Seat depth is the measurement from the front edge of the seat cushion back to the base of the backrest. It has a big effect on how a sofa feels to sit in, since a depth that suits one person's height can leave a shorter person unable to reach the backrest with their feet flat on the floor.
A shallower seat depth, roughly 50 to 55 cm (20 to 22 inches), suits upright, formal sitting, while a deeper seat depth from around 60 cm (24 inches) upward suits relaxed lounging with legs tucked up. Checking seat depth alongside seat height gives a fuller picture of comfort than the overall sofa dimensions alone, as our sofa size guide explains.