Show the formula and working
This carpet calculator works out the size of carpet to buy from a 4 m or 5 m wide roll, plus the underlay, gripper rod and door bar for the room, for homeowners in the UK, Ireland and Europe. It adds a trim allowance to each edge, tries the roll both ways round and keeps the layout that buys the least carpet. It works in metres or feet.
Quick answer: add trim to each edge, choose a roll width that covers one side in one piece, then buy a length equal to the other side plus trim. The carpet calculator shows a 4.2 m by 3.6 m bedroom needs a 4 m by 4.3 m piece (17.2 m²), 15.12 m² of underlay and 14.8 m of gripper.
What Is a Carpet Calculator?
A carpet calculator is a tool that works out what size piece of carpet to buy from a roll of fixed width, along with the underlay, gripper rod and door bar a room needs.
Carpet is cut from the roll, so you always pay for the full roll width times the length you buy. The tool adds a trim allowance to each edge, counts how many roll widths are needed to cross the room, and multiplies that by the other side plus trim. It checks both directions and keeps the one with the smallest area. Underlay equals the room area. Gripper rod equals the perimeter minus doorways.
Broadloom carpet is made in 4 m or 5 m widths in Europe, compared with 12 or 15 feet in the United States, so a US calculator can give the wrong answer for a UK room.
How Do You Use the Carpet Calculator?
- Measure the room length at its longest point, into any alcove, bay or doorway, and enter it.
- Measure and enter the width at its widest point. Switch to feet if you measure in imperial.
- Choose the carpet roll width: 4 m or 5 m. Check which widths your chosen range comes in.
- Set the trim allowance per edge. The carpet calculator uses 5 cm as a sensible default.
- Enter the total width of doorways, which is used for the door bar and taken off the gripper rod.
- Add the carpet price per m² if you want a cost, then read the size to buy, underlay and gripper.
How Much Carpet Do I Need for My Room?
You need one piece of carpet as wide as the roll and as long as the other side of the room plus about 10 cm, as long as one side of the room plus trim fits within the roll width.
That is why the area you buy is always more than the room area. A 3.6 m by 3 m room is 10.8 m², but from a 4 m roll you buy 4 m by 3.1 m, which is 12.4 m². The extra 0.9 m strip along one side is the price of a fixed roll width, not waste you could have avoided.
When neither side fits, you need a join. The carpet calculator then shows two widths and a seam warning. Its total assumes two full length strips, which is the safe maximum; a fitter can sometimes cut a narrower infill from the same order, so ask your retailer before you pay for the second full width.
Should You Buy a 4 m or 5 m Carpet Roll?
Use a 4 m roll when one side of the room is under about 3.9 m, and a 5 m roll when both sides are between about 3.9 m and 4.9 m, because the wider roll avoids a seam.
Those break points include the 5 cm trim on each edge. Not every carpet range comes in both widths, so check before you fall for a colour. The table, run through the carpet calculator with a 5 cm trim, shows how much the choice of width changes the order.
Carpet Calculator Results: 4 m vs 5 m Rolls
| Room | Room area | 4 m roll | 5 m roll | Gripper rod |
|---|---|---|---|---|
| 3 m x 2.7 m | 8.1 m² | 4 m x 2.8 m = 11.2 m² | 5 m x 2.8 m = 14 m² | 10.6 m |
| 3.6 m x 3 m | 10.8 m² | 4 m x 3.1 m = 12.4 m² | 5 m x 3.1 m = 15.5 m² | 12.4 m |
| 4.2 m x 3.6 m | 15.12 m² | 4 m x 4.3 m = 17.2 m² | 5 m x 3.7 m = 18.5 m² | 14.8 m |
| 4.5 m x 4.2 m | 18.9 m² | 4 m x 8.6 m = 34.4 m² (seam) | 5 m x 4.3 m = 21.5 m² | 16.6 m |
| 5.2 m x 4.3 m | 22.36 m² | 4 m x 8.8 m = 35.2 m² (seam) | 5 m x 5.3 m = 26.5 m² | 18.2 m |
| 6 m x 4.8 m | 28.8 m² | 4 m x 9.8 m = 39.2 m² (seam) | 5 m x 6.1 m = 30.5 m² | 20.8 m |
How Do You Measure a Room for Carpet?
Measure the length and width at the widest points, from wall to wall, and include doorways to the middle of the door where the door bar will sit.
- Bay windows: measure to the deepest point of the bay. The carpet is cut to shape on fitting day.
- Alcoves and chimney breasts: measure across the alcoves, not the narrower chimney breast.
- L shaped rooms: measure each part separately and ask whether one piece or a join is cheaper.
- Stairs and landings: these need a different method and are best measured by the fitter.
- Doorways: add the width of every opening to the doorway box for the door bar and gripper.
Measure twice in metres to the nearest centimetre before you use the carpet calculator. Rounding down by even a few centimetres can leave a gap at the skirting that cannot be fixed.
Worked Example: James Carpets a 4.2 m by 3.6 m Bedroom
James in Glasgow is carpeting a double bedroom that measures 4.2 m by 3.6 m with one 0.8 m doorway. His chosen twist pile carpet costs £22 per m² and comes in 4 m and 5 m widths. He enters the room into the carpet calculator with a 4 m roll and a 5 cm trim.
- Layout: roll along the 4.2 m side, one width of 4 m covers the 3.6 m width plus trim
- Length: 1 x (4.2 + 0.1) = 4.3 m, so buy 4 m x 4.3 m
- Carpet bought: 17.2 m², costing £378.40
- Underlay: 15.12 m²
- Gripper: 2 x (4.2 + 3.6) minus 0.8 = 14.8 m
- Door bar: 0.8 m
James also tries the 5 m roll. The tool turns the carpet the other way and suggests 5 m by 3.7 m, which is 18.5 m², so the 4 m roll is cheaper by 1.3 m².
Do You Need New Underlay and Gripper With New Carpet?
Yes, fit new underlay with every new carpet: the Carpet Foundation recommends new underlay every time, says it can increase carpet life by 40 per cent, and warns that some warranties may not apply over old underlay.
Gripper rod can sometimes be reused if it is firmly fixed with sharp pins, but many fitters replace it. The carpet calculator gives the length either way. On concrete floors, gripper is usually glued or fixed with masonry nails, so ask your fitter which type they use.
Comparing floors for the same room? Run the laminate flooring calculator or the vinyl flooring calculator to compare quantities and cost.
What Are Common Carpet Ordering Mistakes?
The most common mistake is ordering by square metres instead of roll width and length, which can leave you with a piece that is the right area but the wrong shape. A carpet calculator that works from roll width avoids this.
- Mixing pile directions at a join, which makes the two pieces look different shades.
- Forgetting that patterned carpet needs extra length to match the pattern at a seam.
- Leaving out doorways in the carpet calculator, so the carpet stops short under the door.
- Buying two rooms separately when one longer cut from the same roll would leave less waste.
Carpet Calculator FAQs
How much carpet do I need for a 4 m by 3 m room?
With a 5 cm trim on each edge, a 4 m by 3 m room fits one width of a 4 m roll. You buy 4 m by 4.1 m, which is 16.4 m². Enter your exact sizes in the carpet calculator to check.
What widths does carpet come in the UK?
Most carpet in the UK and Europe is made in 4 m or 5 m widths. Some ranges come in only one width, so check before you choose. US guides use 12 or 15 foot rolls, which give different answers.
How much extra carpet should I allow?
Allow about 5 cm on each edge, which adds 10 cm to each dimension, for trimming against the skirting. Add more for uneven walls or a patterned carpet. The carpet calculator applies the trim to both directions automatically.
How much underlay do I need?
You need underlay equal to the room area, because underlay is cut to the floor and laid inside the gripper. A 4.2 m by 3.6 m room needs 15.12 m² of underlay, even though the carpet bought is 17.2 m².
How do I work out gripper rod?
Add up the length of all the walls, which is twice the length plus twice the width, then subtract the doorways. A 4.2 m by 3.6 m room with one 0.8 m doorway needs 14.8 m of gripper rod.
What if my room is wider than the carpet roll?
You will need a join. The carpet calculator shows two widths and a seam warning, and prices two full length strips as the safe maximum. Ask the retailer about a 5 m roll, or whether a narrower infill piece can be used.
Checked October 2026. Sources: Wikipedia: Carpet, The Carpet Foundation underlay advice.