The method

How this calculator works

Why the roll width matters

Carpet is sold in long rolls of a fixed width, commonly 4 m or 5 m, and is cut to length. You pay for the length you take multiplied by the full roll width, even if part of the width is cut off or unused, so the amount you order is usually bigger than the floor area.

A room narrower than the roll needs just one strip. A room wider than the roll needs two or more strips joined by a seam, and each strip is cut at full length.

Trying it both ways round

The strips can run along either side of the room, and the two choices can need very different amounts of carpet. In a 4 m by 3 m room with a 4 m roll, running along the 4 m side needs one 4.1 m strip, while running along the 3 m side needs two strips because 4.1 m is wider than the roll.

The calculator works out both layouts and shows the better one. In practice your fitter may choose differently, for example to run the pile towards the light or away from a doorway, so treat the result as the amount to expect.

Allowances, patterns and offcuts

Carpet is cut slightly larger than the room and trimmed to fit at the walls, so the calculator adds an allowance on each edge. A patterned carpet also needs extra length at each seam so the pattern lines up, which the pattern repeat setting covers.

The offcut is the carpet you pay for but do not use. It can be a large share of the order in a room that just misses a roll width, which is why it is worth checking more than one roll width if your carpet comes in several.

Worked example

Example: a 4 m × 3 m room with a 4 m wide roll

  1. Add the trimming allowance. 50 mm on each edge adds 100 mm to each measurement, so the room is treated as 4.1 m × 3.1 m.
  2. Lay the strips along the 4 m side. The 3.1 m needed across fits inside the 4 m roll, so one strip is enough, and its length is 4.1 m.
  3. Lay them along the 3 m side instead. The 4.1 m needed across is wider than the roll, so two 3.1 m strips are needed, which is 6.2 m, so the first layout is better.
  4. Order 4.1 m from the 4 m roll: 4.1 × 4 = 16.4 m². The room is 12 m², so about 4.4 m² is offcut.

Practical answer: Order 4.1 m from a 4 m wide roll, which is 16.4 m² with no seam. Buy underlay for the room area of 12 m², and ask a fitter to check the measurements before you order.

Before you buy

Common measuring mistakes

  • Ordering the floor area, such as 12 m², when you pay for the roll width as well, which here is 16.4 m².
  • Forgetting that carpet needs a trimming allowance at the walls.
  • Not checking which roll widths your carpet is actually sold in, since a different width can change the order a lot.
  • Ignoring the pattern repeat on a patterned carpet, which needs extra length at each seam.
  • Leaving out alcoves, bays, doorways or hallways that will be carpeted as well.
  • Buying underlay by the carpet order instead of the room area.