Carpet Calculator

Carpet Calculator: How Much Carpet Do You Need?

Carpet is the one floor covering you do not buy by the box. It is cut from a wide roll in the shop, so the width of that roll — not just the size of your room — decides how much you need, where the joins fall and how much is left over. This carpet calculator does the roll maths for you: it works out the room area, the carpet needed with waste in square feet and square yards (or square metres), the length to buy off the roll, and the number of runs and seams. If your floor is a hard surface instead, use the Flooring Calculator for laminate, vinyl and tile.

Measure the room at its longest and widest points, skirting board to skirting board, and allow for carpet running into doorways and alcoves. Enter the length and width below, choose the roll width your shop stocks, and set a waste percentage. The results update as you type, so you can compare a 12 ft roll with a 15 ft roll before you commit to either.

Roll widths: why the roll decides the seams

Most carpet in the United States is woven 12 ft wide, and that single fact explains a lot about carpet pricing and fitting. A room no wider than the roll can often be covered in one run, with no seam at all. Some ranges are also made 15 ft wide, which suits large living rooms and open-plan spaces: the wider roll can remove a seam from the middle of the floor, where everyone walks and the light falls.

In the UK and much of Europe the standard roll is 4 m wide — about 13 ft 2 in — with 3.66 m (12 ft) rolls also common. Whichever system you buy in, the rule is the same: if the room is wider than the roll, the carpet arrives in two or more runs side by side, and every join between runs is a seam. Seams are not a fault, but they need planning, because a seam in a doorway or in front of a sofa gets far more wear than one tucked along a wall.

Pile direction, and why offcuts do not patch in

Every run of carpet must be laid running the same way. Carpet pile leans in one direction, a little like the nap on velvet, and two pieces laid opposite ways reflect the light differently, so the join shows as a clear change of shade even when both pieces came off the same roll. Patterned carpet is stricter still: the pattern has to match across the seam, which pushes the cutting plan — and the waste — up further.

That is also why you cannot save money by patching a second run together from offcuts. A strip left over from the first cut may be the right size, but turned round to fit, it will look like a different carpet. A good fitter plans the runs on paper first, keeps the pile running one way, and orders one continuous length for each run.

How much waste to allow

For a plain carpet in a simple rectangular room, a 10% waste allowance is the standard starting point, and it is the default in the calculator above. It covers trimming to the walls, cutting around the hearth and door frames, and the small strip lost where runs are joined and the edges are trimmed back to sound pile.

Raise the allowance to 15–20% in three common cases. Patterned carpet needs the extra for pattern match, because each run may have to start further along the roll so the design lines up across the seam. Stairs need it because every tread and riser is cut separately and the pile must keep running down the flight. Hallways and odd-shaped rooms need it because their long, turning shapes waste far more at the edges than a plain rectangle does. If in doubt, round up: leftover carpet is far cheaper than a second delivery and a mismatched dye lot.

A worked example

Take a bedroom 12 ft wide by 15 ft long, carpeted from a 12 ft roll at 10% waste. The room area is 180 sq ft. Adding waste gives 198.00 sq ft to buy, which is 22.00 sq yd, and dividing by the 12 ft roll width gives 16.5 ft to buy off the roll — one run the full width of the room, so no seams at all. Widen the room to 15 ft by 20 ft and the same sums give a 300 sq ft floor, 330.00 sq ft to buy (36.67 sq yd) and 27.5 ft off the roll, in two runs with one seam.

Buying notes: square yards, underlay and fitting

Carpet is priced per square foot or per square yard in the US — remember that 1 sq yd = 9 sq ft, so the price per square yard should look roughly nine times the price per square foot for the same carpet. In the UK it is priced per square metre. When you compare quotes, convert them all to the same unit first, and check whether each quote is for the carpet alone or a fitted total.

Two costs sit on top of the carpet itself. Underlay (called padding in the US) is sold separately and typically runs about $0.30–0.60 per sq ft; good underlay makes a carpet feel softer and last longer, so it is rarely the place to save. Fitting is also extra, and gripper rods, door trims and delivery may be added on top. Keep the carpet, underlay and fitting as separate lines in your budget and you will not be surprised at the till.

What to calculate next

Carpet is only one floor in the house. These tools cover the jobs that usually come with it:

• Flooring Calculator — boxes of laminate, vinyl or hardwood for hard floors, waste included.
• Square Footage Calculator — the area of rooms and other shapes, in square feet or square metres.
• Cost per Square Foot Calculator — compare quotes and materials at a fair per-foot rate.

Frequently asked questions

Estimates only — have the carpet shop confirm the cut plan against your room before ordering.