How the carpet calculator works
Wall-to-wall carpet, often called broadloom, is cut from a roll of fixed width and sold by the running metre or by the square yard. If the room is narrower than the roll, one piece does the job. If it is wider, a second strip (fitters call it a drop) has to be cut and seamed on, and the unused part of that strip is paid for but thrown away. The calculator therefore tries two directions: strips running along the room and strips running across it.
Carpet to buy = number of strips × (strip length + trim) × roll widthThe number of strips is the room dimension across the roll divided by the roll width, rounded up. Each strip is cut 10 cm (4 in) longer than the room so the fitter can trim it tight against the skirting. Whichever direction gives the shorter cut wins. Underlay is laid in butted sheets, so it only needs the room area plus 5%, and the gripper strip (tack strip) runs along the whole perimeter, minus door thresholds.
Worked example
A lounge measures 5.5 m × 4.5 m, or 24.75 m², and the chosen carpet is on a 4 m roll. Running along the 5.5 m length, the 4.5 m width needs two strips of 5.6 m, a cut of 11.2 m (44.8 m²). Running across, the 5.5 m length needs two strips of 4.6 m, a cut of 9.2 m. The second option wins: 9.2 m × 4 m = 36.8 m² to pay for, one seam, and 12.05 m² of offcut. The same carpet in a 5 m width would need a single 5.6 m cut, only 28 m² and no seam at all. Underlay comes to 26 m² and the gripper to 20 m before subtracting the doorway.
Standard roll widths and what they fit
| Roll width | Where it is common | Rooms it covers in one piece |
|---|---|---|
| 4 m (13 ft 1.5 in) | UK and Europe, most ranges | Bedrooms and lounges up to 4 m wide |
| 5 m (16 ft 5 in) | Europe, selected ranges | Larger lounges and open plan areas |
| 12 ft (3.66 m) | North America, standard | Rooms up to 12 ft on one side |
| 13 ft 6 in (4.11 m) | North America, some ranges | Rooms up to 13 ft 6 in |
| 15 ft (4.57 m) | North America, some ranges | Rooms up to 15 ft |
Underlay usually comes in rolls of about 1.37 m × 11 m (15 m²) in Europe, while North American carpet pad is often sold by the square yard in rolls of around 30 yd².
Things that change the quantity
- Pile direction. Every strip must run the same way or the colour looks different in each piece. That is why the offcut of a second drop can rarely be turned around to fill a gap elsewhere.
- Pattern repeat. Patterned carpet needs each strip to line up with its neighbour. Add one pattern repeat, often 30–90 cm, to every drop.
- Seam placement. Keep seams out of doorways and away from main walkways, and run them towards the main window where possible so light does not cross them. The cheapest layout is not always the best one.
- Stairs and landings. Stairs are measured separately; each step typically takes a piece about 0.5 m (20 in) long covering the tread and riser.
- Bays and alcoves. Measure to the deepest point of a bay window or alcove. The carpet must cover it, even though most of the extra width ends as offcut.
Frequently asked questions
How much carpet do I need for a 12x12 room?
On a 12 ft roll, a 12 × 12 ft room needs one piece about 12 ft 4 in long, so roughly 148 ft² or 16.4 square yards, compared with 16 yd² of floor.
Why do I have to buy more carpet than my room size?
Carpet is cut from a roll of fixed width, typically 4 m or 12 ft. If your room is 4.5 m wide on a 4 m roll, you pay for a second full-width strip even though you only use 0.5 m of it.
How many square yards are in a square metre?
One square metre equals 1.196 square yards, and one square yard is 0.836 m² or 9 ft². To price carpet sold per square yard, divide the square feet by 9.
How much extra carpet should I add for trimming?
Add about 10 cm (4 in) to the length of every strip. Fitters trim it into the gripper against the skirting, and walls are rarely perfectly square.
Do I need new underlay with new carpet?
In most cases yes: old underlay is compressed and shortens the life of the new carpet. Plan about 5% more underlay than the floor area, or 26 m² for a 24.75 m² room.
Last reviewed: 2026-09-27 · How we calculate