What size coffee table, and will it fit?

Length is about two-thirds of the sofa, measured arm to arm. Anything from half to three-quarters reads as deliberate. Below half the table looks stranded in the middle of the floor; above three-quarters it starts to crowd the ends of the seat, and you lose the bit of floor where a side table or a standing lamp wants to go.

Height matches the sofa's seat, or sits up to 5cm under it. This is the one measurement with a wrong answer rather than a range: a table taller than the seat means reaching up for a mug rather than down, and you feel it every evening. UK sofas mostly seat at 42 to 48cm, which is why 40 to 45cm is the standard coffee table height.

Depth follows length rather than being a fixed number. Around half the table's length looks right — roughly 60cm deep on a 120cm table. Much squarer and a rectangular table reads as an oversized side table; much shallower and there is nowhere to put a tray down without it overhanging.

The gap between the sofa and the table wants 35 to 45cm. Under 35cm you cannot get past or sit forward; over 45cm you have to stand up to reach your drink. This calculator uses 40cm, the middle of that range.

Behind the table there has to be somewhere to walk, and this is the part the size rules leave out. 75cm is comfortable for a main route across a room; 45cm is the least that works at all. Give the tool the clear floor in front of your sofa and it caps the depth at what is left once both the gap and the route are taken out — which is how a table that is the right shape on paper turns out to be the wrong one in the room.

Three measurements, and you want a tape rather than an estimate — a 10cm error here is the difference between a table that works and one that goes back.

Sofa length is arm to arm at the widest point, not the length of the seat cushions. On a sofa with rolled or flared arms the widest point is usually a few centimetres above the seat, so measure there. This is the number the two-thirds rule works from, and taking it from the cushions instead is the most common reason a table arrives looking small.

Seat height is the floor to the top of the seat cushion, with the cushion uncompressed — do not sit on it first. On a deep feather-filled sofa this reads high and settles lower in use, so if yours has soft cushions, press gently and take the number it settles at.

Floor depth is the clear run from the front edge of the sofa to whatever faces it: a TV unit, a wall, an armchair, or the point where the route through the room crosses. Measure at the narrowest point rather than the middle, because that is the point the table has to clear.

If you are working around a rug, measure to the rug's edge only if the rug is what people walk round. Most of the time it is not — people walk on rugs — so measure to the real obstruction.

Most rectangular coffee tables sold in the UK land between 100 and 130cm long, 50 and 70cm deep, and 40 and 45cm high. That is not a rule so much as what the market makes, and it is worth knowing because it tells you where you will have a real choice and where you will be hunting.

Under 90cm long, the choice thins out fast and most of what is left is square or round rather than rectangular. Over 140cm, you are into a smaller and more expensive set of pieces, often sold as a 'large' or 'grand' coffee table, and frequently deeper than half their length because they are designed for a room with space around them.

Round tables are usually 80 to 110cm across. A round table of a given diameter takes noticeably less floor than a rectangle of the same length, which is why it is the answer in a tight room even though it gives you less usable surface.

Nests of two or three run smaller again — typically a 60 to 80cm main table with one or two beneath it. They are the honest answer when the floor cannot take a single table, because you get the surface when you need it and the floor back when you do not.

None of these numbers should override what your own sofa says. A 100cm table is the standard size and completely wrong in front of a 260cm sofa.

The two-thirds rule assumes one straight sofa, and most rooms are not that. Two cases come up often enough to be worth the adjustment.

For a corner or L-shaped sofa, measure the longer arm and work from that, then take the result down by about 10 per cent. The short return eats floor that a straight sofa of the same length would leave you, and a table sized to the long arm alone tends to sit too close to the corner to walk round. A square or round table often suits a corner sofa better than a rectangle for the same reason — there is no long edge to line up with, because the seating turns.

For two sofas facing each other, size to the shorter of the two, and check the gap on both sides rather than one: you need 35 to 45cm to each sofa, so the table has to fit in the middle of a span it does not control. A table that works against one sofa and leaves 25cm to the other is worse than a smaller table centred properly.

Sofa and two armchairs is the easiest arrangement to get right and the easiest to overthink. Size to the sofa and ignore the chairs — as long as each chair has something in reach, which usually means a side table rather than a bigger coffee table.

Shape is not only a matter of taste; each one behaves differently in a room, and in a small room the difference is the whole decision.

Rectangular suits a straight sofa and a room with a clear long axis. It gives you the most usable surface for the floor it takes, and it is the easiest shape to buy because it is what most of the market makes.

Round suits a tight room, a room where the route past the table is the route across the room, and a room with children in it. It has no corner to catch a shin on, it lets people cut the corner rather than walk the long way round, and it reads as less bulky than a rectangle of the same footprint. You lose surface area and you lose the ability to line it up with anything.

Oval is the compromise and, in a narrow room, often the right one: it keeps most of a rectangle's surface and length while losing the corners that make a narrow room feel narrower.

Square works in front of a corner sofa and in a room that is close to square itself. In front of a long straight sofa it usually reads as too small even when the footprint is right, because it does not answer the sofa's own shape.