Retaining Wall Calculator
Find how many wall blocks and caps you need for a dry-stacked retaining wall, and how much gravel goes under and behind it. Enter each straight run of the wall, and the calculator counts the courses, including the buried course. For a mortared block wall, use the Concrete Block Calculator.
- Choose US or Metric. You can still change the unit of any single field.
- Enter the Length and Height of the wall. Measure the length along the front of the wall, and the height from the ground in front of the wall to the top of the blocks, without the cap. Use Add section for each straight run, or where the wall steps up or down.
- Keep or change the Buried depth, the depth of block below the ground. Enter 0 for a wall built on a concrete slab.
- Choose the Block size. For another block, choose Custom size and enter its actual height, length, and depth.
- Choose the Caps: caps the same length as the blocks, a cap of another length, or no caps.
- Check the gravel sizes. Leave Base width blank to use the block depth plus 12 in (300 mm in Metric), and clear Base depth or Drainage gravel thickness to leave that gravel out.
- Optionally, enter a price for each item to estimate the cost. Gravel is priced per US ton or per tonne.
Length fields accept whole numbers, decimals, fractions, and mixed
numbers, such as 20, 10.5, 3/4, or
11 1/2. Choose ft + in as the unit to enter
a height such as 2 ft 8 in in two fields.
How to calculate retaining wall blocks
Each section of the wall is the same number of blocks in every course, so the calculator counts one course and the number of courses:
Blocks per course = section length ÷ block length, rounded up
Courses = section height ÷ block height, rounded up + buried courses
Buried courses = buried depth ÷ block height, rounded up
Blocks to buy = sum of (courses × blocks per course) × (1 + waste % ÷ 100), rounded up
This follows Allan Block's Material Estimation Worksheet, which divides the wall length by the block length for the blocks per course, and the wall height, with the buried block included, by the block height for the courses. A part block at the end of a course counts as one block, cut to fit. Allan Block's Residential Retaining Walls guide also counts 7 full-size blocks for each 10 ft of wall: 120 ÷ 18 = 6.67, rounded up.
A height that divides evenly gets no extra course, so 2 ft of 8 in blocks is 3 courses. Otherwise the wall comes out a little taller than you entered, and the breakdown shows the height above the ground. If you think in courses, enter the courses times the block height.
Caps are counted the same way along each section:
Caps = section length ÷ cap length, rounded up
Allan Block's AB Capstone and Pavestone's 4-in Wall Cap are 18 in long, the same as the blocks they go with, so caps the same length as the blocks are one per block in the top course. The height you enter is to the top of the blocks: caps add their own height, such as the 4 in that Allan Block gives for its caps. For caps laid with short and long faces alternating, such as Belgard's XL cap, enter the average of the two lengths as a custom cap length.
Each course of a segmental wall sets back a little from the one below. On a straight wall, that doesn't change the number of blocks. On a curved wall, measure the length along the front of the wall: the upper courses are a little shorter on an outside curve and a little longer on an inside curve.
Block sizes and buried depth
| Block | Depth | Hollow cores |
|---|---|---|
| 8 × 18 in | 12 in | Yes |
| 4 × 11 1/2 in | 7 1/2 in | No |
| 200 × 460 mm | 300 mm | Yes |
Each size is the height × length of the block's face, the way US stores and manufacturers list wall blocks. The 8 × 18 in block is the full-size block of Allan Block's AB Classic and Pavestone's BroadStone 8, about 1 ft² of face. Allan Block recommends larger blocks like this for walls over 3 ft. The 200 × 460 mm block is the same block in Allan Block's metric sizes. The 4 × 11 1/2 in block is the small basic wall block sold at large US stores, made for walls up to 2 ft high. Keystone's similar block is 12 in long, so enter your block's actual size if it differs.
Allan Block notes that its sizes are approximate, and its own estimating table counts 21 blocks for a 30 ft course, one more than 360 ÷ 18. If your blocks are shorter than 18 in, choose Custom size and enter the actual length. Small blocks sold outside the US come in many sizes, such as Apex Masonry's Classic Wall block at 255 × 200 × 125 mm, so enter those as a custom size too.
The buried depth starts at 6 in (150 mm). Allan Block's worksheet, Belgard's Technical Resource Guide, and Pavestone's BroadStone guide all bury at least 6 in of block. That is one 8 in course, or two 4 in courses, since a part course is still a whole block. Some guides bury less for small walls, such as 50 mm in Apex Masonry's guide for walls up to 1 m, and Allan Block adds 1 in of buried block for each foot of wall over 6 ft, so follow the instructions for your block.
Base and drainage gravel
The base is compacted gravel in a trench under the first course. The drainage gravel goes behind the blocks, from the base to the top of the top course, buried courses included:
Base gravel = wall length × base width × base depth
Drainage gravel = sum of (section length × courses × block height) × (drainage thickness + core fill)
Weight = volume × (1 + waste % ÷ 100) × gravel density
- Base width = block depth + 12 in (+ 300 mm in Metric) if you leave it blank. Belgard extends the base at least 6 in in front of and behind the first course, and both Pavestone and Allan Block dig 24 in wide for a 12 in deep block. Allan Block allows a base 18 in wide and 4 in deep for walls under 4 ft, which you can enter instead.
- Base depth = 6 in (150 mm) after compaction, the minimum all three guides give. The base is counted along the wall length only, not the extra at each step of a stepped base.
- Drainage thickness = 12 in (300 mm) behind the blocks, as in Allan Block's and Pavestone's guides. Belgard asks for at least 12 in behind the block or 24 in from its front face, whichever is more, so add to the thickness for a block less than 12 in deep. The calculator fills to the top of the blocks. If you cap the drainage gravel with soil, you'll have a little to spare.
- Core fill. Hollow blocks are filled with the same gravel. Allan Block's worksheet counts 1.4 ft of gravel for each square foot of wall face: 1 ft behind the block and 0.4 ft in the cores. The calculator adds that 0.4 ft (0.13 m) for the 8 × 18 in and 200 × 460 mm blocks. For a custom hollow block, raise the drainage thickness, such as to about 17 in to match Allan Block's total.
The volumes are compacted volumes in place. The calculator weighs them at 120 lb/ft³, the unit weight of rock that Allan Block's worksheet uses to turn those volumes into tons. Compacting gravel doesn't change its weight, so this is the weight to order. Allan Block uses the same wall rock, a compactable aggregate of 1/4 to 1 1/2 in, for the base, the cores, and the drainage. Belgard and Pavestone use 3/4 in minus base material for the base and clean, free-draining stone for the drainage. For another material, ask your supplier for its compacted density and enter it, and price the two gravels separately.
Gravel sold by volume is measured loose, so it takes more than the compacted volume. If you order by the cubic yard or cubic meter, check with your supplier how much to allow. For bagged gravel labeled by weight, such as 50 lb or 20 kg, divide the weight by the bag weight and round up. Bags labeled by volume hold loose gravel, so check the bag or your supplier for how much it compacts. The Road Base Calculator finds the loose volume to order for a compacted base.
The 5% waste allowance follows Allan Block's worksheet and Apex Masonry's guide, which both add 5% to the blocks. The calculator adds it to the caps and gravel too, since Allan Block's figures are the minimum a wall needs.
Examples
A 20 ft garden wall
A wall 20 ft long and 2 ft 8 in high, in 8 × 18 in blocks, has 32 ÷ 8 = 4 courses above the ground and 1 buried course. Each course takes 240 ÷ 18 = 13.33, rounded up to 14 blocks, so the wall is 5 × 14 = 70 blocks, or 74 blocks with 5% added, and 15 caps. The base is 24 in wide and 6 in deep: 21 ft³ with 5% added, or 1.26 US tons. The drainage gravel, with the cores, is 98 ft³, or 5.88 US tons.
A stepped wall
Add a second section 10 ft long and 16 in high where the ground rises. It has 3 courses of 7 blocks, so the wall is 70 + 21 = 91 blocks, or 96 blocks and 23 caps to buy.
Small blocks
A 12 ft wall 16 in high in 4 × 11 1/2 in blocks has 4 courses above the ground and 2 buried courses for the 6 in buried depth. Each course takes 13 blocks, so the wall is 78 blocks, or 82 blocks to buy.
A metric wall
A wall 6 m long and 0.8 m high in 200 × 460 mm blocks has 4 courses above the ground, 1 buried course, and 14 blocks per course: 74 blocks and 15 caps with 5% added. The base, 600 mm wide and 150 mm deep, is 1.09 tonnes, and the drainage gravel is 5.21 tonnes.
FAQ
For a wall 20 ft long and 2 ft 8 in high in 8 × 18 in blocks, 74 with 5% added. Divide the length by the block length and round up for the blocks per course, divide the height by the block height and round up for the courses, add the buried course, and multiply.
At least 6 in (150 mm) of block below the ground, in most installation guides. That's one 8 in course or two 4 in courses, set on a compacted gravel base about 6 in deep.
About 1 ft³ for each square foot of wall face with a 12 in layer, or 1.4 ft³ when the hollow cores are filled too, as in Allan Block's worksheet. At 120 lb/ft³, 1.4 ft³ is about 168 lb.
It depends on where you build. Many places require a permit and an engineer's design for walls over about 4 ft, and some measure from the bottom of the footing. Lower walls with a slope or a load above them can need a design too, so check the rules where you build.
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