Retaining Wall Calculator
Blocks, caps, base stone and drainage stone for a segmental wall.
This retaining wall calculator works out the blocks, caps, base stone and drainage stone for a segmental block wall. Enter the wall size and the block, and you’ll see the numbers update as you type. It flags the height where a wall needs an engineer.
The stone gets the attention here. Blocks get the glory, but the drainage stone behind the wall is the biggest quantity on the order. Did you expect that?
How the Retaining Wall Calculator Works
The calculator divides wall length by block width to get blocks per course. It divides wall height by block height to get courses, adds the buried ones, multiplies, and adds an allowance for cuts and breakage. Stone comes from the trench and the drainage column.
- Base stone is a 6 inch compacted pad, 24 inches wide. Nicolock’s calculator assumes a 6 inch compacted pad about 2 feet wide.
- One course is buried below grade. Nicolock says to bury one full course and set it on the 6 inch base.
- Drainage stone is a 12 inch column behind the wall, the full height. Nicolock and the Landscape Architect worked example both use 12 inches.
- Blocks get 5% extra and the stone gets 10%. Nicolock builds in 10% for both.
- Stone weighs 1.4 tons per cubic yard, a middle value. Use the supplier’s number if it doesn’t match.
With the example values (a 30 foot wall, 3 feet above ground, 12 by 4 inch blocks), the order is about 315 blocks, 32 caps and 5.3 cubic yards of stone, which is 7.4 tons.
Checked Against Two Published Examples
Nicolock’s example is a 24 foot wall, 3 feet high, built from 6 by 16 inch blocks with 10% waste. It lists about 119 blocks and 2.9 cubic yards of drainage stone, which is 4.1 tons. Set the same numbers in this calculator and you’ll get 119 blocks and 2.93 cubic yards.
The Landscape Architect example is a 50 foot wall, 4 feet high, with a 12 inch drainage column. It uses 7.41 cubic yards of drainage aggregate. The calculator gives 7.41 with no allowance and no buried course.
Good sign. The formulas match how contractors count.
Blocks and Stone for Common Wall Sizes
This table comes from the calculator with 12 by 4 inch blocks, one buried course, a 12 inch drainage column and the defaults above, so it matches the tool.
| Wall (length by height) | Blocks (12 by 4 in, 1 buried, 5% extra) | Caps | Stone |
|---|---|---|---|
| 20 by 2 ft | 147 | 21 | 2.7 cu yd |
| 30 by 3 ft | 315 | 32 | 5.3 cu yd |
| 40 by 3 ft | 420 | 42 | 7.1 cu yd |
| 50 by 4 ft | 683 | 53 | 10.9 cu yd |
A 30 foot wall that’s 3 feet high takes 315 blocks, but a 50 by 4 foot wall takes 683. The wall grows longer and taller together, so the count more than doubles. Stone grows the same way, and you’ll feel it at the delivery.
What Does the Buried Course Add?
Every buried course is a full row of blocks nobody sees. Is it worth it? It’s what keeps the wall from sliding. Here’s a 30 by 3 foot wall with none, one and two courses buried.
| Courses buried | Blocks for a 30 by 3 ft wall |
|---|---|
| 0 | 284 |
| 1 | 315 |
| 2 | 347 |
One buried course adds 31 blocks, and two add 63. The block maker’s manual sets the number. It’s usually tied to the wall height.
The Drainage Stone Is the Biggest Quantity
On the 30 by 3 foot example, the drainage column is 4.1 of the 5.3 cubic yards of stone, about three-quarters. Here’s how the column width changes the total.
| Drainage stone behind the wall | Total stone, 30 by 3 ft wall |
|---|---|
| None | 1.2 cu yd, 1.7 tons |
| 6 in | 3.3 cu yd, 4.6 tons |
| 12 in | 5.3 cu yd, 7.4 tons |
| 18 in | 7.3 cu yd, 10.3 tons |
Going from a 6 inch to a 12 inch column adds 2 cubic yards and almost 3 tons. That’s the line that surprises you at the delivery.
Nicolock recommends 3/4 inch clean angular stone with no fines for the column, so ask for it by name.
How Tall Can a Wall Go Without an Engineer?
Nicolock says a segmental wall under 4 feet can be built after taking the list to a local dealer. Over 4 feet, it says the wall usually needs an engineered design, a permit and soil reinforcement.
The Landscape Architect article says reinforcement is needed if conditions require it or the wall is more than 4 feet tall.
Nicolock adds that most places measure the 4 feet from the bottom of the buried course to the top, and that local limits vary, with some lower. That changes the math. Here’s an example. A wall that stands 3.75 feet above ground with one 4 inch course buried measures about 4.1 feet by that rule.
The calculator shows the height from the bottom of the buried course and adds a warning at 4 feet. Check with the local building office before you buy blocks. Don’t skip that call. Walls that hold up a slope, a driveway or a structure deserve the most caution.
For more, concrete retaining walls covers sizing, drainage and limits. Retaining wall cost shows what drives the price per square foot.
How Deep Should the Trench Be for a 4 Foot Wall?
About 12 inches. Why that number? Nicolock says a 4 foot wall needs a trench roughly 12 inches deep, which is 6 inches of compacted base plus one buried course. The Landscape Architect example also gives a trench at least 24 inches wide and 12 inches deep.
The block manufacturer’s manual has the final say, since it sets the base for that block.
A commenter on a DIY wall post in r/landscaping put the base first.
“The base layer is always the most important. Make sure you dig a good footer and compact the ABC in 4-6" lifts instead of filling the whole thing and compacting just the surface.”
– r/landscaping, February 2026 (140 upvotes)
The lift size doesn’t change the stone quantity, and sources differ on it. A trade article on patio bases says 2 to 3 inch layers, while this commenter says 4 to 6 inches for a wall base. Follow the block maker’s manual.
What the Order Still Needs
Stone and blocks aren’t the whole job, and a drain doesn’t come free. Nicolock says its calculator excludes the drain pipe, filter fabric, geogrid and construction adhesive. The Landscape Architect example for a 50 foot wall adds 50 linear feet of drain pipe, two rolls of filter fabric and 41 cap stones with three tubes of adhesive.
The calculator gives the pipe length, which matches the wall length. For fabric, French drain fabric explains which type works and which clogs.
If the wall will also take roof water, the French drain calculator sizes the trench and stone for the outlet.
Retaining Wall Questions
How do I calculate how many retaining wall blocks I need?
Divide the wall length by the block width, and the wall height by the block height. Multiply the two and add 5 to 10% for cuts. A 24 foot wall of 6 by 16 inch blocks, 3 feet high, is 18 blocks across and 6 courses. That’s 108, or 119 with 10% extra.
How much does a 100 foot retaining wall cost?
It depends on the height, the block and the labor. A block counter can’t tell you that. Retaining wall cost breaks down what drives the price per square foot. For materials, enter prices in the optional boxes above.
Do I need drainage stone behind every wall?
Both sources I read include a drainage column behind the wall, 12 inches wide in their examples. Follow the block maker’s manual for the width and the pipe. It’s the final word.
