How to use
- Enter the wall length and the height you'll see above the finished ground.
- Pick a block size or type in your block's height, face length, and depth.
- Leave the buried depth blank to use the usual rule, keep or change the base and drainage depths, then select "Calculate".
How the block count works
A block wall needs its first course partly or fully below ground. Allan Block's installation guide adds 1 inch of buried block for every foot of wall height, and the calculator does the same unless you enter your own depth. Courses = (height above ground + buried depth) ÷ block height, rounded up. Blocks per course = wall length ÷ block length, rounded up.
Example: a wall 20 ft long and 2 ft high with small blocks 4 inches tall and 11.75 inches long. The buried depth is 2 inches, so the wall is 26 inches tall in total, which takes 7 courses. Each course needs 240 ÷ 11.75 = 20.4, so 21 blocks. That's 147 blocks, or 155 with 5% extra, plus 21 caps.
Common block sizes
| Block | Height × length × depth | Face area per block |
|---|---|---|
| Small wall block (Pavestone RockWall Small and similar) | 4 × 11.75 × 6.75 in | About 0.32 sq ft (144 blocks cover 46.5 sq ft) |
| Large wall block (Pavestone RockWall Large and similar) | 6 × 17.5 × 7 in | About 0.73 sq ft (48 blocks cover 34.9 sq ft) |
| Allan Block AB Classic | 8 × 18 × 12 in | About 1 sq ft, roughly 75 lb each |
Sizes and weights vary by brand and plant, so check the label. Each block maker also lists the tallest wall its block can be used for; check that before you buy.
Base and drainage
- Base trench. Unilock says to dig the trench twice as wide as the block is deep. For a 12-inch-deep block that's 24 inches, the same width Allan Block calls for.
- Base gravel. Put at least 6 inches of compacted crushed stone in the trench, according to both Allan Block and Unilock. Compact it in layers.
- Drainage stone. Fill 12 inches behind the blocks with clean crushed stone, up to the top of the wall, as Allan Block's guide describes. This zone lets water drain out from behind the wall.
- Drain pipe. Allan Block requires a drain pipe for gravity walls over 4 feet tall and on sites with poor drainage. Lay it at the back of the trench and vent it to daylight.
- Backfill. Behind the drainage stone, backfill in layers no more than 8 inches thick, and compact each one.
When a wall needs an engineer
The International Residential Code (section R404.4) requires a designed wall when it isn't supported at the top and holds back more than 48 inches of soil, or when it's over 24 inches tall and also holds back extra load, such as a driveway, a slope, or a building. Such walls must be designed for a safety factor of 1.5 against sliding and overturning. Your city may require a permit even for shorter walls. The calculator flags these cases, but your local building department has the final say.
Buying tips
- Buy blocks and caps from the same color run, and keep a few spares.
- Cap units are usually glued down with concrete adhesive. Unilock's guide says to glue the final row or coping.
- Many wall systems sell a matching corner block. If your wall has corners, ask about them.
- Base gravel and drainage stone are sold by the ton. The calculator gives both cubic yards and tons so you can order either way.
Common mistakes
- Setting the first course on dirt. Both Allan Block and Unilock put the first course on a compacted gravel base.
- Skipping the buried course. The installation guides bury part of the first course on every wall, more for taller walls.
- Backfilling the drainage zone with dirt. Use clean crushed stone behind the blocks, as the guides call for, and save the soil for the layers behind it.
- Terracing without a plan. If you're thinking of two shorter walls instead of one tall one, ask the block maker how far apart they need to be.
Sources
Buried depth, trench width for AB Classic, base, drainage zone, drain pipe, and backfill layers: Allan Block residential installation guide. Trench width rule and glued caps: Unilock Project Planning Guide. Code thresholds: International Residential Code section R404.4, as quoted by the City of Grand Prairie, Texas residential retaining wall permit page (2021 IRC). Block sizes: Pavestone RockWall product listings (Home Depot) and an Allan Block AB Classic specification sheet. Gravel weights: Wisconsin DOT (compacted base) and Missouri DOT (loose aggregate).
Related calculators
- Gravel Calculator - more stone for paths and drainage areas
- Topsoil Calculator - soil to fill behind and on top of the wall
- Paver Calculator - a paver patio next to the wall
- Sales Tax Calculator - add tax to the block order
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length by the block length for blocks per course, and divide the total height, including the buried course, by the block height for the number of courses. Multiply the two and add a few percent for cuts.
How deep should the first course be buried?
Allan Block's guide uses 1 inch of buried block for every foot of wall height, on top of at least 6 inches of compacted gravel base. Some manufacturers and local codes ask for more, so check your block maker's instructions.
How tall can a block retaining wall be without an engineer?
Under the International Residential Code, a wall that holds back more than 48 inches of soil, or more than 24 inches with a driveway, slope, or building above it, needs an engineered design. Your city may have stricter rules or require a permit.
How much gravel goes behind a retaining wall?
Allan Block calls for 12 inches of clean crushed stone behind the blocks, up to the height of the wall. For a wall 20 feet long and 26 inches tall in total, that's about 1.6 cubic yards.
Do I need a drain pipe?
Allan Block requires one for gravity walls over 4 feet tall and on sites with poor drainage. It's a good idea on any wall where water collects. The calculator gives the length, which equals the wall length.
Should I use cap blocks?
Caps give the wall a finished top and protect the last course. They're usually glued down with concrete adhesive. The calculator counts one cap per block length unless you enter a different cap length.