Carpet calculator off a 4m or 5m roll

How the carpet calculator works

Carpet is cut off a roll of fixed width, so you buy a length of roll, not a number of square metres. The calculator adds 10cm to the room's length and 10cm to its width for fitting, tries the roll both ways across it, and picks whichever needs the shorter length. If the room is wider than the roll both ways, it works out how many widths are needed and the total length to cut, and warns you there will be a seam.

The 10cm comes from Online Carpets' measuring guide, which says to add at least that for fitting, trimming and walls that are not square. The allowance is fixed, so if a room is badly out of square, add a little to your measurements before you type them in.

A worked example using the calculator's defaults: a bedroom 4.5m by 3.6m, one door, carpet from a 4m roll.

• With the allowance: 4.6m by 3.7m.
• Which way: 3.7m fits across a 4m roll, so the roll runs along the 4.6m side.
• Carpet to order: 4.6m off a 4m roll, which is 18.4m².
• Floor area: 4.5 × 3.6 = 16.2m². So you pay for 2.2m² more than the floor. The calculator shows it as the offcut: 12% of the 18.4m² you buy, or about 14% on top of the floor area.
• Gripper: 2 × (4.5 + 3.6) = 16.2m of wall, less a 0.76m doorway, is 15.44m. In 1.52m lengths, 15.44 ÷ 1.52 = 10.2, so 11 lengths.
• Underlay: 16.2m² ÷ 15.07m² a roll = 1.07, so 2 rolls.
• Door bars: 1.

UK carpet roll widths: 4m and 5m

In the UK, carpet comes in 4m and 5m wide rolls, and not every range comes in both. Online Carpets says you can choose either width but only certain carpets come at 5m, and Flooring Hut's guide to widths says some carpets come in only one. Tapi says 5m is the widest it sells. The calculator offers 4m and 5m, plus Other for any width you type.

You may see 3.66m (12 feet) on American calculators. It is the standard US width, but we could not find a UK retailer stocking it as a normal width, so the calculator leaves it out. If your carpet is 3.66m, choose Other and type 3.66.

The wider roll is not always the better buy. For the bedroom above, a 5m roll laid across the 4.6m side needs 3.7m of length, which is 18.5m². That is 0.1m² more than the 4m roll, because the extra metre of width is cut off and thrown away. Run both widths through the calculator before you pick a carpet that only comes in one.

When the room is wider than the roll

If both sides of the room are wider than the roll, the carpet has to be joined. Online Carpets puts it simply: if the room is over 5m in both directions, you need two pieces. The calculator then shows how many widths are needed and the total length, with a warning about the seam. All the pieces must run the same way.

A lounge 5.6m by 4.8m shows how much the roll width matters here.

• From a 5m roll: 4.9m with the allowance fits across, so 5.7m of length is enough. That is 28.5m², with no seam, for a floor of 26.88m².
• From a 4m roll: neither side fits. The best layout is two widths, each 4.9m long, so 9.8m of length and 39.2m², with a seam.
• The difference: 10.7m² more carpet from the 4m roll. The second 4m piece only has to cover 1.7m, so a strip 2.3m wide is cut off it and paid for. If the carpet comes in 5m, use it.

Pile direction and why it limits the cut

Carpet pile leans one way, and a piece laid the other way round looks like a different shade. DCT's fitting guide warns of exactly that. It is why you cannot turn an offcut sideways to fill a gap, and why every piece in a room, and in rooms that open into each other, should run the same way.

That is also why the order is always longer than the room. The length has to cover the room plus the trimming allowance in one direction, and the pile direction rules out piecing in leftovers from the side. Patterned carpet adds more, since each piece has to line up with the repeat. Check the pattern repeat on the sample and add it to the length for every extra piece.

Which way the pile should face is a fitter's call, and sources do not agree on a rule about windows or doors. What they do agree on is consistency. When there is a join, Online Carpets asks customers to ring before ordering so the pieces are cut with the right orientation and pile direction. Tell your fitter or the shop which way you want the pile before anything is cut.

Gripper, underlay and door bars

Gripper goes round the walls but not across doorways, so the calculator takes the door openings off the perimeter. Online Carpets notes that grippers are not fitted across thresholds. It uses a 762mm door, the typical internal door width in England and Wales according to Howarth Timber. The width is fixed, so for a wider opening the gripper count comes out slightly high, which errs on the safe side.

Gripper comes in 1.52m lengths, sold by Online Carpets in boxes of 100, and Wickes sells 1.53m lengths in packs of 8. The calculator uses 1.52m. The 11 lengths in the bedroom example would be two packs of 8 from Wickes.

For underlay, the calculator divides the floor area by 15.07m², one roll of Cloud 9 Cumulus: Online Carpets lists its Cumulus underlay as 11m by 1.37m. The roll size is fixed, so for another underlay divide the floor area by its roll size. Over underfloor heating, Tapi recommends a maximum of 2.5 tog for the carpet and underlay together. Cumulus alone is 3.1 tog, so pick a thinner underlay there.

Door bars are one per doorway. A 900mm bar, such as Selco's carpet to laminate strip, covers a standard door. Match the bar to the floor on the other side of the door.

• Gripper position: Online Carpets says grippers sit 10mm from the wall. Ulster Carpets' fitting guide, based on BS 5325, puts the gap at two thirds of the carpet's thickness, and recommends new gripper for a new carpet.
• Underlay joins: Ulster says to tape every underlay join with single sided tape.

Measuring a room for carpet

Measure wall to wall at the longest and widest points, including bay windows, alcoves and the space inside each doorway. A room is rarely square, so measure each side twice and use the bigger figure. Do not round down.

For an L-shaped room, measure the full length and full width as if it were a rectangle, and see what the calculator gives. Then ask the shop whether cutting it as two pieces saves carpet. The fitter has to agree, because a second piece means a join, and joins are best kept out of walkways. Tapi suggests putting them near the far wall or under a sofa or wardrobe.

Fitters can use the quote template to set out carpet, underlay, gripper and fitting line by line, and the day rate calculator to check the fitting rate covers their costs.

• Outside its scope: stairs (measured tread by tread), pattern repeats and carpet tiles.
• Cost is up to you. Carpet is priced per m² of what you buy, so the waste figure is the number to watch.
• Length rounding: the calculator rounds up to the next 10cm. Some shops cut to other steps, so check before you order.

Common questions