Standard Seat depth is 20-22″ for a more formal sofa or ~24″ for a lounger or theater room sofa. This is measured from the front of the sofa to the front of the back cushion. The seat depth is tied to the overall depth of the sofa so as one changes, the other will as well. As with seat height, the seat depth should allow the shorter REGULAR user to have their heels firmly on the floor (unless there is an ottoman or your feet are usually pulled up under you).