304 vs 316 Stainless Steel: A2 vs A4, Which Grade Should You Use?
A2 is the fastener name for 304 stainless steel and A4 is the fastener name for 316. Use A2 (304) for indoor fixings and inland outdoor jobs. Use A4 (316) near the coast, on a boat or pontoon, around a pool or hot tub, and consider it where road salt spray reaches. The difference is molybdenum: A4 contains 2-3%, which improves resistance to chloride pitting.
304 vs 316 and A2 vs A4: how the names match up
Fasteners are named under ISO 3506, which groups stainless steels by letter. Sheet, bar and tube are usually named by their steel number instead, either the American 304 and 316 or the European numbers 1.4301 and 1.4401. The British Stainless Steel Association lists these as approximate equivalents, so a bolt marked A4 and a sheet of 316 are the same family of steel.
| Fastener grade (ISO 3506) | Common name | European steel number | Molybdenum | Usual property classes |
|---|---|---|---|---|
| A2 | 304 | 1.4301 | None specified (may be present at the maker's discretion) | A2-70 most common; 50 and 80 also exist |
| A4 | 316 | 1.4401 | 2.00-3.00% | A4-70 most common; A4-80 for higher strength |
The number after the letter is the property class: 70 means a minimum tensile strength of 700 MPa and 80 means 800 MPa. So A4-80 is 316-type stainless at the higher strength class. Class 70 is the most common supply condition. For tightening figures by class, see our stainless steel bolt torque chart.
A2 (304) v A4 (316): which to use where
| Grade | Alloy family | Molybdenum | Best for | Avoid for |
|---|---|---|---|---|
| A2 (304) | Austenitic stainless steel, ISO 3506 designation A2 | None specified | Indoor fixings, workshops, inland decking, fencing and garden structures, general DIY | Coastal exteriors, boats, pools; think twice where de-icing salt spray reaches |
| A4 (316) | Austenitic stainless steel, ISO 3506 designation A4 | 2-3% (part of the alloy specification that separates A4 from A2) | Coastal fencing and gates, boats and pontoons, pools and hot tubs, bathrooms and other wet areas | No functional downside outdoors; it costs more than A2 for the same fastener, so there is no need to pay for it inland |
Strength is not the reason to choose one grade over the other near the coast; corrosion resistance is. The molybdenum in 316 improves its resistance to pitting and crevice corrosion where chlorides are present, which is why it is the usual choice near the sea. For outside work on buildings, BSSA guidance treats 304 as acceptable at most locations and recommends 316 at most marine sites and heavily polluted industrial ones. That is when A2 is the better buy: away from salt, A4 adds cost without a real benefit.
Coastal fencing and gates
For a fence or gate close to the sea, or anywhere salt-laden air reaches it, A4 (316) is the safer choice, because salt spray is a chloride environment and that is where the molybdenum in A4 earns its keep. Simfix stocks A4 hex bolts and set screws, hex nuts and flat washers for this. Cup square (carriage) bolts to DIN 603 now come in A4 as well as A2, so a timber gate or fence frame can be bolted in A4 throughout; the M10 x 100 A4 cup square bolt is one example. For fencing well away from the coast, A2 (304) gives good corrosion resistance at a lower cost.
Boats and pontoons
Use A4 (316) for boat deck fittings, pontoon and jetty hardware, and anything in the splash or spray zone. Simfix stocks A4 hex bolts and set screws, socket cap screws, dome nuts, nyloc nuts and flat washers for this kind of build; the marine-grade fasteners buying guide goes through them in more detail. For fittings that stay permanently underwater, get advice from the fitting or hardware manufacturer. Even 316 is not fully resistant to seawater. The BSSA warns that crevices, seawater left standing on the steel and heated seawater all raise the risk of pitting and crevice corrosion.
Pools and hot tubs
Use A4 (316) stainless steel around a pool or hot tub. BSSA guidance on pool buildings favours 316 over 304 because it holds up better against staining, pitting and crevice attack. Simfix stocks A4 dome nuts, pan tapping screws, and socket countersunk and button screws for pool surrounds, steps and changing-room fittings. One limit matters here: in indoor pool halls, the BSSA says 304 and 316 must not be used for parts vulnerable to stress corrosion cracking where a failure could cause injury, such as load-bearing ceiling fixings. Those need an engineer's specification and a higher alloy.
Decking and garden timber near the sea
Coach screws to DIN 571 now come in A4 (316) as well as A2 (304), for example the 10 x 100 A4 hexagon coach screw, so heavier timber frames near the coast can be fixed in A4. Decking screws to DIN 7997 are not stocked in A4. For deck boards near the sea the screws themselves will be A2, while the Form A and Form C washers used with them come in A4. Away from the coast, A2 (304) gives good corrosion resistance for decking and garden timber.
General outdoor use, inland
For sheds, workshops, general garden structures and DIY jobs more than a short distance inland, A2 (304) is the right choice. It costs less than A4 and gives good corrosion resistance outside salt-exposed environments. Save A4 for the coast, boats and pools, and consider it where road salt spray reaches.
What Simfix stocks in A2 and A4
- A4 (316): hex bolts and set screws, socket cap screws, cup square bolts, coach screws, dome nuts, nyloc nuts, flat and Form C washers, self-tapping screws and more. See the A4 (316) marine grade fasteners and marine, boat and coastal hardware collections.
- A2 (304): the larger range for indoor and inland outdoor work, including decking screws, coach screws, cup square bolts and most of the same bolt, nut and washer types as A4. See the A2 (304) stainless steel fasteners collection.
- Decking and garden timber (decking screws are not stocked in A4; coach screws and washers come in both grades). See decking and garden timber fixings.
- Fencing, gates and outdoor structures (A2 and A4 by product). See fencing, gates and outdoor structures.
- Bathrooms, kitchens and wet areas. See bathrooms, kitchens and wet areas.
- Cup square (carriage) bolts in A2 and A4. See cup square bolts.
- Dome nuts in A2 and A4. See dome nuts.
Order before 3:30pm for next working day dispatch. Next working day delivery is £5.99, free on UK orders over £50, and returns are accepted within 30 days.
Don't mix grades or dissimilar metals in one assembly
Keep one grade per connection: don't put an A2 nut on an A4 bolt, or the other way round. Where a stainless fastener is in direct contact with a different metal, such as aluminium, isolate the two (for example with a washer or coating) rather than leaving bare metal-to-metal contact, to reduce the risk of galvanic corrosion. Our guide to stainless steel screws in aluminium covers this in more detail.
Frequently asked questions
Is A4 stainless steel the same as 316?
In practice, yes. A4 is the fastener grade name used in ISO 3506, and 316 (European number 1.4401) is the steel it corresponds to. The British Stainless Steel Association lists them as approximate equivalents. A4 contains 2-3% molybdenum, which improves its resistance to pitting and crevice corrosion where chlorides are present, compared with A2 (304).
Is A2 stainless steel the same as 304?
Yes, A2 is the ISO 3506 fastener name for 304 stainless steel (European number 1.4301). A2 does not specify any molybdenum, which is the main difference from A4 (316). It gives good corrosion resistance indoors and for most outdoor work inland, away from salt and chlorine.
Should I use 304 or 316 stainless screws for outdoor use?
Inland, 304 (A2) is usually the right choice: it resists corrosion well and costs less. Near the coast, on boats or around pools, choose 316 (A4): its molybdenum improves resistance to pitting where chlorides are present, and BSSA guidance recommends 316 at most marine sites. Consider A4 where road salt spray reaches too.
Is A2 stainless steel marine grade?
No. A4 (316) is the grade usually called marine grade. A2 (304) has no molybdenum specified, and the BSSA does not regard 304 as suitable for use in seawater. Use A4 for boats, pontoons and coastal fixings, while remembering that even 316 is not fully resistant to seawater.
What does A4-80 mean on a bolt?
A4 is the steel grade, the 316-type stainless used for fasteners under ISO 3506. The 80 is the property class, meaning a minimum tensile strength of 800 MPa. A4-70 has a minimum of 700 MPa and is the more common supply condition. The class is a strength designation; corrosion resistance comes from the grade, A4.
Can I use A4 (316) fasteners in an indoor pool hall?
For fittings, surrounds and steps that are cleaned regularly, A4 (316) is the preferred choice over A2. For load-bearing or safety-critical parts in an indoor pool hall, such as ceiling fixings, the BSSA says 304 and 316 must not be used where stress corrosion cracking could cause injury. Those need an engineer's specification and a higher alloy.
Sources
- Stainless steel fasteners, British Stainless Steel Association
- AD 342: Stainless steel fasteners, Steel Construction Institute (New Steel Construction)
- Selection of stainless steels for building external applications, British Stainless Steel Association
- Selection of 316, 304 and 303 types of stainless steels for seawater applications, British Stainless Steel Association
- Stainless steels for swimming pool building applications, British Stainless Steel Association
- Stainless steel grade datasheets, worldstainless (grade sheets by Atlas Steels)
