Carpet Calculator
Works out carpet off a roll, including the waste the roll width forces. Also compares the two laying directions and tells you which costs less.
Length off the roll at the full roll width.
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The formula
buy length = room dimension along the roll × (1 + trim) + doorway allowance area charged = buy length × roll width
Carpet comes off a roll of fixed width, so you pay for the full width however narrow the room is. A 3.2 m room off a 4 m roll wastes 0.8 m down the entire length — which is why running the longer dimension along the roll is usually, but not always, cheaper.
What to watch out for
Carpet is the one floor covering not bought by area:
- You pay for the full roll width. A 3.2 m wide room off a 4 m roll is charged for 4 m, and the spare 0.8 m runs the whole length.
- Direction changes the cost. Which room dimension runs along the roll decides how much is wasted, and the difference is often several square metres.
- Pile direction matters too. Carpet looks different viewed with or against the pile, so the fitting direction is partly an appearance decision, not purely a cost one.
- Rooms wider than the roll need seams. Place them away from the main window and never across a doorway, where they wear fastest.
Frequently Asked Questions
How much carpet do I need?
Work in roll lengths, not area. A 5 × 3.2 m room off a 4 m roll needs about 5.5 m of carpet — 22 m² charged for a 16 m² room, because the roll is wider than the room.
Why am I charged for more carpet than my room?
Because carpet is sold at a fixed roll width. Any room narrower than the roll leaves a strip down the full length, and that strip is charged for whether you use it or not.
Which way should carpet be laid?
Usually with the longer room dimension along the roll, which wastes least. Pile direction also matters for appearance — in a corridor the pile normally runs along it, and in a room away from the main window.