How to use the retaining wall block calculator
- Enter the wall length and the height above ground in feet.
- Enter your block face length and block height in inches from the block's specifications. The defaults are an example block.
- Set how many courses are buried below ground, whether to add cap blocks, and a percent extra for cuts and breakage.
- Enter the gravel backfill width and the base trench depth and width. Read the number of wall blocks, caps and cubic yards of gravel.
Formula
Exposed courses = wall height / block height, rounded up
Courses = exposed courses + buried courses
Blocks per course = wall length / block length, rounded up
Wall blocks = courses x blocks per course x (1 + extra %), rounded up
Gravel in cubic yards = length x height x width / 27, with all dimensions in feet
Backfill height is the full height of all courses. Cap blocks are counted by length and are not part of the wall height.
Worked example
A wall 30 feet long and 3 feet high with 12 x 4 inch blocks needs 36 / 4 = 9 exposed courses plus 1 buried, so 10 courses of 360 / 12 = 30 blocks, or 300 blocks. With 5% extra that is 315, and 32 caps.
Backfill 12 inches wide behind 40 inches of block face is 3.7 cubic yards, and a 24 inch wide, 6 inch deep base is 1.11 cubic yards, so about 4.81 cubic yards of gravel in all.
Use your block's numbers
Segmental blocks come in many sizes and shapes, and manufacturers publish the face dimensions, buried course rule, base requirements and maximum height without engineering. The sizes here are only an example, so replace them with the data sheet for your block. See the concrete block calculator for general masonry and the paver calculator for the base layer approach.
Drainage and backfill
Water behind a wall is the main cause of failure, which is why walls use free draining gravel and often a perforated drain pipe at the base. The gravel quantity is a volume, and the weight depends on the stone, so ask your supplier. The mulch and gravel calculator converts volume to weight and bags.
When a wall needs an engineer
Short garden walls are often built by owners, but taller walls, walls holding up a driveway or a building, and walls on a slope can fail in ways that hurt people. Many local codes require engineered plans above a certain height, and the page warns above 4 feet as a prompt to check with your building department. This tool does not design a wall.
Assumptions and limits
- Block size, buried courses, the extra percent and all gravel dimensions are example values you should replace with your block's instructions.
- The wall is straight with one block thickness and no setback, curves, corners or steps.
- Geogrid, drainage pipe, fabric, adhesive and fasteners are not counted.
- This is a materials estimate, not an engineering design or a code check.
Frequently asked questions
How many blocks do I need for a retaining wall?
Divide the wall length by the block length for blocks per course, divide the height by the block height for courses, multiply, and add extra for cuts. The tool does this and adds the buried course.
How many courses should be buried?
Many block makers call for at least one buried course, but the rule depends on the block and wall height. Follow the manufacturer's instructions and enter their number.
How much gravel goes behind a retaining wall?
It depends on the design. An example layout uses a strip of free draining gravel about a foot wide behind the wall, but your block instructions or engineer decide. Enter that width.
Do I need a permit?
Often for taller walls, and sometimes for any wall near a boundary or structure. Ask your local building department before you start.