Seat height is measured from the floor to the top of the seat cushion in its normal, settled state. It determines how easy a sofa is to sit down on and stand up from, which matters especially for taller people or anyone with knee or hip discomfort.
Most sofas sit somewhere between about 38 and 45 cm (15 to 18 inches), with lower seat heights giving a more relaxed, lounging feel and higher seat heights making it easier to get up. Seat height works together with seat depth and cushion firmness to determine overall comfort, so it is worth checking all three, as covered in our sofa size guide.