm |
| M4 | 7 mm | 7 mm |
| M5 | 8 mm | 8 mm |
| M6 | 10 mm | 10 mm |
| M8 | 13 mm | 13 mm |
| M10 | 17 mm | 16 mm |
| M12 | 19 mm | 18 mm |
| M14 | 22 mm | 21 mm |
| M16 | 24 mm | 24 mm |
| M20 | 30 mm | 30 mm |
| M24 | 36 mm | 36 mm |
Washer Dimensions (Inside Diameter / Outside Diameter / Thickness)
| Size | DIN 125 flat | DIN 9021 fender |
|---|---|---|
| M3 | 3.2 / 7 / 0.5 mm | 3.2 / 9 / 0.8 mm |
| M4 | 4.3 / 9 / 0.8 mm | 4.3 / 12 / 1.0 mm |
| M5 | 5.3 / 10 / 1.0 mm | 5.3 / 15 / 1.2 mm |
| M6 | 6.4 / 12 / 1.6 mm | 6.4 / 18 / 1.6 mm |
| M8 | 8.4 / 16 / 1.6 mm | 8.4 / 24 / 2.0 mm |
| M10 | 10.5 / 20 / 2.0 mm | 10.5 / 30 / 2.5 mm |
| M12 | 13 / 24 / 2.5 mm | 13 / 37 / 3.0 mm |
| M16 | 17 / 30 / 3.0 mm | 17 / 50 / 3.0 mm |
| M20 | 21 / 37 / 3.0 mm | 22 / 60 / 4.0 mm |
A Few Things Worth Knowing Before You Order
Check the head stamp. A legitimate A4 stainless bolt should have "A4" or "A4-70" or "A4-80" stamped on the head, often with a manufacturer's mark. If a bolt is being sold as marine grade 316 and the head is blank, I'd ask questions. Nuts are usually marked too, though the stamp can be tiny.
A4 is not magnetic, mostly. 316 stainless is generally non-magnetic, but cold forming during manufacturing can make the head or threads slightly magnetic. A weak pull on a magnet doesn't automatically mean you've got the wrong alloy. A strong grab is a different story.
Don't mix metric and imperial in the same hole. Some sizes are awfully close. An M8 bolt and a 5/16" bolt are within a hair of each other in diameter, and an M8 nut will sometimes start on a 5/16"-18 bolt before it binds up. Forcing it wrecks the threads on both. If a nut isn't spinning on freely by hand for the first few turns, stop and check what you've got.
Match the whole joint. If you're replacing a bolt, replace the nut and washer to the same standard and the same A4 material. Mixing a 316 bolt with a plated steel nut is asking for rust streaks and galvanic corrosion. Our Galvanic Series chart is a good gut check if you're not sure.
Still Not Sure What You've Got?
If you're holding a fastener and can't figure out what it is, send us a photo with a ruler or caliper in the shot, along with the thread diameter, length, and any markings on the head. And if you've got a parts list full of DIN numbers, just send it over. We'll tell you what we have on the shelf in 316/A4 and what we can source. That's what we're here for.
You can browse our current metric 316/A4 stainless fastener line here, or submit an RFQ for bulk or hard to find sizes.