How to Calculate Carpet Cost — Step by Step

The short answer

To work out what carpeting a room costs, measure the room, multiply length by width toget the area, add a waste allowance of 10 percent, and price the carpet, the padding andthe fitting on that billable area. For a 12 by 10 foot room that gives 120 square feetof room and 132 square feet bought, which comes to $871.20 in total.

The three numbers that matter

The first number is the room area, which is length times width. A room 12 feet long and10 feet wide is 120 square feet. The second number is the billable area, which is theroom area plus waste, and waste is the part most people forget. The carpet roll is 12feet wide, the pattern has to line up across seams, and the cuts around doorways leavepieces too small to use. Installers add 10 percent for a plain carpet in a square room,so 120 square feet becomes 132.

The third number is the combined rate, which is the carpet price plus the padding priceplus the install price, all per square foot. At the defaults those are $4.50 plus $0.60plus $1.50, which is $6.60 a square foot. The total is the billable area times thecombined rate: 132 times $6.60 is $871.20. The waste factor is the part that changes with the room, so a square room takes 10 percent and a patterned carpet or an awkward room takes 15 percent or more.

The method

The method is three multiplications and an addition. Step one: multiply the room lengthby the room width to get the room area in square feet. Step two: multiply the room areaby one plus the waste as a decimal, so 1.10 for 10 percent, to get the billable area.Step three: multiply the billable area by each of the three rates, carpet, padding andinstall, to get the three costs. Step four: add the three costs to get the total. The check is the cost per square foot of room, which is the combined rate times the waste factor.

Step by step

Step 1: find the room area. Measure the room wall to wall at the longest points, infeet, and multiply length by width. For a 12 by 10 foot room that is 12 times 10, whichis 120 square feet.

Step 2: add the waste. Multiply the room area by 1.10 for the standard 10 percent wasteallowance. For this room that is 120 times 1.10, which is 132 square feet. The wasteadds 12 square feet, which is the carpet that ends up as offcuts and trimming around thedoorways.

Step 3: price the carpet. Multiply the billable area by the carpet price per squarefoot. At $4.50 a square foot that is 132 times $4.50, which is $594.00. This is thebiggest of the three costs, and it moves the most when you change the carpet.

Step 4: price the padding and the fitting. The padding is the billable area times thepadding price, so 132 times $0.60 is $79.20. The fitting is the billable area times theinstall price, so 132 times $1.50 is $198.00. If a quote says padding is included, enterzero for the padding rather than paying for it twice.

Step 5: add the three costs and check. $594.00 plus $79.20 plus $198.00 is $871.20. Thecheck is the cost per square foot of room: $871.20 divided by 120 is $7.26, which is thecombined rate of $6.60 times the waste factor of 1.10.

The worked example

A homeowner is carpeting a 12 by 10 foot bedroom and the shop quotes $4.50 a square footfor the carpet, $0.60 for the padding and $1.50 for the fitting. The room area is 12times 10, which is 120 square feet. The billable area is 120 times 1.10, which is 132square feet. The carpet costs 132 times $4.50, which is $594.00. The padding costs 132times $0.60, which is $79.20. The fitting costs 132 times $1.50, which is $198.00. Thetotal is $871.20, or $7.26 per square foot of room.

A bigger room, and other round numbers

Some rooms are worth knowing by heart. A 15 by 12 foot room is 180 square feet, and with10 percent waste it needs 198 billable square feet, which costs $1,306.80 at the samerates. A carpet at $6.00 a square foot instead of $4.50 costs $792.00 for the materialalone on the 12 by 10 room, and the total rises to $1,069.20. With 15 percent wasteinstead of 10, the billable area is 138 square feet and the total is $910.80.

Working backwards from a budget

To find the biggest room a budget buys, divide the budget by the combined rate, thendivide by the waste factor. The combined rate is $6.60 a square foot, so $1,000 dividedby $6.60 buys 151.52 square feet of carpet. Divide that by 1.10 for the waste and thebiggest room it covers is 137.74 square feet, about a 12 by 11.5 foot room.

Common mistakes

The most common mistake is pricing the room area instead of the billable area. Payingfor 120 square feet instead of 132 saves $79.20 on paper and leaves you 12 square feetshort on the day, which is a strip of bare floor and a second delivery. The secondmistake is forgetting the padding and the fitting, which are $277.20 between them onthis room, or nearly a third of the total. The third mistake is measuring the room atits shortest points instead of its longest. The fourth is paying for padding twice, oncein the carpet price and once as a separate line.

The verdict

A 12 by 10 foot room costs $871.20 to carpet: 120 square feet of room, 132 square feetbought, $594.00 of carpet, $79.20 of padding and $198.00 of fitting. Measure the room,add 10 percent for waste, and price the three parts on the billable area. Check thenumbers yourself on the carpet cost calculator,work out how much paint the same room needs with the paint coverage chart, or check the tile sizes withthe tile size chart.