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Carpet calculator

Work out how much carpet to buy for a room, including L-shapes and alcoves. You see the length to order off a 4 m and a 5 m wide roll, how many seams there are, how much is waste, and the underlay, gripper and door bars.

Fitter unrolling grey carpet in a bedroom beside gripper rods and a plug socket

Measure at the longest and widest points, wall to wall.

Both are standard. We show the other width too.

doorways

On the roll label. 15 m² is common for foam underlay.

Your result

Carpet to buy

12 m²

3 m off a 4 m wide roll

Length off the roll
3 m4 m wide
Floor area
12 m²
Waste
0%offcuts you pay for
Seams
0each area comes from one piece
On a 5 m roll instead
15 m² (3 m long, 20% waste)buys more carpet
Underlay rolls
115 m² each, for 12 m² of floor
Gripper rod
13.3 medge of the room minus doorways
Door bars
1each at least 762 mm long
  • Pile and pattern direction can force the carpet to run the other way, which can mean more carpet. Ask the shop or fitter to confirm the length before you order.

Add this to your project shopping list, then print it or take it to the merchant.

Open shopping list

How to use the carpet calculator

  1. Measure the room wall to wall at its longest and widest points, and type in the length and width.
  2. L-shaped room, bay or alcove? Switch on L-shaped room or alcove and add the other areas, up to three in all.
  3. Pick the roll width: 4 m or 5 m. We also show the other width, so you can compare.
  4. Leave Also work out underlay, gripper and door bars on, then enter the doorways, their width and the underlay roll size.
  5. Check the carpet to buy, the waste and the seams. Then tap Add to shopping list or Share link to send it to your shop or fitter.

Why use this calculator

It shows the real amount you pay for. Carpet comes in rolls 4 m or 5 m wide, so you pay for the full width of every metre you cut, even where the room is narrower. That is why you often buy more than the room area. The calculator shows the waste as a percentage.

It compares both roll widths. A 4 × 3 m room is 12 m² on a 4 m roll, with nothing wasted. On a 5 m roll it is 15 m². A 6 × 5 m room needs a seam on a 4 m roll, but no seam on a 5 m roll. You can see which buys less carpet.

It adds the bits that go under and round the edge: underlay rolls, gripper rod for the edges, and a door bar for each doorway. The working is shown under How we worked it out.

How we worked it out

Your numbers, step by step. This updates as you change the calculator.

  1. Floor area = 4 m × 3 m = 12 m²
  2. 4 m roll, Room: 4 × 3 m. Run the roll along the 3 m side with no seam: 1 × 3 = 3 m
  3. 4 m roll: 3 m off the roll × 4 m wide = 12 m² to buy
  4. 5 m roll, Room: 4 × 3 m. Run the roll along the 3 m side with no seam: 1 × 3 = 3 m
  5. 5 m roll: 3 m off the roll × 5 m wide = 15 m² to buy
  6. Waste on the 4 m roll = (12 − 12) ÷ 12 = 0%
  7. Underlay = 12 m² ÷ 15 m² per roll, rounded up = 1 roll
  8. Edge of the carpeted floor = 14 m
  9. Gripper = 14 m − 1 doorway × 0.76 m = 13.24 m
  10. Door bars = one for each doorway = 1

The formula

Carpet is bought by the length off a roll of fixed width, not by the area of the room.

  • Floor area: length × width, added up for each area you enter.
  • Length off the roll: for each rectangle, the roll can run along either side. If the other side is wider than the roll, you need two or more strips (seams). We pick the way that uses the fewest metres, then the fewest seams.
  • Carpet to buy: length off the roll × roll width.
  • Waste: (carpet to buy − floor area) ÷ carpet to buy.
  • Underlay: floor area ÷ roll size, rounded up.
  • Gripper rod: the distance round the carpeted floor, minus the doorways.
  • Door bars: one for each doorway.

A 6 m by 5 m room on a 4 m roll: along the 6 m side you would need 2 strips of 6 m, which is 12 m. Along the 5 m side you need 2 strips of 5 m, which is 10 m. We use 10 m, so 10 × 4 = 40 m² to buy, with one seam.

What we assume

  • Standard roll widths are 4 m and 5 m. We do not offer 3 m or 2 m, which only come in some ranges.
  • Each area is planned as its own piece. This is the safe way to order an L-shape.
  • Underlay is 15 m² per roll by default, which is common for foam underlay. Rolls of 10 m² are also common. Change it to match your roll.
  • A doorway is 762 mm wide, the most common internal door width in England and Wales. Change it if yours is different.
  • Where extra areas join the main room, we take the shorter joining edge off the gripper length, twice. If your areas join differently, measure round the real edge.
  • We do not work out the carpet cost or stairs.

Tips

  • Measure at the longest and widest points. Walls are rarely straight, and carpet is trimmed to fit, so you need to cover the biggest size of the room.
  • Ask the shop or fitter to confirm the length before you order. Carpet with a pile or a pattern may have to run one way, which can mean more carpet.
  • If the result shows a seam, ask your fitter where it will run before they start.
  • Compare both widths before you choose. The wider roll is not always the better buy: for a 4 × 3 m room the 4 m roll wastes nothing. Use the square metre calculator to check other areas of the house.
  • Compare a few prices for the same carpet. Check the price per m², and then check how many m² you actually need to buy.

FAQs

How much carpet do I need for a 4 m by 3 m room?
12 m² on a 4 m wide roll, which is 3 m off the roll, with no waste and no seam. On a 5 m wide roll it is 15 m², so 3 m² of waste. The 4 m roll is the better fit for this room.
What width do carpet rolls come in?
4 m and 5 m are the standard widths, and some ranges also come in 2 m or 3 m. You pay for the full roll width of whatever length you cut. Check the roll width of your carpet before you order.
Why do I need to buy more carpet than the room area?
Because carpet is sold off a roll that is 4 m or 5 m wide. A 6 m by 5 m room is 30 m², but on a 4 m roll you need 10 m of carpet, which is 40 m², and one seam. On a 5 m roll you need 6 m, which is 30 m², with no seam.
How much extra carpet should I allow for waste?
There is no fixed percentage, because waste depends on the room and the roll width. We work out the exact length of carpet off the roll, and show the waste as a percentage. For 4 × 3 m on a 5 m roll it is 20%.
How do I measure for carpet in an L-shaped room?
Split the room into rectangles and measure each one. Our calculator takes up to three areas and plans a piece for each. A fitter can sometimes get two areas from one strip, so ask for their cutting plan.
How much underlay and gripper rod do I need?
One roll of 15 m² foam underlay covers a 4 m by 3 m room (12 m²). For gripper, it is the distance round the room minus the doorways. For the same room with a 762 mm doorway, that is 13.3 m, plus one door bar.

Last reviewed: 6 November 2026 · How we calculate · Report an error