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 meet 18/8 constantly, whether you have noticed or not: cookware, mixing bowls, sinks, water bottles, cutlery 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 stabilises 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 re-forms 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 stabilises 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 need 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 cutlery, 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 made — 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?

Our frying pans have a tri-ply clad body: a 304 (18/8) stainless steel cooking surface, an aluminium core and a 430 stainless steel exterior. The cooking surface is the same grade this guide describes, and it 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 or flake. The chromium oxide passive layer is the protection, and it renews itself whenever the surface is scratched. That coating-free construction is the basis of the 5-Year Limited Warranty.

The same grade runs across the SEIDO Cookware range: it is the frying pans' cooking surface and the steel of the containers in the stainless steel lunch box 3-pack. The full material story is on our materials and safety page.

304 vs 316 stainless steel: what does molybdenum add?

316 is the other stainless grade you will see on food containers and bottles. Like 304, it is an austenitic stainless steel. The difference is one added element: about 2% to 3% molybdenum.

The ASTM A240 standard sets 316 at roughly 16% to 18% chromium, 10% to 14% nickel and 2% to 3% molybdenum. 304 has no molybdenum requirement. It carries a little more chromium, from about 17.5% to 19.5%, and less nickel, from 8.0% to 10.5%.

Molybdenum strengthens the passive layer against chlorides, the salts that cause pitting. Engineers compare grades with a figure called the pitting resistance equivalent number. It weights molybdenum at about 3.3 times chromium, so 316 scores several points higher than 304.

That extra resistance matters in chloride-heavy places: seawater, coastal fittings, swimming pool equipment, and some chemical and pharmaceutical plants. It is also used in some food-processing lines that handle hot brines for long periods.

A kitchen frying pan or a lunch box container does not live in those conditions. Food sits in contact for hours, not months, and the steel gets washed and dried between uses. For ordinary cooking and food storage, 304 is the standard food-contact grade, and 316 is not needed.

One habit matters for both grades. Do not leave salty liquids or undissolved salt sitting in the steel for long periods. Rinse and dry it. That protects 304 and 316 alike.

• 304 (18/8): about 17.5–19.5% chromium, 8.0–10.5% nickel, no molybdenum requirement
• 316: about 16–18% chromium, 10–14% nickel, 2–3% molybdenum
• Choose 316 for long exposure to seawater, brines or other strong chloride sources
• For everyday cooking and food storage, 304 is the standard food-contact grade

Can you tell 18/8 from 18/10 or 316 by looking at it?

No. 18/8, 18/10 and 316 look the same once they are polished or brushed. A two-point change in nickel does not produce a colour difference you can see.

A magnet does not settle it either. All three grades are austenitic, and in the annealed state they are essentially non-magnetic. But pressing, drawing and bending can make 304 weakly magnetic in the worked areas, so a slight pull on a formed bowl or rim does not mean the steel is a different grade.

Many frying pans add a magnetic stainless layer on the outside, often a 400-series ferritic grade, so the base works on induction hobs. That exterior can be strongly magnetic while the cooking surface is still austenitic. A magnet test tells you about the layer the magnet touches, not the cooking surface.

The reliable ways to confirm a grade are a chemical analysis or the steel producer's paperwork. For a finished product, that paperwork is the mill certificate.

How do you read a stainless steel mill certificate?

A mill certificate, also called a mill test report or an inspection certificate, is the steel producer's record for a specific batch of steel. It is the document behind a grade claim such as '304' or '18/8'.

Start with the certificate type. Many mills, including most in Europe, issue certificates to EN 10204; mills working to American specifications issue a mill test report against a product standard such as ASTM A240 for plate, sheet and strip. An EN 10204 type 3.1 certificate reports test results for the actual batch supplied. It is signed by the manufacturer's authorised inspection representative, who is independent of the manufacturing department. It is still the producer's own document, not a third-party certificate.

Next, find the heat number. A heat is one melt of steel. The heat number ties the certificate to the coil or sheet it describes, so the results apply to that steel and no other.

Then check the standard and grade. A line such as 'EN 10088-2, 1.4301' or 'ASTM A240, Type 304' names the specification the steel was made to. The certificate should also show whether the steel met it.

The chemical composition table is the heart of the document. Look for chromium (Cr) and nickel (Ni). A 304 steel reading about 18% chromium and 8% nickel is 18/8. A reading of about 10% nickel is 18/10. A molybdenum (Mo) value of 2% or more points to 316.

Most certificates also list mechanical properties, such as tensile strength, yield strength, elongation and hardness. These show how the sheet behaves when it is formed. They are useful to manufacturers, but they do not change the grade.

• Certificate type: an EN 10204 3.1 certificate or an ASTM mill test report covers the batch supplied, issued by the producer rather than a third party
• Heat number: links the results to one melt of steel
• Standard and grade: for example, EN 10088-2, 1.4301, or ASTM A240, Type 304
• Chemistry: Cr about 18% and Ni about 8% means 18/8; Mo of 2% or more means 316
• Mechanical properties: strength and elongation for forming, not a grade test

Is 18/8 stainless steel food safe?

18/8 (304) is one of the most widely used food-contact stainless steels in the world. You will find it in commercial kitchens, dairy equipment and home cookware.

Two cautions apply when you read a listing. First, 'food grade' is a trade term, not a grade designation, so it helps to know the actual grade: 304 or 18/8 tells you more than 'food grade' or 'surgical grade' alone. Second, a grade describes the material, not the finished product — how an item is made and finished matters too.

The wider safety question, including coatings and what the surface is made of, is covered in our guide to whether stainless steel cookware is safe.