Carpet Calculator
Work out how much carpet to buy from the roll for a room — the linear length, the area bought and the waste — from the room size and roll width.
How much carpet off the roll, and how much is waste.
How the carpet calculator works
Carpet comes on rolls of fixed width — 3.66 m (12 ft) is standard, 4 m and 5 m also exist. The room is covered by strips running along its longer side; the number of strips is the shorter side divided by the roll width, rounded up, and the length to buy is strips × the longer side. Because you pay for the full roll width, the area bought is usually more than the room area; that difference is the waste.
Rooms wider than one roll need a seam; a fitter will place it away from the doorway and the main light.
Formula: strips = ⌈shorter side ÷ roll width⌉ · length to buy = strips × longer side
Worked examples
| Inputs | Carpet to buy (linear) | Note |
|---|---|---|
| 5 × 4 m room, 3.66 m roll | 10 | 2 strips → 10 linear m, 45% waste |
| 5 × 3.5 m room, 3.66 m roll | 5 | fits in one width: 5 m |
| 16 × 13 ft room, 12 ft roll | 32 | 2 strips → 32 linear ft |
FAQFrequently asked questions
Why is the waste so high?
Because the roll is a fixed width: a 4 m wide room on a 3.66 m roll needs a second strip, of which only 34 cm is used. Fitters sometimes cut the second strip into pieces to reduce it.
Which way should the carpet run?
Strips run along the longer side so seams are as few and as short as possible, and pile direction should be consistent across seams.
Does the calculator include underlay?
No — underlay is cheaper and sold by area; use the room area row plus 5%.
What roll widths exist?
3.66 m (12 ft) is most common; 4 m and 5 m rolls are made for some ranges. A wider roll can remove a seam and cut the waste.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Brick Development Association — brick sizes and quantities — 215 × 102.5 × 65 mm, 60 bricks per m²
- UK Government — Approved Documents (Building Regulations) — Building Regulations guidance
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.