Sizing · 4 min read
Sofa Depth: What the Number Means When You Sit Down
Overall depth and seat depth are different numbers. The one that decides how a sofa feels.
Two sofas can both be listed at 860 mm deep and feel nothing alike. Depth is the most misread number in furniture, because the number published is almost never the number you sit on.
Three different depths
- Overall depth. Front of the seat to the back of the frame. This is the number in the specification, and it is the one you need for planning the room.
- Seat depth. Front edge of the seat cushion to the face of the back cushion. This is what your legs actually get.
- Usable depth. Seat depth after the back cushion is compressed by somebody leaning on it. Slightly more than seat depth, and the reason soft-backed sofas feel deeper than they measure.
The gap between the first and the second is usually 250 to 350 mm. So a sofa with an overall depth of 860 mm typically sits somewhere around 520 to 600 mm of seat depth, depending entirely on how thick the back is built.
If a seller only gives overall depth, ask for the seat depth. It is not a trick question and any workshop knows the answer.
What seat depth does to how you sit
| Seat depth | How it sits |
|---|---|
| 450 to 500 mm | Upright. Easy to get out of. Good for small rooms, eating on your lap, older users. |
| 500 to 560 mm | The general-purpose range. Back supported, feet on the floor, works for most adults. |
| 560 to 620 mm | Relaxed. Comfortable with knees bent up or a cushion behind the back. |
| 620 mm and over | Lounging furniture. You sit in it, not on it. Shorter users need a cushion to reach the backrest. |
There is no correct number, only a correct number for the people using it. A 600 mm seat is luxurious if you are 1.85 m and want to watch a film, and a trap if you are 1.60 m and want to read upright.
Depth and height work together
Seat depth only makes sense alongside seat height. A deep seat that is also low means your knees end up above your hips, which is comfortable for an hour and hard to get out of. A deep seat at a normal height leaves your feet dangling unless you sit forward, which puts you off the backrest entirely.
The pairing that works for most people is a seat around 400 to 450 mm high with 500 to 560 mm of depth. Change one and you usually have to change the other.
The two-finger test
In a shop or at home with a sofa you already own, this takes ten seconds.
- Sit with your back properly against the backrest, not perched forward.
- Put your feet flat on the floor.
- Reach down to the front edge of the seat with two fingers.
Two to three fingers between the seat edge and the back of your knee is right. No gap means the edge is cutting into your calves. A hand-width gap means you are not using most of the seat you paid for.
Planning the room
Use overall depth for the floor plan, and add to it.
- Allow 400 mm between the front of the sofa and a coffee table, so people can get past and put their feet somewhere.
- Allow 750 to 900 mm for a walkway in front.
- Leave 30 to 50 mm behind the sofa if it goes against a wall. Pushed hard against plaster it marks the wall and traps dust.
A sofa 1620 mm wide and 860 mm deep therefore wants roughly 1.7 by 1.3 m of room once you count the walkway, more if there is a table in front.
What else changes the feel
Cushion fill matters as much as the measurement. A firm cushion holds you at the depth it measures. A soft one lets you sink backwards and the seat feels deeper than the tape says. Loose cushions can be moved, plumped and swapped end for end, which keeps a sofa even over years. Fixed cushions cannot, and they show their favourite seat first.
Covers that unzip are the other quiet advantage. Depth is about comfort for the first week. Being able to take covers off and wash them is comfort for the next decade.
Common questions
- What is a good seat depth for a sofa?
- Around 500 to 560 mm suits most adults sitting upright with their back supported and their feet on the floor. Above about 600 mm you are designing for lounging, and shorter people will need a cushion behind them to reach the back.
- Why is the advertised depth bigger than the seat?
- Advertised depth is the whole footprint, from the front of the seat to the outside of the back frame. The back cushion and the frame behind it eat 250 to 350 mm of that. A sofa listed at 860 mm deep does not give you 860 mm to sit on.
- How do I test seat depth before buying?
- Sit with your back against the backrest. Two to three fingers should fit between the front edge of the seat and the back of your knee. If the edge presses into your calves the seat is too deep. If your thighs hang over with nothing under them it is too shallow.
- Does a deeper sofa fit more people?
- No. Width decides how many people sit down. Depth decides how they sit. A deep two-seater is still a two-seater, it just takes more floor.