You can pick a sofa that is the right width, the right colour and the right price, and still end up perching on the edge of it or sinking so far back that your feet leave the floor. The number that decides that is easy to miss: seat depth, the distance from the front edge of the seat cushion to the backrest. It is not the same as the sofa's overall depth, and it is worth knowing before you sit down in one.
Shop listings usually give the overall depth, which includes the back cushions, the frame and often the arms' curve. A sofa that is 90 cm deep overall might only have a seat that is 55 cm deep. Seat depth is the part your legs feel, so if a listing gives it, use it. If it doesn't, the overall depth is a rough guide only, and it does the other job: it tells you how much floor the sofa takes up.
Below, a side view of a sofa with a seated person. Change the height of the person and the depth of the seat. The grey gap behind the knee is the thing to watch: too small and the edge of the seat presses into the back of your legs, too large and you either slide forward or need pillows to be supported. The person's proportions are rough averages, so treat the picture as a way to see the trade-off, not as a measurement of you.
Sofa makers and furniture guides describe a similar spread. Roughly 50 to 60 cm is a standard seat. Under about 53 cm counts as shallow, and over about 61 cm as deep, with seats of 70 cm and more sold as lounge sofas. Shorter people, and anyone who likes to sit upright, tend to be happier at the shallow end, since their feet reach the floor and their back reaches the backrest. Taller people, roughly 180 cm and up, usually want a deeper seat for their legs. Anyone who likes to curl up or lie along the sofa will like depth, as long as they can get out of it again.
Seat height matters alongside depth. A typical seat is around 43 to 46 cm from the floor, and the deeper the seat, the better it feels when it sits a little lower. People with knee or hip trouble usually find a shallower seat easier to get up from.
Seat depth is easy to check on a sofa you already own, and that is the best reference you have. Sit on it the way you usually do, back against the backrest, then measure from the backrest to the front edge of the cushion with a tape. Compare the number with the sofa you are considering, not the picture.
Then think about the room as well as the seat. Measure the overall depth including the back, and leave room in front of it for legs and a walkway; the rest of the floor plan decides whether a deeper sofa fits. Measure every doorway, hallway and stairwell on the way in, and the narrowest point of any turn, because a deep sofa is harder to angle through than a slim one. If you can, sit in the sofa before you buy, or in a similar one at a friend's, for a few minutes with your feet on the floor.
A sofa that is slightly too deep or too shallow is not a reason to replace it. Cosy is not about buying more, and there are small fixes first.
The ranges above are an average, and bodies vary. If you are mostly going to lounge, read with your legs up or nap, a deep sofa you sink into can be exactly right; if you have guests who sit upright, a shallower one is kinder. Households are mixed too, so a shallow end and a chaise end on the same sofa can suit two people at once. Trust the sit test over the table: if it feels good for twenty minutes, it is probably fine.