How does the carpet calculator decide how much to buy?

Carpet is sold off a roll of fixed width, so you cannot buy an exact rectangle — you buy a length of the full roll width. The tool tries two orientations: running the roll length along the room length, and running it across the width with pieces joined. Each needs a whole number of full-width strips, so it rounds up, multiplies by the roll width to get the area bought, and keeps whichever orientation buys less carpet.

Why is there always waste with carpet?

Because the roll width rarely matches your room. A 3.6 m wide room cut from a 4 m roll leaves a 0.4 m strip of offcut down one side that you have paid for but may not use. That unavoidable offcut, plus a waste percentage for trimming, door reveals and pile matching, is why the area you buy is larger than the floor area.

What roll width should I choose?

Read it from the carpet you are buying. Broadloom is most commonly sold in 4 m and 5 m widths in the UK and Europe, and in 12 ft (and sometimes 13.5 ft or 15 ft) widths in the US. The quick-pick buttons cover the usual sizes; a width that just exceeds your room dimension wastes the least.

Does the pile direction matter?

Yes. Carpet pile lies in one direction and should run the same way across a room and down a flight of stairs, or joins and patches will look different in the light. Forcing a single pile direction is one reason a particular orientation can cost more, which the with-roll and across-roll comparison in the results makes visible.

How does the stair option work?

Each step needs carpet for the horizontal tread (the going) plus the vertical riser, so the runner length per step is tread plus rise. The tool multiplies that by the number of steps to get the linear run, then by the roll width for the area, and adds it to the room order. Add a little extra waste if your stairs turn or have a landing.

Should I include fitting and underlay in the price?

The optional fitting and underlay field is charged per square metre of room floor area, separate from the carpet itself which is charged on the area bought. Quotes vary by region and by whether you need gripper rods, door bars or old carpet uplift, so treat the total as a planning estimate and confirm with your fitter.

Carpet Calculator — Gera Tools

A carpet calculator that works out how much carpet to buy — and roughly what it will cost — for a room and, optionally, a staircase. Unlike a tile or paint sum, carpet is not bought by the exact square metre: it comes as broadloom, long rolls of a fixed width (most often 4 m or 5 m). You buy a length off that roll, so the real question is how to lay your room out across the roll width with the least offcut waste. This tool does that layout for you in metric or imperial, compares two cutting orientations, adds stairs, and turns the result into square metres, square yards and a cost.

How it works

The room needs to be covered by full-width strips of carpet. There are two ways to orient them, and the tool computes both:

Length along the roll — strips run the full length of the room. It works out how many strips (each the roll width) are needed to span the room width, rounds that up to a whole number, and multiplies by the room length.
Width along the roll — strips run across the room and are joined to make up the length. Same idea with the dimensions swapped.

strips        = ceil( room dimension / roll width )
length pulled = strips × the other room dimension
area bought   = roll width × length pulled

Each orientation gives a different area bought, because the leftover offcut down the wider edge differs. The calculator keeps the cheaper of the two, adds any stair run, applies your waste percentage, then prices the result per square metre or square yard. Fitting and underlay, if entered, are charged on the room floor area rather than the area bought.

Worked example

A bedroom 5 m long by 3.6 m wide, carpeted from a 4 m roll with 10% waste at £22/m²:

Length along the roll: the 3.6 m width needs ceil(3.6 / 4) = 1 full-width strip, each 5 m long, so you pull 5 m of 4 m roll = 20 m² bought.
Width along the roll: the 5 m length needs ceil(5 / 4) = 2 strips, each 3.6 m, so 7.2 m of roll = 28.8 m² — clearly worse.
The tool keeps the first orientation: 20 m², of which 18 m² is floor and 2 m² is offcut waste. Adding 10% gives 22 m² to buy.

At £22/m² that is an estimated £484 of carpet, before any fitting and underlay. Switch on a 13-step staircase with a 25 cm tread and 18 cm riser and the tool adds 13 × 0.43 m = 5.59 m of runner, about 22 m² more carpet off the same roll.

Reference — roll widths and broadloom

In the UK and Europe carpet broadloom is overwhelmingly sold in 4 m and 5 m widths; the US uses 12 ft as standard, with 13.5 ft and 15 ft available in some ranges. Because a roll is continuous in length, the trade buys linear metres of a known width — exactly what the length pulled figure above shows. A square yard is 0.8361 m², which is why older stock or US pricing per square yard reads cheaper than the same carpet priced per square metre; the tool converts so you compare like with like. Always treat the result as a minimum: buy from one batch for a consistent dye lot, keep pile running one way, and hold the offcut back for future repairs.

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