Basic construction component. Purveyor of Hofstadter's Law; you will always need more of these than you expect, even if you account for Hofstadter's Law.
Size & Weight
A single steel plate weighs 20kg, and is equal to 1 small light armor block. A small light armor block has a surface area of 6x(0.5m x 0.5m) = 6x(0.25m) = 1.5m2. Given these values, plus the generally accepted density of steel of 7.85g/cm3 , we can calculate the thickness (or "guage") of the steel plate using this formula .
weight (kg) = density (g/cm3) x thickness (mm) x width (m) x length (m)
Rearranging to obtain thickness:
thickness (mm) = weight (kg) / density (g/cm3) / width (m) / length (m)
thickness = 20kg / 7.85g/cm3 / 0.5m / 3m - (using length = 3m, because this is equal to 6x 0.5x0.5 plates placed in a line)
thickness = 1.698514mm = ~1.7mm
Theses calculations hold true for the large light armor block as well, where the 25-fold increase in steel plate count reflects the 25-fold increase in surface area.
It should be noted that the accepted definition of "steel plate" is steel with a guage greater than 6mm. Anything from 0.2mm to 6mm is "steel sheet" (<0.2mm is "steel foil"). Given that the steel plate in Space Engineers has a guage of around 1.7mm, it is actually "steel sheet", not "steel plate".
Ship and Station part Recipes
These are ship and station part recipes that use Steel Plates: