Carpet Calculator - Work Out Carpet and Underlay for a Room (UK)
Give us your room size and the roll width, and we'll work out the carpet and underlay you need, allowing for the fixed widths carpet comes in.
Interested? Then → Quotato for Flooring Fitters · More advice on the Blog & Chips guides.
How to use this calculator
Enter your room length and width in metres, then choose the roll width your carpet comes in (usually 4m or 5m). Because carpet is sold in fixed-width rolls, we work out the length you buy off the roll, plus matching underlay.
How we calculate this
- Carpet run (m) = room length × 1.1 (10% for trimming/matching), rounded up to 0.1m.
- Carpet area (m²) = run × roll width - you pay for the full roll width even if your room is narrower.
- Underlay (m²) = room length × width × 1.05.
- If room width exceeds the roll width, the carpet won't fit in one piece and a seam is needed.
Worked example
A 4m × 3m room with a 4m roll: run is 4 × 1.1 = 4.4m × 4m roll = 17.6 m² of carpet. Underlay is 4 × 3 × 1.05 = 12.6 m². The room (3m) is under the 4m roll, so no seam.
UK notes & gotchas
- UK carpet is sold in 'broadloom' rolls, typically 4m or 5m wide, so you almost always pay for some offcut.
- If your room is wider than the roll, a seam is unavoidable - plan it for a low-traffic spot.
- Stairs, bays and thresholds add extra; always have a fitter measure before ordering.
FAQ
Why do I pay for the full roll width?
Carpet is made in fixed-width rolls, so any width narrower than the roll leaves an offcut you still pay for.
When do I need a seam?
If your room is wider than the roll width (4m or 5m), the carpet can't cover it in one piece. We flag this in the results.
Do I need underlay too?
For most carpets, yes - it adds comfort and life. We calculate it from room area plus 5%.
Which roll width should I choose?
Pick the width the carpet you like is sold in. A 5m roll can reduce seams in wider rooms.