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Work out how much carpet to buy, plus the underlay, gripper rod and door bars. Carpet is cut from rolls 4 m or 5 m wide, so the calculator finds the cheapest way to cut your room from the roll and shows where any joins will go.
Carpet To Buy
0m²
Check the measurements above. The result updates once they look right.
| Piece | Size (Width × Length) | Notes |
|---|
Each piece is the full roll width. Lengths include the trim allowance.
| Roll | To Buy | Waste | Joins |
|---|
Same room. Your choice is highlighted.
Carpet is not sold by the square metre of floor. It is cut from a roll of fixed width, so you pay for the full width of every piece even if part of it is trimmed off. This calculator works the way a carpet shop does: it fits your room to the roll and tells you how much you actually have to buy.
The result shows the carpet to buy, the length to cut off the roll, the joins, the offcut waste, the underlay and the gripper rod. A plan drawing shows how the carpet is laid and where any join falls.
Most carpets sold in the UK come in 4 m and 5 m widths, and some woven and wool carpets are made 3.66 m (12 ft) wide. You can buy any length you like, but only the full width. That is why the width of the roll matters so much.
| Roll width | Best for | Notes |
|---|---|---|
| 4 m | Rooms up to about 3.9 m wide | The most common width. Many ranges only come in 4 m. |
| 5 m | Rooms 3.9 to 4.9 m wide | Often avoids a join in larger living rooms. |
| 3.66 m (12 ft) | Some wool and woven carpets | Narrower, so wider rooms are more likely to need a join. |
A room 3.5 m wide fits a 4 m roll with room to spare, but a room 4.2 m wide needs either a 5 m roll or two pieces of 4 m carpet joined together. The calculator works out both and shows the difference in the Compare Roll Widths table.
The calculator adds a trim allowance of 10 cm to each cut, which gives the fitter enough to trim the edges neatly against the skirting. You can change this in the optional settings.
Before you order, it is worth having the retailer measure too. Independent retailers that belong to The Carpet Foundation follow a code of practice approved by the Chartered Trading Standards Institute, which covers advice, fitting and complaints.
The calculator tries the roll both ways: with its length running along the room, and with it running across. With Best Fit it chooses the layout that needs less carpet, and if both are the same it chooses the one with fewer joins. When a room is exactly the roll width, the carpet can still be fitted in one piece, but with no trim margin on that side. Best Fit only uses this exact fit when the other way would need a join.
For an L-shaped room, it compares two options. The first is one big piece covering the whole L, with the corner cut away. The second is two separate pieces, one for each part of the room, which adds a join but can save a lot of carpet. For stairs, it works out how many step-sized pieces fit across the roll and how many rows of them you need.
The floor is 14.7 m². The 3.5 m width plus 10 cm fits on a 4 m roll, so you need one piece 4.3 m long: 4 m × 4.3 m = 17.2 m² of carpet, with no join. About 2.5 m² is trimmed off. The bedroom also needs about 15.4 m² of underlay, which is two 15 m² rolls, and around 14.6 m of gripper rod for one door.
On a 4 m roll, the room is too wide for one piece. Two pieces 4.6 m long give 36.8 m² with one join. On a 5 m roll, one piece 5.1 m long gives 25.5 m² with no join. At £20 per m², the 5 m roll saves about £226 and avoids a seam, so check whether your carpet comes in 5 m.
The main part is 5 m × 3 m and the leg is 2 m × 1.5 m. One piece covering the whole L on a 4 m roll would take 36.8 m². Cutting two pieces, 4 m × 5.1 m for the main part and 4 m × 1.6 m for the leg, needs just 26.8 m², with one join where the leg meets the main room.
Each step has a 250 mm tread and a 200 mm riser, so each piece needs about 0.5 m of carpet including the wrap over the nosing. The stairs are 0.9 m wide, so four pieces fit across a 4 m roll. Thirteen steps need four rows, which is 2.1 m off a 4 m roll, or 8.4 m².
These figures use Best Fit and a 10 cm trim allowance. The cheaper width for each room is the smaller of the two figures.
| Room | Size | 4 m roll | 5 m roll |
|---|---|---|---|
| Hallway | 1 × 4 m | 16.4 m²* | 5.5 m² |
| Box room | 2.5 × 3 m | 10.4 m² | 13 m² |
| Small bedroom | 3 × 3 m | 12.4 m² | 15.5 m² |
| Bedroom | 3 × 4 m | 16.4 m² | 15.5 m² |
| Double bedroom | 3.5 × 4.2 m | 17.2 m² | 18 m² |
| Square room | 3.8 × 3.8 m | 15.6 m² | 19.5 m² |
| Living room | 4 × 4.5 m | 18.4 m² (exact fit) | 20.5 m² |
| Large lounge | 4.5 × 5 m | 36.8 m² (1 join) | 25.5 m² |
| Open-plan room | 5 × 6 m | 40.8 m² (1 join) | 30.5 m² (exact fit) |
An exact fit means the room is the same as the roll width, so the carpet goes wall to wall with nothing spare on that side. *A 1 m × 4 m hallway can be cut across a 4 m roll for just 4.4 m², but only as an exact fit. Choose Across The Width as the roll direction to see it, and ask your fitter whether the walls are square enough.
Each step needs enough carpet to cover the tread, run down the riser and wrap over the nosing. Measure:
A typical UK step needs about 0.5 m of carpet. Fitters often cut several step pieces side by side across the roll, so a straight flight of 13 steps usually needs 2 m or so off a 4 m roll. Winders and curved steps need more, because each step is a different shape. Measure landings separately as a small room.
| Item | How it is worked out | Sold as |
|---|---|---|
| Underlay | Floor area plus 5% for trimming | Rolls of about 15 m² |
| Gripper rod | Room perimeter minus about 0.8 m per doorway | Lengths of 1.52 m (5 ft) |
| Door bars | One for each doorway | Single, double or Z-bar, depending on the floor next door |
| Stair gripper | About two strips per step, one on the tread and one on the riser | Lengths of 1.52 m, cut to fit |
Underlay is laid in the same direction as the carpet, but its joins should not line up with the carpet joins. Good underlay makes a carpet feel softer and helps it last longer, while a firm, dense underlay is better for stairs and busy hallways.
Every piece of carpet has a pile direction. Run your hand over it: one way feels smooth and the other feels rough. All the pieces in a room must point the same way, otherwise the joins show as a change in shade.
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Measure the room's length and width, add about 10 cm to each for trimming, then fit the room to the roll width. The carpet you buy is the full roll width times the length you cut, not just the floor area.
In the UK, most carpets come in 4 m and 5 m widths. Some wool and woven carpets come in 3.66 m (12 ft).
A 4 m × 4 m room exactly matches a 4 m roll, so one piece 4 m × 4.1 m (16.4 m²) covers it with no join, but with nothing spare on one side. A 5 m roll gives a trim margin: 5 m × 4.1 m = 20.5 m².
A typical step needs about 0.5 m of carpet. A straight flight of 13 steps usually needs around 2 m off a 4 m roll, as several step pieces fit side by side across the roll.
Allow about 10 cm on each cut for trimming. You do not need a percentage on top, because the roll width already gives you extra on the other side.
Underlay covers the floor area plus about 5% for trimming. It usually comes in rolls of about 15 m², so a 4 m × 3.5 m room needs one roll.
Measure around the edge of the room and take off about 0.8 m for each doorway. Gripper comes in 1.52 m lengths, so divide by 1.52 and round up.
Only if the room, plus the trim allowance, is wider than the roll in both directions. Choosing a 5 m roll often avoids a join in rooms between 4 m and 5 m wide.
Yes. Switch to Imperial and enter feet. Roll widths stay in metres because UK carpets are sold that way, and results show both square metres and square feet.
Roll widths of 4 m and 5 m are the common UK standard. The calculator adds a 10 cm trim allowance to each cut, 5% to the underlay, and leaves 0.8 m of gripper out at each doorway. Gripper is counted in 1.52 m lengths and underlay in 15 m² rolls. Reviewed November 2026.