Carpet is not sold by the square metre
It is priced by the square metre, which is why every other carpet calculator gets this wrong. It is sold by the length off a roll of fixed width.
A roll is normally 4m or 5m wide in Britain and Australia and 12ft or 15ft in North America, and it cannot be split lengthways. So you buy a rectangle of roll width by however many metres you need off it, and you pay for that rectangle whatever shape your room actually is.
A 3m by 5m room is 15 square metres of floor. Off a 4m roll you take 5 metres, and you pay for 20 square metres. The extra metre runs the whole length of the room and gets cut off and binned. That is not a markup, it is the material.
So a floor-area calculator can quote you a quarter under what the shop charges, and why people think they have been overcharged when they have not.
Which way round it goes changes the price
This is the part worth knowing before you go in.
A 5m by 3m room off a 4m roll: run the carpet across the 3m and you need 5 metres, so 20 square metres. Run it the other way and you need two drops of 3m, so 24. Same room, same carpet, four square metres of difference for nothing but which way it was unrolled.
This works out both orientations and takes the cheaper one. What it cannot decide for you is whether the cheaper way is acceptable, because the pile direction has to run the same way through the room, and on a patterned carpet the pattern has to run the right way up. Tell the fitter which way you have costed it.
The roll width matters more than the price per metre
A room 4.2m wide is the worst case on a 4m roll: you need two full drops, and nearly half the second one is waste. On a 5m roll the same room takes one drop and no seam.
So a dearer carpet on a 5m roll can genuinely come out cheaper fitted than a cheaper one on a 4m roll. Ask what widths a range comes in before you fall in love with it, because many only come in one.
Seams
A seam is needed when the room is wider than the roll in both directions. It is normal, and a good fitter's seam is close to invisible on a plain carpet.
Where it goes matters. Keep it out of the main walking line, because that is where it will wear and eventually show. Keep it away from a window, because light raking across a seam picks it out when nothing else would. And on a patterned carpet the pattern has to be matched across it, which needs extra length again, sometimes a full pattern repeat.
Measuring
Measure the widest point in each direction, not the average, and include any doorway, bay or alcove the carpet has to run into. Carpet is cut to the largest rectangle it needs to cover, and a chimney breast does not reduce that.
Add about 5 per cent for trimming. Rooms are rarely square, and a fitter needs something to trim to rather than exactly enough. If the walls are visibly out, make it ten.
What is not in this number
Underlay, gripper rod, door bars, and fitting. Underlay comes in its own roll widths, usually different from the carpet, so it is a separate sum. Fitting is normally priced by the room or by the square metre and often includes taking the old carpet away, which is worth asking about because tip charges are not nothing.
Good underlay is the single biggest thing you can do for how a carpet feels and how long it lasts. A mid-range carpet on good underlay beats an expensive one on thin foam.
Common questions
How is carpet measured and priced?
By the length taken off a roll of fixed width. You pay for a rectangle of roll width by that length, whatever shape the room is, so a 3m by 5m room off a 4m roll uses 15 square metres and is charged for 20. Britain and Australia stock 4m and 5m; North America stocks 12ft and 15ft. Switch the units at the top and the roll widths change with them.
Why am I paying for more carpet than my room?
Because the roll cannot be split lengthways. If your room is 3.6m wide and the roll is 4m, the extra 0.4m runs the whole length of the room and is cut off and thrown away. It is not a mistake or a markup, it is how the material is made.
Does it matter which way round the carpet is laid?
Enormously, for cost. Running it across a room rather than along it can change the length needed and therefore the price. This works out both ways and takes the cheaper one. The pile direction and any pattern have to agree with the choice.
What are the standard carpet roll widths?
4m and 5m in Britain and Australia, 12ft and 15ft in the United States and Canada. They are not roundings of each other: 4m is 13ft 1in and 12ft is 3.658m, so a room that lays out with no waste on one is wasteful on the other. 13ft 2in also turns up, and it is 4m sold under an imperial name. Some ranges only come in one width, and the width often matters more to the bill than the price per square metre does.
Do I need a seam in my carpet?
Only if the room is wider than the roll in both directions. Then two drops are joined, and the seam should sit out of the main walking line and away from raking light. Patterned carpet has to be matched across a seam, which needs extra again.