How to Calculate Deck Cost — Step by Step

The short answer

To work out what a deck costs, measure the length and the width, add 10% extra for cuts and waste, and price the decking and the permit. For a 20 by 20 ft deck that gives an area of 400.00 square feet and a billable area of 440.00 square feet, which is $15,400.00 of decking at $35.00 a square foot, plus $250.00 for the permit, which is $15,650.00 in total.

The four numbers that matter

The first number is the size, in feet, measured across the deck. For this deck that is 20 ft long and 20 ft wide. The second number is the decking rate, which is about $35.00 a square foot for decking installed, including the boards, the joists and the labour. The third number is the permit, which is about $250.00. The fourth number is the extra for cuts and waste, which is 10% for a normal deck.

The area is the length times the width, which is 20 times 20, which is 400.00 square feet. The billable area is the area plus 10% extra, which is 400 times 1.10, which is 440.00 square feet to order. The decking cost is the billable area times the decking rate, which is 440.00 times $35.00, which is $15,400.00.

The method

The method is three parts and an addition. One: multiply the length by the width, to get the area. Two: multiply the area by one plus the extra, to get the billable area. Three: multiply the billable area by the decking rate, to get the decking cost. Four: add the permit, to get the total.

Step by step

Step 1: find the area. Measure the deck in feet, with a tape measure, and multiply the length by the width. For this deck that is 20 times 20, which is 400.00 square feet. A deck that follows a slope is measured in pieces, and the pieces are added up, because the straight-line distance is not the deck.

Step 2: find the billable area. Multiply the area by one plus the extra. For this deck that is 400 times 1.10, which is 440.00 square feet to order. The extra is 10% for a normal deck, and 15% for a deck with a diagonal pattern.

Step 3: price the decking. Multiply the billable area by the decking rate. For this deck that is 440.00 times $35.00, which is $15,400.00. The decking rate is the boards, the joists and the labour, so a builder quote and a boards-only price are not the same number.

Step 4: add the permit. Add the permit to the decking cost, to get the total. For this deck that is $15,400.00 plus $250.00, which is $15,650.00. The permit is one flat fee in this model: a town that charges by the square foot will charge more for a bigger deck.

Step 5: check the cost per square foot. Divide the total by the area, to get the number to compare between decks and between quotes. For this deck that is $15,650.00 divided by 400, which is $39.13 a square foot all-in. The per-foot cost before the permit is 1.10 times $35.00, which is $38.50.

Step 6: count the boards and the budget. The billable area is the boards to order, in square feet. For this deck that is 440.00 square feet of boards. And a $10,000 budget minus the $250 permit leaves $9,750, which divided by 38.5 is 253.25 square feet of deck.

The worked example

A homeowner is building a 20 by 20 ft deck, and the decking costs $35.00 a square foot, the permit is $250.00, and the extra is 10%. The area is 20 times 20, which is 400.00 square feet. The billable area is 400 times 1.10, which is 440.00 square feet to order. The decking cost is 440.00 times $35.00, which is $15,400.00. The total is $15,650.00, or $39.13 a square foot. The deck takes 440.00 square feet of boards, and it comes on a truck, not in a car boot.

A smaller deck, and other round numbers

A 15 by 20 ft deck is 15 times 20, which is 300.00 square feet, so the billable area is 330.00 square feet, the decking is $11,550.00, and the total is $11,800.00, or $39.33 a square foot. Decking at $25.00 a square foot costs $11,000.00, and the total is $11,250.00, or $28.13 a square foot. With no extra for cuts and waste the billable area is 400.00 square feet, the decking is $14,000.00, and the total is $14,250.00, or $35.63 a square foot. A deck with no permit is $15,400.00, or $38.50 a square foot. A 10 by 12 ft deck is 120.00 square feet, the billable area is 132.00 square feet, the decking is $4,620.00, and the total is $4,870.00, or $40.58 a square foot.

Working backwards from a budget

To find how much deck a budget covers, subtract the permit from the budget, then divide what is left by the per-foot cost. The per-foot cost is 1.10 times $35.00 of decking, which is 38.5. A $10,000 budget minus the $250 permit leaves $9,750. Divide that by 38.5, and it is 253.25 square feet of deck. A $20,000 budget buys 512.99 square feet.

Common mistakes

The most common mistake is ordering the deck area, not the billable area. A 400 square foot deck is 440.00 square feet of boards to order, not 400, so ordering the deck area leaves the last board short by 40 square feet. The second mistake is forgetting the permit, which is $250.00. The third is measuring the straight-line distance instead of the deck. The fourth is guessing the extra: 5% looks the same as 10% on the day, but the last board comes up short. The fifth is comparing a boards-only price with a builder quote, which includes the joists and the labour.

The verdict

A 20 by 20 ft deck costs $15,650.00: 440.00 square feet of boards, at $35.00 a square foot, plus a $250.00 permit. Measure the length and the width, add 10% extra, and price the decking and the permit. Check the numbers on the deck cost calculator, the deck vs patio cost comparison, or the paver patio cost calculator.