Retaining Wall Block Calculator
This retaining wall block calculator gives the blocks and caps a wall needs, plus the base stone and drainage gravel behind it, the two orders that make up the job.
Blocks and Caps
Base and Drainage Stone
The trench under the wall and the drainage chimney behind it. Usually the bigger of the two orders.
Retaining Wall Cost
How to use the Retaining Wall Block Calculator
Enter the length and the exposed height, what you will see above grade. The buried courses are added separately, because a retaining wall always starts below grade and forgetting that is a course short on every wall.
Then do the Base tab. The trench under the wall and the clean stone behind it are usually more material than the blocks, and they are what stops the wall from moving.
Anything over about 4 feet, or holding a slope, a driveway or a structure, needs an engineer. This tool counts material; it does not design a wall.
Retaining Wall Cost covers blocks, caps and base stone in one figure. The buried course is counted in with the rest, which is why the block total usually comes to one course more than the wall you can see.
Retaining Wall Block formula
courses = ceiling(total height ÷ block height)blocks per course = ceiling(wall length ÷ block face width)blocks = courses × blocks per coursecaps = blocks per coursebase stone = length × trench width × base depthdrainage stone = length × drain width × wall height
| Face size | Face area | Blocks per sq ft | Courses per foot of height |
|---|---|---|---|
| 12 × 4 in | 0.33 sq ft | 3.00 | 3.0 |
| 12 × 8 in | 0.67 sq ft | 1.50 | 1.5 |
| 16 × 6 in | 0.67 sq ft | 1.50 | 2.0 |
| 18 × 8 in | 1.00 sq ft | 1.00 | 1.5 |
Blocks are counted by face area, not by volume, because only the face shows and the depth is fixed by the product.
Worked example
A 30 ft wall, 3 ft exposed, 12 × 4 in blocks, one buried course, capped, 5% waste:
- Exposed courses: 3 ft ÷ 4 in = 9 courses
- Plus 1 buried = 10 courses, so total height is 3 ft 4 in
- Blocks per course: 30 ft ÷ 12 in = 30
- Blocks: 10 × 30 = 300, plus 5% = 315
- Caps: 30
And the stone: a 24 in trench 6 in deep is 30 ft³, and a 12 in drainage chimney 3 ft 4 in tall is 100 ft³. That is 4.8 cubic yards against 315 blocks, about 6.5 tons of stone to deliver.
What the result means
The stone is the wall. Blocks stack; the compacted base stops them settling and the drainage chimney stops water pressure pushing them over. A wall that fails almost always failed on drainage, not on blocks.
Bury the first course. One course below grade for a low wall, and roughly one course for every eight above that on taller ones. It is what keeps the toe from kicking out.
Setback eats depth at the top. Most segmental blocks step back about an inch per course, so a ten-course wall leans back the better part of a foot. That is by design, it puts the wall into the slope, but it changes where the top of the wall actually lands.
Common mistakes
- Not burying the base course. The single most common cause of a wall that leans forward within a few seasons.
- Backfilling with the soil you dug out. Clay behind a wall holds water and pushes. Use clean angular stone for the first foot behind the blocks.
- Skipping the drain pipe. The stone chimney needs somewhere to drain to, or it just becomes a taller reservoir.
- Building over four feet without an engineer. Most jurisdictions require design and a permit above that, and geogrid reinforcement beyond it.
- Counting blocks and forgetting caps. Caps are a separate unit, usually a different price, and there is one course of them.
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length by the block face width to get blocks per course, divide the total height including buried courses by the block height to get the number of courses, then multiply. A 30 foot wall 3 feet 4 inches tall in 12 by 4 inch blocks needs 300 blocks before waste.
How deep should the base course be buried?
At least one full course below grade, and roughly one course for every eight courses of wall above it. The base course sits on compacted stone, not on soil, and it is what keeps the bottom of the wall from kicking out.
How much gravel do I need behind a retaining wall?
A chimney of clean angular stone about 12 inches wide running the full height of the wall. For a 30 foot wall 3 feet 4 inches tall that is 100 cubic feet, or about 3.7 cubic yards, plus the base trench underneath.
How thick should the base under a retaining wall be?
Six inches of compacted crushed stone is typical for a low wall, in a trench about twice the block depth wide. Taller walls need more. The base must be level and compacted in lifts, everything above it inherits its accuracy.
How high can I build a retaining wall without an engineer?
Commonly four feet measured from the bottom of the base, but this varies by jurisdiction and is lower where the wall carries a slope, a driveway or a structure above it. Check locally before building, permits are usually required above that height.
Do I need caps on a retaining wall?
Not structurally, but they finish the top, shed water off the back of the wall and are usually glued down with concrete adhesive. You need one cap course, so the cap count equals the blocks in one course.
Sources and assumptions
Block face dimensions are the common segmental retaining wall sizes, and the two most widely used are both in the list. Versa-Lok Standard has a 16 × 6 in face, 0.667 sq ft, or 1.5 blocks to the square foot of wall. Allan Block AB Classic has an 18 × 8 in face, exactly 1.0 sq ft, one block to the square foot. There is no single industry figure for blocks per square foot, which is why the face size is an input. Confirm it and the unit weight with your supplier.
Base trench and drainage stone dimensions follow NCMA and Allan Block guidance: a leveling pad “not less than 6 in.” deep and roughly 12 in wider than the block depth, with a twelve inch drainage zone behind the units. Embedment follows Allan Block’s rule of about 1 in of buried depth per foot of wall height, minimum 6 in; NCMA states it by exposed height and slope instead, and is less conservative on level ground.
Stone tonnage is estimated at 2,835 lb per cubic yard for crushed stone, consistent with the gravel calculator on this site. Loose stone density varies with moisture and gradation.
This tool estimates material only. It is not a wall design. Walls over about four feet, walls carrying a surcharge, and walls on a slope require engineering and usually a permit.
For walls beyond a few courses, the reference works are the CMHA Design Manual for Segmental Retaining Walls and Allan Block’s best practice guide.