How to use the retaining wall calculator
- Enter the wall. Type the length of the wall and its full height, including the part that will be below the ground.
- Enter the block. Pick the size of the block face or type your own. Type the depth of the block from front to back and, if the wall has caps, the length of one cap block.
- Read the blocks and the gravel. The result shows the blocks to buy, the courses, the cap blocks, and the gravel for the base pad and for drainage behind the wall.
What is a retaining wall calculator?
A retaining wall calculator is a tool that estimates the materials for a wall that holds back soil. This one is for a segmental retaining wall: a wall of concrete blocks stacked without mortar on a gravel base. It counts the blocks in the face of the wall, the cap blocks along the top, and the gravel under and behind the wall.
It gives quantities only. It does not check whether a wall of that height is safe.
Features
- The number of retaining wall blocks from the wall length and height
- Three example block sizes and a field for your own block
- The number of courses and the blocks in each course
- Cap blocks from the length of one cap
- Gravel for the base pad and drainage gravel behind the wall, in cubic yards and tons
- An adjustable waste allowance and a cost from your own price per block
- A live drawing of the wall with every block, a 3D view, a PDF of the estimate and a WhatsApp share button
What you can use it for
- A garden wall that holds back a slope
- A raised bed or a terraced yard
- A low wall along a driveway or a path
- Ordering the gravel for the base and the drainage
How the retaining wall is calculated
- Blocks = wall length × wall height × blocks per square foot × (1 + waste ÷ 100), rounded up.
- Blocks per square foot = 144 ÷ (block length × block height), both in inches.
- Cap blocks = wall length ÷ cap length, rounded up.
- Base pad gravel = wall length × (block depth + 12 in) × 6 in.
- Drainage gravel = wall length × wall height × 12 in.
Worked example: a wall 20 ft long and 3 ft high
- Face area: 20 × 3 = 60 square feet.
- Blocks of 16 in × 6 in: 144 ÷ 96 = 1.5 per square foot, so 60 × 1.5 = 90 blocks. With 5% waste: 95 blocks.
- Courses: 36 in ÷ 6 in = 6. Blocks in each course: 240 in ÷ 16 in = 15.
- Base pad for a block 12 in deep: 20 ft × 2 ft × 0.5 ft = 20 cubic feet = 0.74 cubic yards, which is 1.37 tons at 1.85 tons per cubic yard.
- Drainage gravel: 20 ft × 3 ft × 1 ft = 60 cubic feet = 2.22 cubic yards, which is 3.18 tons at 1.43 tons per cubic yard.
These are the numbers the calculator shows when the page opens.
The base pad and the drainage gravel
The Concrete Masonry & Hardscapes Association's Guide to Segmental Retaining Walls (SRW-TEC-005-09) says the leveling pad of crushed stone should be at least 6 inches thick and typically extends 6 inches in front of and behind the bottom block. It also says the gravel fill should extend a minimum of 12 inches behind the blocks. The calculator uses these minimums.
The tons use two weights from the Washington State DOT Design Manual: 1.85 tons per cubic yard for the base pad, which is compacted, and 1.43 tons per cubic yard for the drainage stone, which is loose crushed stone. Your supplier's stone can differ.
Enter the full height of the wall, including the bottom course or courses that sit below the ground. The buried depth depends on your block maker's instructions, which this site does not have.
What the calculator includes
- The wall is a straight rectangle with one height. Curves, corners, steps in height and tapered blocks are not counted.
- Geogrid, drain pipe, filter fabric, adhesive for the caps and soil backfill are not included.
- The gravel uses the association's minimum sizes. Your block maker or engineer may ask for more.
Is it private?
Yes. The calculation and the PDF are made in your browser. The numbers you type are not sent to our server and are not saved.
Tips for an accurate estimate
- Choose the block size you will buy, because the face size sets the count.
- Set the first course on a level, compacted leveling pad.
- Order the drainage gravel with the blocks; it goes behind the wall as you build.
- Get engineering for tall walls or walls that hold up a driveway, a slope or a building.
- Keep a few spare blocks for cuts at the ends and for later repairs.
Related calculators
Frequently asked questions
- How many retaining wall blocks do I need?
- Multiply the wall length by its height to get the face area, then multiply by the blocks per square foot. A wall 20 ft long and 3 ft high with 16 inch by 6 inch blocks needs 90 blocks, or 95 with 5% waste.
- How do I calculate blocks per square foot?
- Divide 144 by the length times the height of the block face in inches. A 16 × 6 block gives 1.5 per square foot and an 18 × 8 block gives 1.
- How much gravel do I need behind a retaining wall?
- The Concrete Masonry & Hardscapes Association says the gravel should extend at least 12 inches behind the blocks. For a wall 20 ft long and 3 ft high that is 60 cubic feet, which is 2.22 cubic yards.
- How thick should the base of a retaining wall be?
- The Concrete Masonry & Hardscapes Association's guide says the leveling pad should be at least 6 inches thick and typically extends 6 inches in front of and behind the bottom block.
- How many cap blocks do I need?
- Divide the length of the wall by the length of one cap and round up. A 20 ft wall with 16 inch caps needs 15 caps.
- Does the calculator tell me how high I can build?
- No. The safe height depends on the block, the soil and what is above the wall. Follow the block maker's instructions and your local building rules, and use an engineer for taller walls.
Sources
Every figure this calculator uses comes from the published sources below. The formulas are on this page, so any result can be checked by hand.
- Concrete Masonry & Hardscapes Association, SRW-TEC-005-09, Guide to Segmental Retaining Walls. Leveling pad and drainage gravel sizes behind the wall.
- Washington State Department of Transportation, Design Manual M 22-01, Exhibits 620-1 and 620-2. Dry weights in tons per cubic yard of gravel, crushed stone, ballast, sand and hot mix asphalt.