How to Calculate Retaining Wall Cost — Step by Step

The short answer

To work out what a retaining wall costs, measure the length and the height, add 10% extra for cuts and waste, and price the blocks, the gravel and the delivery. For a 30 by 4 ft wall that gives a face area of 120.00 square feet and a billable area of 132.00 square feet, which is $3,300.00 of blocks at $25.00 a square foot, plus $150.00 for base gravel and $75.00 for delivery, which is $3,525.00 in total.

The five numbers that matter

The first number is the size, in feet, measured along the wall and up the face. For this wall that is 30 ft long and 4 ft high. The second number is the block rate, which is about $25.00 a square foot for blocks installed, including the blocks and the labour. The third number is the base gravel, which is about $150.00. The fourth number is the delivery, which is about $75.00. The fifth number is the extra for cuts and waste, which is 10% for a straight wall.

The face area is the length times the height, which is 30 times 4, which is 120.00 square feet. The billable area is the face area plus 10% extra, which is 120 times 1.10, which is 132.00 square feet to order. The blocks cost is the billable area times the block rate, which is 132.00 times $25.00, which is $3,300.00.

The method

The method is three parts and two additions. One: multiply the length by the height, to get the face area. Two: multiply the face area by one plus the extra, to get the billable area. Three: multiply the billable area by the block rate, to get the blocks cost. Four: add the gravel and the delivery, to get the total.

Step by step

Step 1: find the face area. Measure the wall in feet, with a tape measure, and multiply the length by the height. For this wall that is 30 times 4, which is 120.00 square feet. A wall that follows a slope is measured in pieces, and the pieces are added up, because the straight-line distance is not the wall.

Step 2: find the billable area. Multiply the face area by one plus the extra. For this wall that is 120 times 1.10, which is 132.00 square feet to order. The extra is 10% for a straight wall, and 15% for a curved wall.

Step 3: price the blocks. Multiply the billable area by the block rate. For this wall that is 132.00 times $25.00, which is $3,300.00. The block rate is the blocks and the labour, so a builder quote and a blocks-only price are not the same number.

Step 4: add the gravel and the delivery. Add the base gravel and the delivery to the blocks cost, to get the total. For this wall that is $3,300.00 plus $150.00 plus $75.00, which is $3,525.00. The gravel and the delivery are one flat fee each in this model: a wall that needs more gravel for a deeper base will cost more.

Step 5: check the cost per square foot. Divide the total by the face area, to get the number to compare between walls and between quotes. For this wall that is $3,525.00 divided by 120, which is $29.38 a square foot all-in. The per-foot cost before the gravel and the delivery is 1.10 times $25.00, which is $27.50.

Step 6: count the blocks and the budget. The blocks to order are the billable area times two, because each 12 by 6 in block covers half a square foot. For this wall that is 132.00 times 2, which is 264 blocks. And a $5,000 budget minus the $150 gravel and the $75 delivery leaves $4,775, which divided by 27.5 is 173.64 square feet of wall.

The worked example

A homeowner is building a 30 by 4 ft retaining wall, and the blocks cost $25.00 a square foot, the gravel is $150.00, the delivery is $75.00, and the extra is 10%. The face area is 30 times 4, which is 120.00 square feet. The billable area is 120 times 1.10, which is 132.00 square feet to order. The blocks cost is 132.00 times $25.00, which is $3,300.00. The total is $3,525.00, or $29.38 a square foot. The wall takes 264 blocks, and they come on a pallet, not in a car boot.

A smaller wall, and other round numbers

A 20 by 4 ft wall is 20 times 4, which is 80.00 square feet, so the billable area is 88.00 square feet, the blocks are $2,200.00, and the total is $2,425.00, or $30.31 a square foot. Blocks at $18.00 a square foot cost $2,376.00, and the total is $2,601.00, or $21.68 a square foot. With no extra for cuts and waste the billable area is 120.00 square feet, the blocks are $3,000.00, and the total is $3,225.00, or $26.88 a square foot. A wall with no delivery is $3,450.00, or $28.75 a square foot. A 25 by 5 ft wall is 125.00 square feet, the billable area is 137.50 square feet, the blocks are $3,437.50, and the total is $3,662.50, or $29.30 a square foot.

Working backwards from a budget

To find how much wall a budget covers, subtract the gravel and the delivery from the budget, then divide what is left by the per-foot cost. The per-foot cost is 1.10 times $25.00 of blocks, which is 27.5. A $5,000 budget minus the $150 gravel and the $75 delivery leaves $4,775. Divide that by 27.5, and it is 173.64 square feet of wall. An $8,000 budget buys 282.73 square feet.

Common mistakes

The most common mistake is ordering the face area, not the billable area. A 120 square foot wall is 132.00 square feet of blocks to order, not 120, so ordering the face area leaves the last block short by 12 square feet. The second mistake is forgetting the gravel and the delivery, which are $225.00. The third is measuring the straight-line distance instead of the wall. The fourth is guessing the extra: 5% looks the same as 10% on the day, but the last block comes up short. The fifth is comparing a blocks-only price with a builder quote, which includes the labour.

The verdict

A 30 by 4 ft retaining wall costs $3,525.00: 132.00 square feet of blocks, at $25.00 a square foot, plus $150.00 of gravel and $75.00 of delivery. Measure the length and the height, add 10% extra, and price the blocks, the gravel and the delivery. Check the numbers on the retaining wall cost calculator, the deck cost calculator, or the concrete cost calculator.