What Is 18/8 Stainless Steel — and Why Is It Also Called 304?

What does the 18/8 in stainless steel mean?

The numbers are a recipe. 18/8 names the two alloying elements that define the steel: about 18% chromium and 8% nickel, with the balance mostly iron plus small amounts of carbon, manganese, and silicon. That composition places the alloy in the austenitic family of stainless steels — the group known for formability, toughness, and corrosion resistance.

The same steel carries different names depending on which naming system you meet it in. "18/8" is the traditional shorthand built directly from composition. "304" is the grade designation from the American SAE/AISI numbering system. European standards list it as 1.4301, and fastener markings abbreviate it as A2. All of these point to the same material.

You encounter 18/8 constantly: cookware, mixing bowls, sinks, water bottles, flatware, and commercial food-processing equipment. It became the default food-contact stainless steel because it pairs reliable corrosion resistance with the formability manufacturers need to press, draw, and weld it into finished goods.

• 18 = approximately 18% chromium — the element that makes steel "stainless"
• 8 = approximately 8% nickel — the element that stabilizes the steel's structure
• Same grade, other names: 304 (SAE/AISI), 1.4301 (European standard), A2 (fastener marking)

What does chromium do in stainless steel?

Chromium is what earns the "stainless" name. Once chromium content passes a threshold of roughly 10.5%, the chromium atoms at the steel's surface react with oxygen in the air and form an extremely thin, invisible film of chromium oxide. Metallurgists call this the passive layer, and it is the mechanism behind nearly everything stainless steel is known for.

The passive layer seals the iron beneath it away from air and moisture, so the reactions that produce rust never get started. Crucially, the layer forms spontaneously wherever chromium meets oxygen — which means it self-repairs. Scratch the surface, and a fresh oxide film forms across the newly exposed metal within moments. The protection is built into the alloy itself, not applied on top of it.

This is also why stainless steel offers resistance rather than immunity. The passive layer can be challenged by prolonged exposure to chlorides — which is why undissolved salt left sitting in cold water can pit any stainless steel, whatever the brand. Ordinary cooking, washing, and scrubbing pose no such problem: the layer withstands them, and reforms wherever it is disturbed.

What does nickel do in stainless steel?

Chromium alone would make steel corrosion-resistant, but the resulting alloy would have a different internal crystal structure — harder to form, easier to crack. Nickel's job is structural. At around 8%, it stabilizes what metallurgists call the austenitic crystal structure, keeping it in place all the way down to room temperature.

That structure is why 18/8 behaves the way it does in a factory and in a kitchen. Austenitic steel can be deep-drawn into pans and bowls without splitting. It is tough rather than brittle, it does not require hardening heat treatments, and it holds its toughness in the cold as well as the heat. Nickel also contributes to the bright, easily polished surface that stainless cookware is known for.

In short: chromium protects the steel, nickel makes it workable and durable. The 18 and the 8 are a partnership, and neither number works as well without the other.

Is 18/10 better than 18/8?

18/10 stainless steel contains the same chromium as 18/8 and roughly two percentage points more nickel. Both are food-grade austenitic stainless steels, both form the identical self-repairing chromium-oxide passive layer, and both belong to the same broad 304 family of alloys.

The extra nickel in 18/10 modestly improves corrosion resistance and gives a slightly brighter polish, which is why 18/10 is common in flatware — where finish is the selling point. For a cooking surface, the practical difference is small. Both grades handle food contact, both resist staining in ordinary kitchen use, and both depend on the same chromium chemistry for their protection.

Neither number is a quality score by itself. How a pan is fabricated — the gauge of the steel, the quality of the finish, the consistency of the forming — matters at least as much as the 8-versus-10 nickel distinction. Treat 18/8 and 18/10 as two entries in the same food-grade family, not as a ranking.

How does this relate to SEIDO Cookware?

SEIDO skillets are made from 100% 304 (18/8) food-grade stainless steel — the same grade this article describes. The cooking surface is uncoated: it is the bare 18/8 steel itself, without synthetic coatings, and made without intentionally added PFAS.

Because the surface is the metal rather than a layer applied to the metal, there is nothing to wear through, flake, or lose performance over time. The chromium-oxide passive layer is the protection, and it renews itself for the life of the pan. That coating-free construction is the basis of the 5-Year Warranty.

The same steel runs across the SEIDO Cookware range, including the stainless steel lunch box 3-pack. The full material story is on the materials and safety page.