How much carpet do I need?
Carpet is not sold by the square metre you walk on. It is cut off a roll that is 4m or 5m wide, and you pay for the full width however narrow your room is. So the quantity to order is a length in running metres — and the honest cost is the price of that whole rectangle, offcut included. This calculator works out both, and where a seam has to fall if the room is wider than the roll.
Step 1: the room, in metres
Measure the longest and widest points, into doorways and bay windows, not the middle of the room. Round up to the nearest 10cm. A fitter will measure it again, but getting it close tells you whether you are on the right side of a roll width — which is the decision that moves the price.
Step 2: the roll width decides the waste
A 4.2m wide room off a 4m roll needs a second piece and a seam. A 3.6m wide room off the same roll wastes 0.4m across the whole length — real material you pay for and throw away. The calculator tries the roll both ways round, picks the layout with fewest seams, and tells you the square metres you are buying against the square metres you are laying.
Step 3: price the roll, not the floor
Multiply the running metres by the roll width to get the area you are charged for. A 5m × 3.5m room is 17.5 m², but off a 4m roll you buy 5.5m × 4m, which is 22 m². At £25/m² that is a £112 difference, and it is why a quote comes back higher than the sum people do in their heads.
4m or 5m roll: what each costs for the same room
Running metres include 10% for squaring up and door recesses, and the roll is turned whichever way buys less material — which is what the calculator does. The same carpet price per m² applies in both columns, so the difference is material you pay for and bin.
| Room | Floor area | 4m roll | 5m roll | Cheaper width |
|---|---|---|---|---|
| 5.0 × 3.5m | 17.5 m² | 5.5m = 22.0 m² | 3.9m = 19.5 m² | 5m |
| 5.0 × 4.0m | 20.0 m² | 5.5m = 22.0 m² | 4.4m = 22.0 m² | Either |
| 5.0 × 4.5m | 22.5 m² | 9.9m = 39.6 m², 1 seam | 5.0m = 25.0 m² | 5m |
| 6.0 × 4.8m | 28.8 m² | 10.6m = 42.4 m², 1 seam | 6.6m = 33.0 m² | 5m |
| 3.0 × 2.4m | 7.2 m² | 2.7m = 10.8 m² | 2.7m = 13.5 m² | 4m |
The 4m roll only wins in a room where 4m of width already covers a whole dimension — the small bedroom in the last row. Anything over 4m across needs the 5m roll or a seam, and the seam costs more than the wider roll.
Seams, pile and which way the carpet runs
Where a seam should go
If the room is wider than the roll you will have a join. Put it where it gets the least light and the least traffic — along the line of a wall rather than across the middle of the floor, and never across a doorway. A well-made seam in a cheap carpet is less visible than a badly placed seam in an expensive one.
Pile direction
Carpet has a nap, and two pieces laid with the pile running different ways will look like two different colours whatever the seam is like. Every piece in one room runs the same way, and ideally the pile leans away from the main light source and away from the door you walk in through. It also limits how the roll can be turned: the cheapest cut is not always the one you can lay, and it is why offcuts from one room rarely work as patches in another.
Patterned carpet costs more than plain
A pattern has to match across a seam and at the doorway, so the fitter cuts to the pattern repeat rather than to your room. Add the repeat length to every piece — on a large repeat that can be another half metre per join. If you are comparing a patterned carpet against a plain one, compare the metres ordered, not the shelf price.
Underlay, gripper and the bits on the quote
Underlay is not where to save money
Carpet underlay does most of the work you feel underfoot, and a cheap carpet on good underlay outlasts a good carpet on thin foam. It is sold by the roll — commonly 15 m² — and cut to the room, so unlike the carpet itself it wastes very little. Thicker is not automatically better: anything much over 11mm moves too much under a door and on stairs.
Gripper rod and door bars
Carpet is stretched onto gripper rod pinned around the perimeter, minus the doorways. Measure the perimeter, take off the door openings and divide by the length the rod comes in. Each doorway then needs a bar or threshold matched to the floor on the other side — carpet to laminate, carpet to tile and carpet to carpet are three different profiles.
Fitting, and lifting the old one
Carpet goes down fast — a fitter covers more ground in a day than with any hard floor — so fitting is usually the smallest line on the quote. Taking up and tipping the old carpet and underlay is often charged separately, and door trimming is extra again because a new carpet on new underlay can be 10mm thicker than what came out.
Stairs and landings
Stairs are measured per step, not by area
Each step takes the tread depth plus the riser height plus about 50mm to wrap the nose — call it 0.5m of carpet length per step on a typical UK staircase, off the full roll width. Thirteen steps is therefore around 6.5m of running carpet, and the width of the staircase barely changes it. Stairs also have to be one continuous piece where possible, so they are cut before the landing, not from what is left.
Order the stairs and landing together
Buying them separately is how people end up with two batches that do not match on the one surface where the join is most visible. Work out the landing with this calculator, add the stair length, and order the total as one piece. For the rest of the house, the flooring calculator prices all six materials, and the laminate flooring calculator gives pack counts if the downstairs is going hard.
Next steps
For what carpet costs against laminate, LVT and real wood per square metre, fitted, see UK flooring costs per m². If you only need the room area in square metres, the square metre calculator converts between metres and feet, and budgeting a renovation puts the floor into the whole-project number.