Regular ground flat stock has the most tightly controlled tolerances of thickness and width. It may be referred to as on-size, standard, or precision ground flat stock. There are differences between manufacturers but regular tolerance is usually +/-.001″ nominal thickness (for fractional inch sizes) and +.05mm/-.00 nominal thickness (for metric sizes). Some manufacturers produce a looser thickness tolerance of +/-.002″ when the piece is greater than 1.25″ thick or longer than 36″.
The width tolerance is +.005″/-.000″ or +.2mm/-.00mm depending on the unit of measure. When the thickness and width are the same, the pieces are referred to as squares and both dimensions have tolerances of +/-.001″ or +.05mm/-.00.
Oversize GFS has extra grind stock on the thickness and width dimensions. The tolerance for both dimensions is usually +.010/+.015″. The extra grind stock enables the end-user to cut the GFS, heat-treat it, and grind it to a precise desired dimension.
Regardless of whether a piece is regular or oversize, the squareness tolerance specification is the same. The edge-to-surface squareness is .003″ per inch, or .075mm per 25mm. The end-to-surface squareness is .004″ per inch, or .100mm per 25mm.
Cold drawn flat stock is not precision ground and it is similar to drill rod. But the shape is rectangular, not round like ordinary drill rod, so the pieces are categorized with GFS. The thickness and width tolerances that result from the cold drawing process are similar to regular tolerance. They are +/-.0005 to +/-.0015 depending on the thickness and width dimensions of the piece. There is no squareness tolerance specification for cold drawn flat stock.
Length tolerances also vary by manufacturers but in general, all flats are at least 1/8″ longer than nominal.