Carpet Calculator
Enter your room dimensions and pick a carpet roll width — the calculator returns the number of drops, linear metres of roll, m² of carpet to order, gripper rod perimeter, and total cost. Works for UK 4 m / 5 m rolls and US 12 ft / 15 ft rolls.
Carpet to order (m²)
22
- Floor area (m²)
- 20
- Floor area (ft²)
- 215.28
- Drops needed
- 1
- Drop length (m)
- 5
- Linear metres of roll
- 5.5
- Gripper rod perimeter (m)
- 18
- Total carpet cost
- £550.00
- Cost per floor m²
- £27.50
Drops = ⌈short room side ÷ roll width⌉ (always at least 1). Each drop runs the length of the longer side so seams (if any) follow the longer dimension. Linear metres of roll = drops × drop length × (1 + waste %), and carpet area ordered = linear metres × roll width. Default 10 % waste covers trimming and pile-direction allowance; raise to 15–20 % for patterned carpet.
How to use this calculator
Measure the room's longest wall and the wall at right angles to it, both at floor level (skirting to skirting). Use metres — if you have feet, multiply by 0.3048. Pick the roll width that matches what your retailer stocks: 4 m is the UK default, 5 m is the UK wide-roll option (preferred for rooms over 4 m on the short side), 12 ft (3.66 m) is US standard, 15 ft (4.57 m) is US wide. Set a waste allowance — 10 % is the typical default for plain carpet (covers the 5–10 cm trimming margin the fitter cuts off each drop and a bit of pile-direction matching); bump to 15–20 % for patterned carpet where the pattern must match across seams. Enter the price per m² of the carpet itself (underlay, gripper rod, and fitting are usually quoted separately by the retailer). The result panel shows the m² to order, drops needed, linear metres of roll, and the cost.
How the calculation works
Carpet ships on rolls of a fixed width, so for a room wider than the roll you need multiple "drops" — strips of carpet laid side-by-side with seams between them. The standard calculation lays drops along the longer side of the room so seams run the long way (visually less obvious and fewer seams overall). The number of drops is the short room side divided by roll width, rounded up; one drop is the minimum even when the room is much narrower than the roll. Each drop runs the full length of the long side, plus the waste allowance for trimming. Linear metres of roll = drops × drop length × (1 + waste %); the m² of carpet ordered = linear metres × roll width. Gripper rod runs round the room perimeter — 2 × (length + width) — though the fitter normally subtracts the width of any door openings on site.
Worked example
A 5 m × 4 m bedroom, UK 4 m roll, 10 % waste, £25 per m². Short side = 4 m, long side = 5 m. Drops = ⌈4 ÷ 4⌉ = 1 (the room fits one drop of carpet — no seams). Drop length = 5 m. Linear metres of roll = 1 × 5 × 1.10 = 5.5 m. Carpet area to order = 5.5 × 4 = 22 m². Total cost = 22 × £25 = £550, or £27.50 per square metre of floor (the difference is the trimming waste). Gripper rod perimeter = 2 × (5 + 4) = 18 m. For comparison, a 5 m × 5 m room on the same 4 m roll would need 2 drops × 5 m × 1.10 = 11 m of linear roll = 44 m² of carpet (£1,100) — switching to a 5 m roll cuts that to 1 drop × 5.5 m = 27.5 m² (£687.50), saving £412.50.
Frequently asked questions
How much carpet do I need for a 12 × 12 ft room?
A 12 × 12 ft room is 3.66 m × 3.66 m = 13.4 m² (144 ft²). On a US 12 ft (3.66 m) roll it fits in one drop of exactly 3.66 m, so with 10 % waste you order ~4 m of linear roll = ~14.7 m² (158 ft²) — about £/$ 370 at £25/m². On a UK 4 m roll, the short side (3.66 m) still fits in one drop, drop length is 3.66 m, linear metres = 3.66 × 1.10 = 4.03 m, carpet area = 16.1 m² (173 ft²) — slightly more waste because the roll is wider than the room is short.
Why does the calculator use the longer side as the drop length?
Two reasons. First, fewer seams: laying drops along the long axis means at most one seam down the middle of the room rather than multiple cross-room seams. Second, pile direction: carpet has a pile direction (the fibres lean one way) and changing pile direction across a seam shows up as a colour shift under most lighting. By running all drops the long way, the pile stays consistent. The fitter will rotate this for you on site if a feature wall or window means the pile should run differently — but the m² ordered is the same either way.
What waste % should I use?
For plain, untextured carpet on a rectangular room, 10 % is the figure most UK and US retailers build into their online calculators. It covers the 5–10 cm trim each drop needs to land flush against the skirting, plus pile-direction allowance. Bump to 15 % if the carpet has a visible texture or twist that you want consistent. Go to 20 % for patterned carpet (geometric, Persian-style, or strong stripes) where the pattern has to repeat exactly across any seam — the offcuts to land the pattern can be substantial. Pull down to 5 % only if the room is a clean rectangle that fits one drop with room to spare and you accept slightly less trimming margin.
Does the m² figure include underlay and gripper rod?
No — the headline figure is carpet only. Underlay (foam or rubber, sold by the m² in 10–15 m rolls) needs the floor area, not the carpet area: a 5 × 4 m room needs 20 m² of underlay regardless of roll width. Gripper rod is sold by the linear metre (usually in 1.2 m or 1.5 m sticks) and runs round the perimeter — the calculator shows the perimeter figure (2 × length + 2 × width) so you can divide it by the gripper stick length to get the count. Retailers quote underlay and gripper rod separately, plus fitting; together they typically add 30–50 % to the carpet price alone.
What if my room is L-shaped or has alcoves?
Break the room into rectangles and run each one through the calculator. For an L-shape with the carpet running in one direction, the longer leg sets the drop length and you can usually cut the shorter leg from the same drop's offcut — but only if the offcut is wide enough. The safest method is to run each rectangle separately on the same roll width, sum the linear metres, then add an extra 5–10 % "join" allowance on top to be sure pattern and pile match across the bend. For alcoves under 1 m deep, just measure the room to the widest point and let the alcove offcut fall where it will; the existing waste % covers it.
Why does the cost change so much between a 4 m and 5 m roll?
Because of how drops round up. A room with a short side of 4.5 m on a 4 m roll needs ⌈4.5/4⌉ = 2 drops; on a 5 m roll it needs ⌈4.5/5⌉ = 1 drop. Two drops doubles the linear metres (and therefore the carpet area and cost) compared to one. The break-even is the roll width: any room with a short side ≤ roll width fits in one drop. UK retailers stock both 4 m and 5 m rolls for exactly this reason — for rooms 4.0–5.0 m on the short side, the wider roll is usually 20–40 % cheaper despite the higher price per m² because you avoid the second drop entirely.