How much carpet you need to buy
Carpet is sold from a roll of fixed width, so buying it is different from simply measuring the floor. You pay for the full width of the roll for every foot you cut off, and you need extra for trimming and fitting. This calculator takes your room length and width, a waste allowance, the roll width and your price per square yard, and returns the room area, the carpet to buy, the length to order from the roll, the number of seams and the estimated cost.
The calculations
- Room area = length × width, in square feet. Divide by 9 for square yards.
- Waste-adjusted area = room area × (1 + waste %).
- Roll cut. The carpet can run along the length or the width of the room. For each direction, strips = the room dimension across the roll divided by the roll width, rounded up, and roll length = strips × the other dimension. The calculator picks the direction that uses the least roll length.
- Carpet to buy = the larger of the waste-adjusted area and the area of the roll cut (roll length × roll width). Roll length to order = carpet to buy / roll width.
- Seams = strips − 1 in the direction chosen. Cost = carpet to buy in square yards × price per square yard.
Worked example
A 15 ft by 12 ft room with 10% waste, a 12 ft roll and $24 per square yard, which are the default inputs. Room area = 15 × 12 = 180 sq ft, or 20 sq yd.
With waste, 180 × 1.10 = 198 sq ft. Running the carpet along the 15 ft length needs ceil(12 / 12) = 1 strip of 15 ft, so 15 linear feet of a 12 ft roll (180 sq ft). Running it the other way would need 2 strips of 12 ft, which is worse. The roll cut (180 sq ft) is below the waste-adjusted area (198 sq ft), so you buy 198 sq ft = 22 sq yd, ordering 198 / 12 = 16.5 linear feet, with no seam. The cost is 22 × $24 = $528.
Tips and common mistakes
- Measure the widest points. For alcoves, closets and doorways, add them to the length and width or calculate them as separate rectangles and add the results.
- Mixing square feet and square yards. Prices per square yard need the area divided by 9. Using square feet by mistake will overstate the cost ninefold.
- Ignoring roll width. A room 13 ft wide on a 12 ft roll needs a second strip, which adds a seam and a lot of wasted carpet. Compare a different roll width if your supplier offers one.
- Too little waste for patterns. Patterned carpet must be matched across seams, which needs extra length. Use a higher waste percentage and ask the installer.
- Forgetting underlay, gripper and labour. The cost shown is for carpet only.
Treat the result as a planning estimate. An installer will measure the room and plan seam positions and pile direction, which can change the final quantity.