Carpet calculator

Carpet is not bought by the square foot

Whatever the price tag says, carpet comes off a roll of fixed width, cut into lengths called drops. A room narrower than the roll takes one drop; a wider one takes two side by side, with a seam between them. So the question is not “how many square feet” but “how many drops, how long, and which way round” — and the answer is usually well over the floor area.

This page lays every room both ways and keeps the one that buys less carpet, with fewer seams breaking a tie. Each worked example below uses the default trimming allowance of 3 inches at every wall.

The second row is where the usual shortcut goes wrong. Floor area plus 10% says 277 sq ft; the roll needs 348, because carpet is cut in whole roll widths: the second drop has to cover only 6 ft 6 in of the room, and the other 5 ft 6 in of its 12 ft — nearly half — has nowhere to go.

A room as wide as the roll

The last two rows are the same room. It is exactly 12 ft wide, which looks like a perfect fit for a 12 ft roll — until you allow for trimming. The carpet installation standard asks for “at least 3 inches along each wall for trimming”, because walls are rarely straight. On a 12 ft roll that leaves no trim at the side walls, so the room takes two drops and a seam.

On a 13.5 ft roll the same room takes one drop, no seam, and less carpet. A room that is just about a roll wide is where a wider roll pays for itself. Set the allowance to zero if your fitter tells you otherwise; it is only a default.

It is not only the edge cases. A 14 × 10 ft room on a 12 ft roll is one drop of 14 ft 6 in — 174 sq ft. The same room on a 15 ft roll, laid across instead, is one drop of 10 ft 6 in — 157.5 sq ft, about a tenth less carpet. So the page tries every roll width a shop stocks against your rooms and, when another one buys less, says so with a button to switch to it.

Which way the pile runs

Carpet pile leans, and it catches the light differently from each direction. The installation standard asks that adjacent pieces of the same carpet run the same way. Every drop in a room here comes off the roll the same way round, so that holds within each room automatically.

Between rooms it is your call. Tick run every room the same way and the page lays every room in the same direction — whichever buys less carpet overall — so the pile runs straight through the doorways. It assumes you measured each room’s length the same way round, and it usually costs a little more carpet.

The same standard asks for seams to be kept to a minimum, to run the length of the room and with the main traffic, not across the light and not perpendicular to a doorway. A calculator cannot see your windows or your traffic, so the room-by-room plan shows which way each room is laid and where its seams are, for you to check against the room.

Square feet, square yards and square metres

The big US stores tag carpet per square foot; carpet shops quote per square yard; British shops price per square metre. A square yard is nine square feet, so a square-yard price typed into a square-foot field makes the bill nine times too big. The price unit on this page is yours to choose and is always shown next to the price.

Even the stores cross the units. One large retailer’s page for a 12 ft carpet shows “$1.00 /sq. ft.” next to “($12.00 /square yard)” — but a dollar a square foot is nine dollars a square yard. The twelve dollars is one foot cut off a 12 ft roll.

What this version does not do yet

• Fill gaps from offcuts. The offcut is real carpet, and a fitter will often use it for a closet or a doorway. Doing that properly means keeping the pile direction, which is what makes it a planning problem rather than arithmetic — so the offcut is shown here, not spent.
• Stairs, which are measured and wasted differently.
• Patterned carpet, which needs extra length on every drop to match the pattern at the seams.