Need to calculate carpet needed? Carpet Calculator answers in seconds. Just enter Room Length (m) and Room Width (m); Carpet updates the result as you type. The answer from Carpet arrives with its working attached. Carpet shines when speed matters. No account is needed for Carpet. The browser does the arithmetic. Carpet is part of the Construction collection on CalcProMaster, alongside free carpet calculator, free online carpet calculator and more. Carpet shows formula, assumptions and result together. Carpet answers the next Construction question you meet.
What the defaults give
A room 5 m by 4 m is 20 m², and with a 10% waste allowance you need about 22 m² of carpet.
Carpet is sold by the square metre but cut from fixed-width rolls, so the waste figure is not padding — it is what the roll width forces you to buy.
Carpet area and the waste allowance
Ordered area = Room area × (1 + Waste %)
A 12 × 12 ft room is 144 sq ft; at 10% waste you order 144 × 1.10 = 158.4 sq ft.
| Room size | Area | +10% waste |
|---|---|---|
| 10 × 14 ft | 140 sq ft | 154.0 sq ft |
| 12 × 12 ft | 144 sq ft | 158.4 sq ft |
| 15 × 15 ft | 225 sq ft | 247.5 sq ft |
| 20 × 15 ft | 300 sq ft | 330.0 sq ft |
Waste covers seams, doorways and pattern matching — stairs need noticeably more. Related: room area guide and construction guides.
The arithmetic
Area = length × width. 5 × 4 = 20 m². Add 10%: 20 × 1.10 = 22 m².
In imperial, 5 m is 16.4 ft and 4 m is 13.1 ft, giving 215 sq ft — about 10.76 sq ft per square metre.
Worked example
Room 5 m × 4 m
Area = 20 m²
Waste 10% = 2 m²
Order ≈ 22 m²
Roll width decides what you actually buy
Broadloom comes in fixed widths — commonly 4 m or 5 m in metric markets, 12 ft or 15 ft in imperial. A 4 m wide roll covers this room in one piece with no join, which is the ideal. A 3.66 m (12 ft) roll does not, so the fitter either joins two pieces or runs the carpet the other way, and either choice changes the quantity.
That is why the area can be 20 m² and the order 26 m²: the second figure is what the roll geometry allows.
What to add beyond area
- Underlay — measured on the same area, usually sold in rolls that also create offcuts.
- Gripper and threshold bars — priced by the linear metre around the perimeter.
- Stairs — measured by tread and riser, and often requiring a wider roll than the floor.
- Pattern repeat — a patterned carpet consumes extra to match the design across joins.
Ordering
Have the fitter confirm the quantity from a roll plan rather than from the area alone. Buying to the square metre and discovering the roll width does not fit is the most common way a carpet order goes wrong.
Frequently Asked Questions
Why 22 m² for a 20 m² room?
Because 10% is allowed for trimming, doorways and roll-width waste. 20 × 1.10 = 22 m², and carpet is bought in whole units.
How much waste should I allow?
10% for a plain carpet in a rectangular room. Increase it for patterned carpet, rooms with many corners, or where the roll width forces a join.
Does roll width change the quantity?
Yes, sometimes substantially. A room wider than the roll needs two pieces, and the offcut is paid for — which is why a 20 m² room can require 26 m² of carpet.
What is 20 m² in square feet?
About 215 sq ft, using 10.76 sq ft per square metre. With waste, roughly 237 sq ft.
Do I measure under furniture?
Measure the whole floor and let the fitter decide, but note fixed units such as kitchen cabinets — carpet is not usually laid under them.
How many square metres is a 12 ft by 10 ft room?
About 11.15 m², since 3.66 × 3.05 = 11.15. With 10% waste you would order roughly 12.3 m² — and check the roll width, because 3.66 m of carpet across a 3.05 m room leaves usable offcuts.
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