Hot rolled plate thickness tolerance follows EN 10029 (Class A, B, C, D) or ASTM A480; cold rolled sheet and strip follow EN 10259 or A480. The class chosen decides how much under-thickness is permitted, which matters for pressure calculations and for weight-based pricing.
Flatness is a separate tolerance and is where most complaints arise. Specify the flatness class and whether the plate will be laser cut, since heat-affected plate can move. For laser cutting, ask for stress-relieved or leveller-flattened plate.
Weight is calculated as thickness × width × length × 7.93 g/cm³ for austenitic grades (7.7 for ferritic, 7.8 for duplex). Mills sell on theoretical or actual weight; confirm which, because the difference can be several per cent.