Carpet Calculator Explained: Drops, Rolls, Waste & Cost

Three measurements, one rounding rule, and the rest is bookkeeping

Ordering carpet feels like it should be a single multiplication — length times width gives floor area, multiply by £/m², done. It almost is. The wrinkle is that carpet comes off a roll of fixed width, and any room wider than the roll has to be made up of two or more strips laid side by side. Those strips are called drops, and how many you need depends on a ceiling-function calculation that produces sudden cliffs in the cost as room dimensions cross roll widths. The carpet calculator handles that in one step; this article is the long version of what it does.

We walk the drop count formula, why every fitter lays drops along the longer wall, the practical difference between 4 m and 5 m rolls in the UK and 12 ft and 15 ft rolls in the US, where the 10 % waste default actually comes from, and what is missing from the headline m² figure (underlay, gripper rod, and fitting, mostly). At the end: how to handle L-shaped rooms and alcoves, and the sense-check that catches almost every measurement mistake before the order goes in.

What a carpet calculator actually does

Three inputs, five outputs. The calculator takes the room length, room width, and roll width, plus a waste percentage and a £/m² rate. It returns the floor area, the number of drops, the drop length, the linear metres of roll to order, the carpet area billed by the retailer, and the cost. The gripper rod perimeter comes out as a sixth value — sold separately by the retailer, so worth pulling out of the same calculation.

The formulas are:

Floor area = length × width (the rectangle of actual floor, in m²)
Drops = ⌈short side ÷ roll width⌉, minimum 1
Drop length = long side (in m)
Linear metres of roll = drops × drop length × (1 + waste %)
Carpet area to order = linear metres × roll width
Gripper perimeter = 2 × (length + width)
Cost = carpet area × price per m²

The ceiling brackets ⌈ ⌉ around the drops calculation are the important bit. A room with a short side 0.1 m wider than the roll rounds up to two drops, doubling the linear metres of roll compared to a room that fits in one. That step change is why the ratio of carpet area ordered to floor area is rarely close to 1.10; for a single-drop room it might be 1.10, but for a room just over the roll width it can be 2.0 or worse.

Why drops run along the longer wall

Every fitter lays drops with the length of the drop running along the longer wall of the room. Two reasons. First, fewer seams: a room narrower than the roll has zero seams; a room one drop wider than the roll has exactly one seam running down the long axis of the room. Rotating the drops 90° to run along the short axis would give a room of the same dimensions multiple cross-room seams. Fewer seams cost less in carpet (you cross less of the room boundary with drop edges), look better, and last longer because seams are the points that fail first under traffic.

Second, pile direction. Carpet has a pile direction — the fibres lean one way, and the carpet looks slightly different colour depending on which way you look down the pile. Two drops laid with opposite pile directions show a visible colour shift across the seam, the kind of thing that vanishes when the room is empty and reappears the moment a floor lamp is switched on. Running all drops the long way keeps every drop's pile direction identical, so the colour reads as one piece of carpet. The retailer marks the pile direction on the back of the roll for exactly this reason; the calculator assumes you are following that convention, which is how it derives the drop count from the short side rather than the long side.

Worked example: a 5 m × 4 m bedroom

A 5 m × 4 m bedroom, UK 4 m roll, 10 % waste, £25/m² carpet. Plugging straight in:

Floor area = 5 × 4 = 20 m² (215 ft²)
Short side = 4 m, long side = 5 m
Drops = ⌈4 ÷ 4⌉ = 1 (the room fits in one drop — no seam)
Drop length = 5 m
Linear metres of roll = 1 × 5 × 1.10 = 5.5 m
Carpet area to order = 5.5 × 4 = 22 m²
Cost = 22 × £25 = £550 (£27.50 per floor m²)
Gripper perimeter = 2 × (5 + 4) = 18 m

Now change one dimension. A 5 m × 4.2 m bedroom — only 20 cm wider on the short side — on the same 4 m roll:

Floor area = 5 × 4.2 = 21 m²
Drops = ⌈4.2 ÷ 4⌉ = 2
Drop length = 5 m
Linear metres of roll = 2 × 5 × 1.10 = 11 m
Carpet area to order = 11 × 4 = 44 m²
Cost = 44 × £25 = £1,100 (£52.38 per floor m²)

Same retailer, almost the same room, double the carpet bill. That cliff is the case for stocking the wider 5 m roll: on the same 4.2 m short side, a 5 m roll fits in one drop, linear metres becomes 5.5 m, carpet area is 27.5 m², cost is £687.50 — saving £412.50 against the 4 m roll for the same room. Wider rolls usually carry a 5–15 % premium per m², which the saved second drop more than absorbs whenever the short side falls in the 4.0–5.0 m range. Read the calculator output for both roll widths before ordering whenever the room could plausibly go either way.

Where the 10 % waste default comes from

The 10 % waste figure is not arbitrary. It bundles three allowances that every fitter takes off a freshly delivered roll:

Trimming margin (5–6 %): the fitter cuts 5–10 cm off each drop where it meets the skirting, so the carpet lands flush rather than buckling against the wall. On a 5 m drop, that is 5–10 cm × 2 walls = 10–20 cm, or roughly 2–4 % of the drop. Across the cuts at both ends and the trimming at door thresholds, it rounds to 5–6 % of the linear metres.
Pile-direction allowance (2–3 %): if the room has an alcove or a door reveal, the offcut that fits into it has to come from a piece of carpet laid with the same pile direction. That means cutting it from the drop rather than from random leftover material, which loses a few percent.
Squaring (1–2 %): rolls do not always cut dead-square at the factory, so the fitter takes a thin sliver off the leading edge to land a clean 90° before laying the first drop.

Push to 15 % when the carpet has a visible texture or twist that you want consistent — the fitter sometimes has to discard a few more centimetres to find a clean stretch. Push to 20 % for patterned carpet (geometric, Persian-style, strong stripes) where the pattern must align across any seam. Pattern-matching forces the fitter to cut the second drop with an offset of up to a full pattern repeat — for a 60 cm geometric repeat on a 5 m drop, that is up to 12 % wasted on the seam-match drop alone. The 20 % default is a safe round figure that lands roughly in the middle of what real patterns cost in offcuts.

UK and US roll widths in practice

Four widths cover almost every domestic carpet sold in 2026:

4 m (13 ft 1 in): the UK default. Stocked by every UK retailer in twist, loop, and saxony constructions. Fits rooms up to 4.0 m on the short side in one drop.
5 m (16 ft 5 in): the UK wide-roll option. Less stock variety than 4 m but available in most retailers' mid-range and above. Fits rooms up to 5.0 m on the short side in one drop — covers almost every domestic room without seams.
12 ft (3.66 m): the US default. Stocked by all major US carpet retailers. Fits rooms up to 12 ft on the short side in one drop.
15 ft (4.57 m): the US wide-roll option. Less common than 12 ft but standard in higher-end ranges. Useful for US bedrooms with a short side between 12 and 15 ft.

Specialty 2 m and 3 m runner widths exist for hallways and stair carpet, but the calculator is built around the four roomwide standards. For runners and stair carpet, calculate the floor area separately and order to the linear metre at the runner width.

What the headline m² figure leaves out

The carpet area is carpet only — it does not include the items that go between the carpet and the subfloor or around the room edge. Three line items the retailer will quote separately:

Underlay

Foam or rubber, 9–11 mm thick, sold in 10–15 m rolls at 1.37 m width. Underlay is sized to the floor area, not the carpet area: a 5 × 4 m room needs 20 m² of underlay regardless of how the carpet is cut. UK prices in 2026 run £3–8 per m² for domestic PU foam, £8–15 per m² for rubber crumb, with similar US prices in dollars. Quality matters more than thickness — a dense 9 mm PU foam outperforms a soft 12 mm equivalent for resilience and roughly doubles the carpet's useful life over a thin or worn underlay. Always lay new underlay with new carpet unless the existing underlay is recent and shows no compression or damp.

Gripper rod

Thin strips of plywood with angled metal pins, nailed or glued around the perimeter of the room to grip the carpet edge when it is stretched into place. Sold by the linear metre in 1.2 m or 1.5 m sticks, typically £0.50–1.50 per metre. The calculator output gives the full perimeter; subtract the width of door openings (typically 76–82 cm each) where the carpet runs across a threshold rather than into a gripper. Or order the full perimeter and let the fitter cut to length on site — the offcuts are 5–10 cm long and not reusable elsewhere.

Fitting and door bars

Most UK retailers quote fitting at £4–8 per m² for a standard room, with door bars (the metal or wood transition strip at each doorway) at £8–20 each. US fitting runs $4–10 per yd² with door reducers at $10–25 each. For a 20 m² room with one doorway, fitting plus a door bar lands around £90–180. The full job — carpet, underlay, gripper, fitting, door bar — typically costs 1.4× to 1.6× the headline carpet price for a standard domestic room.

L-shapes, alcoves, and other non-rectangles

The calculator assumes a clean rectangle. For anything else, two strategies:

Decompose into rectangles. An L-shape splits into two rectangles along the inside corner. Run each through the calculator at the same roll width. If both rectangles fit within one drop on the same roll, sum the linear metres — the fitter cuts both pieces from one continuous run with one seam at the inside corner. If one rectangle needs two drops and the other one, add the linear metres directly and let the fitter find the seam. Add 5–10 % on top as a join allowance to be sure pattern and pile match across the bend.

Measure to the widest point. For alcoves under about 1 m deep — bay windows, chimney breasts, built-in wardrobes — measure the room to its widest point and let the alcove offcut fall into the standard 10 % waste. This costs a few percent more in carpet than decomposing strictly but saves the seam, and the offcut is usually large enough to lay into the alcove without piecing.

For stairs, measure each tread (run + nosing, typically 26–28 cm) and each riser (typically 18–20 cm) and add them up — a 13-stair flight is roughly 6 m linear at 0.65–0.70 m wide. Stair carpet is normally a runner width (60 cm or 70 cm), not a roomwide roll, and is priced and ordered separately from the rooms it joins.

Common mistakes

Measuring after the carpet is delivered. Always measure twice and once more before placing the order. Carpet is cut from the roll on receipt of the order, so a remeasure afterwards is a re-cut on a new piece — full price, no refund on the original.

Forgetting door thresholds. The room measurement should run to the threshold (the back of the door frame) on each door, not to the open doorway. Carpet tucks under the door threshold; the threshold is where it stops.

Running the long axis across the room. See the pile direction discussion above. If you order with the drops running short-way to save a tiny amount of carpet, expect a visible colour shift across seams under most lighting. Fitters will rotate this convention only if the room has a feature wall the customer wants the pile running toward.

Forgetting the underlay. The headline m² figure is carpet only. Add underlay to the floor area, not the carpet area. A common over-order is to buy underlay to the carpet area and end up with 10 % too much.

When to bring in a professional measure

Most major UK and US retailers offer free professional measurement on rooms over a minimum size, and the measurer's figures are usually a few percent more conservative than a DIY measure — they will add gripper margins and pile-direction offcuts the homeowner would miss. For a single rectangular room with no alcoves, the calculator is enough. For multi-room installations, L-shaped rooms with alcoves, stairs that turn, or any room over about 25 m² where a wasted drop costs hundreds of pounds, the free measure is worth the appointment slot.

Related calculators

Carpet Calculator — m², drops, roll length and cost from room dimensions
Square Footage Calculator — Area from length × width in any unit
Tile Calculator — Tiles needed and cost for a floor or wall area
Drywall Calculator — Sheets, mud and screws for a room
Brick Calculator — Bricks, waste and cost for a wall

Frequently asked questions

How do I measure a room for carpet?

Measure at floor level, skirting to skirting, along the longest wall and the wall at right angles to it. Use a steel tape, not a soft tape, and pull it tight against the skirting so you are not catching the bevelled edge. Add 5–10 cm to each dimension if you intend to tuck the carpet into a door threshold or under a transition strip. For rooms with alcoves under 1 m deep, measure to the widest point and let the alcove offcut fall into the waste allowance; for deeper alcoves or L-shapes, break the room into rectangles and run each through the calculator separately. Always re-measure once before placing the order — the £/$ cost of a re-cut drop is many times the cost of a second measurement.

Why does carpet need to be ordered in drops?

Carpet ships on rolls of a fixed width — 4 m and 5 m are the UK standards, 12 ft (3.66 m) and 15 ft (4.57 m) are the US standards. Any room wider than the roll needs two or more strips of carpet laid side by side, with a seam between each strip. Each strip is called a drop. The drop count is the short side of the room divided by the roll width, rounded up: ⌈short_side ÷ roll_width⌉, with a minimum of 1. A 3 m × 4 m room on a 4 m roll fits in one drop and has no seams; a 5 m × 4 m room on the same roll needs two drops and one seam.

What waste percentage should I use?

The default 10 % covers two things: the 5–10 cm strip the fitter trims off each drop so it lands flush against the skirting, and pile-direction allowance for matching offcuts in alcoves or door reveals. For plain, untextured carpet on a clean rectangle, 10 % is the figure UK and US retailers build into their own online tools. Push to 15 % for textured or twist carpet where you want pile direction consistent across any seams. Push to 20 % for patterned carpet (geometric, Persian-style, strong stripes) where the pattern must repeat exactly across a seam — the offcuts to land the pattern can be substantial. Drop to 5 % only on a single-drop rectangular room where you accept thinner trimming margins.

Why is the cost so much higher on a 4 m roll than a 5 m roll for some rooms?

Because the drop count rounds up. A room 4.5 m on the short side needs ⌈4.5 ÷ 4⌉ = 2 drops on a 4 m roll, but ⌈4.5 ÷ 5⌉ = 1 drop on a 5 m roll. Two drops doubles the linear metres of roll and therefore the cost. The break-even is exactly the roll width: a room with a short side equal to or less than the roll width fits in one drop. UK retailers stock both 4 m and 5 m rolls (and US retailers stock both 12 ft and 15 ft) for exactly this reason — for rooms with a short side just over 4 m, the wider roll is often 20–40 % cheaper despite the higher headline £/m² rate, because the second drop is avoided entirely.

Does the m² figure include underlay and gripper rod?

No. The carpet area is carpet only. Underlay (foam or rubber) is sold by the m² in 10–15 m rolls and is sized to the floor area, not the carpet area — a 5 × 4 m room needs 20 m² of underlay regardless of how the carpet is cut. Gripper rod is sold by the linear metre in 1.2 m or 1.5 m sticks and runs round the room perimeter; the calculator gives the perimeter figure so you can divide by stick length. Retailers quote underlay and gripper separately, plus fitting; together they typically add 30–50 % to the carpet price alone for a fully fitted job.

Can I lay carpet over an existing carpet or old underlay?

Not over an existing carpet — the pile compresses unevenly, the layers move relative to each other under foot traffic, and seams pop. Over old underlay it depends: if the foam is still resilient, has no damp patches, and shows even compression, a fitter may agree to reuse it; if any of those fail, lift and replace. Underlay is cheap relative to the carpet that sits on it (£3–8 per m² versus £20–60 per m²) and replacing it gives the new carpet the right surface to glue, stretch, and feel correct underfoot. Carpet over underlay over a clean, level subfloor (boards or screed) is the standard build-up; anything else is a compromise.

How long does fitted carpet last?

Domestic-grade twist or loop carpet on a busy hallway: 5–8 years before it looks tired. The same carpet in a low-traffic bedroom: 12–15 years. Wool blends and heavier polypropylene constructions sit at the top of those ranges; thin polyester carpets at the bottom. The single biggest determinant is the underlay — a quality 9–11 mm PU foam underlay protects the pile from compression and roughly doubles useful life over a thin or worn underlay. Other factors: pile direction (fitters lay it running away from the main light source so footprints show less), pile height (deep piles look luxurious but flatten faster in traffic lanes), and whether shoes come off at the door.

How much does carpet cost per square metre?

In the UK in 2026, budget polypropylene twist carpet runs £8–15 per m², mid-range polypropylene or polyester twist £15–30 per m², wool-blend twist £30–60 per m², and pure wool or specialty patterned carpet £60–150+ per m². US prices are broadly comparable in dollars. These figures are carpet only — add 30–50 % for underlay, gripper rod, door bars, and fitting on a typical retailer-fitted job. The single biggest swing in finished cost is roll width on rooms in the 4.0–5.0 m range, because of the drop-count step covered above.

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Informational only. Not personalised financial, legal, or tax advice.