Carpet Calculator: How Much Carpet Do I Need, 4 m or 5 m?

Carpet is cut off a roll in fixed widths, so you pay for the full width of whatever length you order, not for the shape of your room. That is why two identical rooms can produce very different bills: order off the wrong roll width and a third of what you pay for goes in the skip. Put your room size in below and this works out the length to order, which roll width wastes least, where the seam should fall, and the underlay and gripper to go with it.

Shops quote carpet per square metre but charge for the full roll width, so the price you are given and the price you pay only match in a room that happens to be exactly as wide as the roll. Your results include a table of what the same room costs off each width, which is the one number a showroom rarely volunteers.

How the calculation works

Carpet comes off the roll in one piece. For a room narrower than the roll, you buy a length equal to the room length and the surplus width is cut off and binned. For a room wider than the roll, you buy two or more lengths and the fitter joins them, which is why a room 4.6 m wide is so much dearer to carpet than one 3.9 m wide.

• Trimming allowance. Fitters need spare material to scribe against skirting, chimney breasts and door frames. Ten centimetres on each measurement is the usual working allowance.
• Roll widths. Most UK broadloom comes in 4 m and 5 m. A 2 m width exists on some ranges and suits narrow rooms, halls and landings, but it is far less widely stocked.
• Which way the roll runs. A room can often be covered either by running the carpet down its length or by turning it and running it across. The calculator tries both for every roll width and keeps the best.
• Seams. The recommendation is always the cheapest plan that needs no join at all, because a join is permanent and waste is only money. Where a seamed plan would be meaningfully cheaper, it is shown underneath as an option rather than pushed on you.
• Pattern repeat. Where widths are joined, one full repeat is added per extra width so the pattern can be matched.

Where the seam should go

If your room needs two widths, tell the fitter where you want the join before they cut. A seam should run in the same direction as the main light from the window, not across it, or it will catch the light and show. Keep it out of the doorway and out of the main walking line, and if there is a sofa or a bed, run it underneath. Pile direction matters just as much: every piece must be laid with the pile running the same way, or the two halves will read as different shades.

Underlay, gripper and door bars

Underlay is sold by the roll rather than cut to size, and a common roll covers about 15 m², so the calculator rounds up to whole rolls. Gripper rod comes in 1.22 m lengths and goes round the perimeter of the room apart from the doorways, where a door bar or threshold strip takes over. Budget one bar per doorway. If you are laying over floorboards, add a roll of board paper or hardboard sheets, because draughts and dirt come up through the gaps and will shadow the seams within a year.

Prices quoted per square yard

Some showrooms and older price lists still work in square yards. One square metre is 1.196 square yards, so multiply a per-yard price by 1.196 to get the per-square-metre figure this calculator expects. A price that looks cheap against the shop next door is often just the same carpet quoted in the smaller unit.

Stairs are measured differently

This calculator covers rooms, not staircases. For a straight flight, measure the tread depth plus the riser height on one step, add about 5 cm for the nosing, then multiply by the number of steps to get the length you need; the carpet width comes from the widest tread. Winders and half landings need a template, and most fitters will measure a staircase free. If you are carpeting stairs and a landing together, ask for both to be cut from one length so the shade matches.