“Stainless” is not one thing. Two bolts can both be stamped stainless, sit in the same bin, and hold up completely differently once they are outdoors. The difference between 304 and 316 is small on paper and large in the field, and it comes down to one question: where is the fastener going to live?
Order 316 for a dry indoor bracket and you have over-built the joint. Order 304 for a dock railing and you have a callback waiting to happen. Here is how to tell which grade a job actually calls for.
The 60-second difference
- 304 (you will also see it marked 18-8): roughly 18% chromium and 8% nickel. It is the workhorse stainless, with strong corrosion resistance across most everyday environments.
- 316: the same base as 304, plus 2 to 3% molybdenum. That one addition is what gives 316 its resistance to salt, chlorides, and pitting.
- Both grades are austenitic and similar in strength. The decision between them is about corrosion, not load. If you need high clamp strength, that is a different conversation (Grade 8 or structural), not 304 versus 316.
Sourcing stainless for a specific job? Browse K2's stainless hex bolts by size and finish. Browse stainless hex bolts →
Quick clarification: what is 18-8?
Buyers get tripped up when a spec says 18-8 and the drawing says 304. For almost every practical purpose, they are the same thing. “18-8” names the family (18% chromium, 8% nickel), and 304 is the specific grade most 18-8 hardware is made to. If your spec calls for 18-8, a 304 fastener meets it.
Match the grade to the environment
This is the whole decision. Start with where the fastener sits and work back to the grade.
- Indoor, dry, or mild conditions: 304 is enough. Brackets, enclosures, general assembly, machinery that stays inside.
- General outdoor, away from the coast: 304 still handles it. Rain and humidity are not a problem for 304 on their own.
- Coastal, marine, or salt spray: step up to 316. Salt air is chloride exposure, and chlorides are exactly what the molybdenum in 316 defends against.
- Chlorine, pool, washdown, or chemical exposure: 316. Anywhere chlorides concentrate, 304 will start to pit.
- Road salt and winter de-icing: treat it like the coast. If a fastener sees road spray through a Canadian winter, that is chloride exposure, and 316 is the safer call.
The tell that catches people out: tea-staining
Near salt air, 304 develops a brown surface discoloration called tea-staining. It is not structural failure, but it looks like rust, and it will fail a visual inspection on a railing or an architectural job. 316 resists it. So if appearance matters and the job is anywhere near the coast, 316 earns its place for looks alone, before you even get to corrosion life.
Do not accept “stainless” as a spec
The single most useful habit here is to stop treating “stainless” as an answer. The grade is what matters, and the grade belongs on the spec, not in your head. Every K2 stainless product page lists the material and grade, so you can confirm 304 or 316 on purpose instead of hoping the bin was right.
Confirm the grade before the PO goes out. Every K2 stainless bolt lists its material and full spec. View a 316 stainless spec →
The bottom line
Use 304 for indoor and general outdoor work. Move to 316 the moment salt, chlorides, or road spray enter the picture. When you are near water and genuinely unsure, step up to 316. The grade is not about the bin it came from, it is about where the fastener has to live.