5-Ply or Tri-Ply? How to Read the Ply and Steel Grade Claims on a Pan Listing

What does the ply number on a listing count?

Ply is the count of metal layers bonded together. On a clad pan, stainless steel sits on the outside faces and a more conductive metal, usually aluminium, sits in the middle. Aluminium spreads heat well, and stainless steel gives a durable cooking surface.

Tri-ply, or 3-ply, is steel, aluminium, steel. 5-ply usually adds two more layers, often alternating steel and aluminium, and 7-ply and higher counts divide the stack further. The number tells you how many layers there are. It does not tell you how much of each there is.

What doesn't a ply number tell you?

Two pans can both say 5-ply and behave completely differently, because the count leaves out most of what decides how a pan cooks. These are the gaps a ply number leaves:

• How thick the core is. The aluminium does the heat spreading, so its total thickness matters more than how many layers it is split into. A tri-ply pan with a thick core can outperform a 5-ply pan with thin ones.
• Where the layers run: through the whole body, or only in a disc under the base.
• How well the layers are bonded. A poorly bonded pan can separate at the base, whatever its count.
• What the layers are made of. A count without materials says little about the cooking surface or the outside of the pan.
• How much the pan weighs. Across pans of the same diameter, weight is a rough guide to how much metal, and how much stored heat, there is.

Is it a 5-ply body or a 5-layer base?

'5-ply' and '5-layer base' are not the same claim. The first should mean the layers run through the whole pan, base and walls. The second can mean a layered disc fixed to an otherwise single-layer body.

Fully clad construction runs the layers up the walls. An encapsulated or disc base puts the extra metal only on the bottom. A good listing says which one it is, with words such as 'fully clad' or 'clad through the sides'. If a listing only describes the base, assume the walls are single-layer steel.

Thickness figures have the same catch. Thickness is usually given in millimetres, or as a gauge number for sheet metal, where a higher gauge means thinner metal. Check whether the figure describes the base or the wall: on a disc-base pan the two can differ a lot.

What do the steel grade numbers on a listing mean?

Grade numbers describe what the steel is made of. Type 304 is the most common austenitic stainless steel used for food contact. It is often written as 18/8, which means roughly 18% chromium and 8% nickel by weight.

Chromium is what makes stainless steel stainless: it reacts with oxygen to form a very thin, stable oxide film that resists corrosion and re-forms if the surface is scratched. Nickel keeps the steel in its austenitic structure, which gives 304 its toughness and formability and makes it essentially non-magnetic in its annealed state.

18/10 means the same family of steel with about 10% nickel. Both are sold under the 304 designation, because the standard allows a nickel range. The ratio on a label should match the ratio on the mill paperwork: if a listing says 18/10 and the certificate reads 8% nickel, the listing is wrong.

• 304 or 18/8: about 18% chromium and 8% nickel, non-magnetic when annealed
• 18/10: the same family, with about 10% nickel
• 430 or 18/0: about 16 to 18% chromium, little or no nickel, magnetic
• 'Surgical grade': a marketing phrase, not a standard grade designation

Which layer of the pan does the grade describe?

On a clad pan, each layer can be a different metal, so a single grade on a listing describes one layer, not the whole pan. The layer that touches food is the one that matters for food contact, and 304 (18/8) is the grade to look for there.

The outside is often a different steel. Induction hobs need a magnetic base to work, and 304 is largely non-magnetic, so many clad pans use a magnetic grade such as 430 on the outside; that is where the induction property usually comes from. Check the maker's stated hob compatibility rather than inferring it from a grade.

A listing that names only one grade for a clad pan has told you about one layer. A useful listing names the cooking surface, the core and the exterior.

What does a mill certificate prove, and what doesn't it?

A mill certificate is issued by the steel mill for a batch, or heat, of metal. It lists the measured chemical composition and usually mechanical properties such as tensile strength and hardness. It is good evidence of what the raw sheet is made of.

It does not describe the finished pan. It says nothing about how the layers were bonded, how the handle was fixed or how the pan behaves in an oven. Any heat figure on it refers to the sheet under the mill's test conditions, not to a pan with a handle, rivets and a lid.

It also covers only the heat it names. A certificate for one blank size does not automatically cover a different blank or a different production run. When a brand shows one, check that the product, size and batch line up.

Which cookware claims should you question?

Some phrases appear on listings without any test behind them. They are not always false, but they are claims to question rather than accept. The useful habit is to ask what specimen was tested — the metal, the finished pan or one part — and under what conditions.

• A layer count with no materials and no core thickness
• A base construction described as if it were the whole pan
• Handles described as staying cool, with no measured handle temperature or time
• 'Surgical grade' with no grade number
• A nickel ratio on the label that differs from the mill certificate
• An oven temperature stated for a whole pan and lid when only the pan body was rated
• Warranty wording with no stated term or conditions

How does SEIDO describe its own frying pans?

We write each claim with its scope beside it. 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 uncoated and made without intentionally added PFAS. The pans work on gas, electric, ceramic and induction hobs, come in 20 cm (8 in), 24 cm (9.5 in) and 28 cm (11 in), and each is backed by a 5-Year Limited Warranty.

The oven rating carries its scope on the same line: oven-safe to 260°C (500°F) for up to 1 hour. It covers the pan body, handle and lid. Our lunch box container's oven rating likewise names the container alone, because it was lab-tested without its lid. Where a claim is not yet backed by a document, it stays off the page.