The Difference Between Metal Sheet and Plate

The difference between a metal sheet and metal plate can be defined by thickness. If the thickness of the metal is 6mm (¼”) or more, then it is considered a plate. Plate thickness is typically measured in millimeters. If the thickness is less than 6mm (¼”), it is considered sheet. Sheet is typically measured in gauges.

Sheet and Plate Production

When molten metal exits the blast furnace it is continuously cast into billets, and slabs. Billets are commonly used to form bar stock and wire while slabs will be used to produce plate and sheets. Slabs exit the casting process in thicknesses often exceeding 100mm. In order to make a usable metal sheet or plate, this thickness needs to be reduced.

To reduce and most importantly, to standardize the thickness along the full length of the metal product, the metal is rolled, either black or bright. If the metal is rolled while it is hot (above its recrystallization temperature) it is called black. The metal is fed through rollers that gradually reduce the thickness of the metal until it reaches the desired thickness. This thickness can be as little as 3 hundredths of a millimeter.

If the final metal product has thickness above 6mm (¼”) then it is known as metal plate and is part of the structural metal family. Any products thinner than this are considered sheet metal.

Bright drawn is when the material either needs increased strength, or when very precise thickness is required. The thickness reduction at each pass is lower than when produced black, so the process takes longer when compared to black steel.

Measuring Sheet

It is important to note that when measuring sheet product, gauges are neither standard nor metric, and have independent values. A conversion chart should be used to convert gauge to mm.

There are a variety of gauge systems used today, with different metal types having different gauge systems. For example, an 18 Ga. Sheet of aluminum measures 1.02 mm thick. While an 18 Ga. Sheet of another steel measures 1.21 mm. The two metals have different gauge systems resulting in different thicknesses for the same gauge. Visit this article for sheet metal gauge charts.