Retaining Wall Calculator
Courses are the wall height divided by the block height, rounded up. Blocks per course are the length divided by the block length, rounded up. Caps are the length divided by the cap length. Base gravel and drainage stone are length times the width and depth you type. No block size is stored as a product. The fields start at a size you can edit.
Last reviewed: 2026-10-28 · How we calculate
How we calculate
Courses = ceil(height ÷ block height). Blocks per course = ceil(length ÷ block length). Blocks = courses × blocks per course. Caps = ceil(length ÷ cap length). Base volume = length × base width × base depth. Drain volume = length × drain width × drain depth. Lengths of the block and the cap are metres. Depths of the stone are millimetres.
Worked example
A 20 ft wall, 3 ft high, with 16 × 6 in blocks, is 6 courses and 90 blocks.
Blocks and gravel for a garden wall
A low garden wall is blocks, caps, a gravel base, and drainage stone behind the face. This page counts those from the length and height you enter and from the block size. It does not design the wall for the soil it holds. Buried courses that you still have to buy should be inside the height. Geogrid and the drain pipe are not in the block count.
Measuring a retaining-wall face
Retaining-wall counts are modular. Courses are the wall height divided by the block height, and blocks in a course are the wall length divided by the block length, both rounded up. If the length does not divide evenly, the last block is a cut. Corners and curves waste more. This formula has no waste field. The block size on the form is a common unit to edit. The block in the yard is the one that matters, including a rear lip if that lip is part of the course height the maker specifies.
How the face becomes blocks and gravel
Base and drain are trenches you describe. A 4 m wall with a 0.5 m wide base at 150 mm is 0.30 cubic metres. A 0.3 m wide drain at 200 mm is 0.24 cubic metres. Those widths and depths are not a structural section. A low garden wall and a wall holding a drive are not the same. This page will not tell you which. It also will not convert the stone to tonnes. Take the cubic metres to the gravel calculator and type the density your supplier quotes, or leave that density at the gravel page's editable 1,500 kg per cubic metre if you accept that field default as a starting point and not a certificate.
Ordering blocks, caps, and drainage stone
Cinder blocks and clay bricks are different modules on their own pages. A footing under a mortared wall is the footing calculator. Drainage pipe, fabric, and geogrid are not volumes of stone and are not estimated. If the first course is buried, include it in the height or you will be one course short. Caps that turn a corner may need a corner cap the straight division does not know about.
Checking a wall order
There is no price per block. Count the units, check the course height against a sample stack, and then ask for a price. A setback batter, where each course steps back, shortens the upper courses. This formula keeps every course the same length. If the batter is large, shorten the upper length yourself and run the courses as separate walls.
Retaining-wall fields and starting values
Wall length starts at 20 ft. Wall length in feet. Switch to m / cm for metres. Wall height starts at 3 ft. Wall height in feet. Switch to m / cm for metres. Block length starts at 16 in. Block length in inches. Block height starts at 6 in. Block height in inches. Cap length starts at 16 in. Cap length in inches. Base width starts at 2 ft. Base width in feet. Switch to m / cm for metres. Base depth starts at 6 in. Base depth in inches, or millimetres on the metric toggle. Drain stone width starts at 1 ft. Drain width in feet. Switch to m / cm for metres. Drain stone depth starts at 6 in. Drain depth in inches, or millimetres on the metric toggle.
Retaining-wall blocks for a straight face
Blocks start at 16 × 6 in. Caps and the gravel base use the form’s other imperial defaults. Those sizes are a starting point, not a wall design.
| Face | Blocks | Caps | Base gravel |
|---|---|---|---|
| 10×2 ft | 32 | 8 | 0.23 m³ |
| 20×3 ft | 80 | 16 | 0.46 m³ |
| 30×4 ft | 161 | 23 | 0.69 m³ |
Buried courses, geogrid, and the pipe are not in the block count.
Common mistakes
- Using a catalogue block you are not buying. Measure the unit.
- Leaving buried courses out of the height and then running short.
- Treating the base width as an engineered heel.
- Expecting wall drainage pipe or fabric in the stone volume.
- Reading the volumes as tonnes. Density is not on this page.
- Ignoring a corner where two faces share a block.
Frequently asked questions
How many retaining wall blocks do I need?
Divide the height by the block height and the length by the block length, round each up, and multiply. The example is 40 blocks.
How many caps?
Divide the length by the cap length and round up.
How much gravel is under the wall?
Length times the base width times the base depth you enter.
Does this design the wall?
No. It counts units and stone volumes from sizes you type. Tall walls need an engineer.
Where is the mortar?
Segmental retaining blocks are often dry-stacked. If you need mortar bags, use the mortar calculator with a coverage you type.
Can I weigh the gravel?
Not here. The gravel calculator will turn the cubic metres into kilograms after you enter a density.