Carpet Calculator – How Much Carpet Do I Need?
Enter your room size and we'll work out the best way to cut it from a 4m or 5m roll, plus underlay, gripper and door bars — with a cost estimate.
Carpet Calculator
How it cuts from the roll
You need
4 × 4.6m to buy
18.4 m² off the roll for a 16.2 m² room
- Carpet area to buy
- 18.4 m²
- Underlay
- 2 rolls
- Gripper lengths
- 11
How we worked this out
Your shopping list
19 × m² of carpet (4m roll × 4.6m)
Room is 16.2 m² — you buy the full roll width.
£342
2 × Rolls of underlay
17.01 m² needed at 15 m² per roll.
£210
11 × 1.52m gripper rod lengths
15.37m of perimeter, less doorways.
£13.20
1 × Door bars / threshold strips
One per doorway, matched to the flooring next door.
£12.00
Carpet to buy off a 4m roll (100mm trimming allowance)
| Room size | Drops | Roll length | Carpet bought | Room area |
|---|---|---|---|---|
| 3.0 × 2.5 m | 1 | 3.1 m | 12.4 m² | 7.5 m² |
| 3.6 × 3.0 m | 1 | 3.7 m | 14.8 m² | 10.8 m² |
| 4.5 × 3.6 m | 1 | 4.6 m | 18.4 m² | 16.2 m² |
| 5.0 × 4.2 m | 2 | 10.2 m | 40.8 m² | 21.0 m² |
| 6.0 × 3.8 m | 1 | 6.1 m | 24.4 m² | 22.8 m² |
How this calculator works
Carpet is not sold by the square metre of floor — it is cut across a 4m or 5m roll, so you pay for the full width however narrow the room. The trick is choosing which way round to lay it. We test both orientations and pick whichever buys the least carpet.
Before doing that we add 100mm to each room dimension for trimming, because no room is perfectly square and a fitter needs something to cut back. If the room is wider than the roll, a second drop is needed and there will be a seam — we flag that clearly.
Underlay is measured as the actual floor area plus 5%, and comes in rolls of about 15m². Gripper rod runs round the perimeter minus the doorways, in 1.52m lengths, and each doorway gets a threshold bar.
Worked example: a 4.5m × 3.6m living room
With the trimming allowance the room is 4.6m × 3.7m. Laid one way, 3.7m fits inside a 4m roll in a single drop, so you buy 4m × 4.6m = 18.4m². Turned 90 degrees, 4.6m needs two drops of 3.7m — 7.4m of roll, or 29.6m². The first option saves 11m² of carpet, around £200 at £18 per m².
Add 17m² of underlay (two rolls), 11 gripper lengths and one door bar, and the whole room comes to roughly £470 in materials. Fitting is usually charged separately at £4–£6 per m², plus uplift and disposal of the old carpet.
Getting the measurements right
Measure at the widest points, including into doorways and any alcoves or chimney breast returns — the fitter cuts the shape out of a rectangle, so it is the widest dimension that matters, not the average. For a bay window, measure to the furthest point of the bay.
Take the measurement in two or three places across the room. Older UK houses are regularly 50mm wider at one end than the other, and you want the biggest number. Write your measurements down as length × width and keep the same order throughout so you do not accidentally rotate the room.
Pile direction matters on a seamed job and on stairs. Carpet laid with the pile leaning down the flight wears better and looks the right shade from below, which is why fitters usually run stair carpet in one continuous piece rather than in treads.
Tips from the trade counter
- Spend on the underlay. An 11mm PU underlay under a mid-range carpet feels better than an expensive carpet on thin foam.
- Let the fitter measure before you order. Most retailers measure free, and they carry the risk if it is short.
- Keep offcuts. Larger pieces make door mats or repair patches if something gets burned or stained.
- Check the doors clear. New carpet and underlay can add 15–20mm; an internal door often needs a few millimetres off the bottom.
- Order the same batch. Dye lots vary — a top-up strip from a later batch rarely matches.
Fitting, fire and rented homes
Carpet fitting itself is not covered by Building Regulations, but if you are laying carpet in a rented property or an HMO, the flooring and any underlay in escape routes may need to meet fire performance requirements. Landlords should check the specification before ordering — and check with your local council where an HMO licence applies.
Frequently asked questions
What width does carpet come in?
UK carpet is sold off 4m and 5m wide rolls. A 4m roll covers most bedrooms in one piece; 5m suits wider living rooms and landings.
How much extra carpet should I allow?
Add 100mm to each room dimension for trimming, and more if the room is out of square or has a bay. Fitters will nearly always ask for that little extra.
Do I pay for the full roll width?
Yes. Carpet is cut across the roll, so you buy the full 4m or 5m width by the length you need. A 4.5m × 3.6m room off a 4m roll means buying 4m × 4.6m, even though the room is 16.2m².
Do I need new underlay?
Almost always. Good underlay makes cheap carpet feel expensive, adds insulation and doubles the life of the pile. Reusing flattened old underlay is a false economy.
How many gripper rods do I need?
Gripper comes in 1.52m lengths and runs round the whole perimeter except across doorways. Divide the perimeter (less door widths) by 1.52 and round up.
How much carpet do stairs need?
Allow about 0.5m of carpet length per step for a standard tread and riser, so a typical 13-step UK flight needs around 6.5m off the roll. Winders and a landing need more.
Can I avoid a seam?
Only if the room fits within the roll width. If a seam is unavoidable, put it out of the main line of sight and away from doorways, and keep the pile running the same direction on both pieces.