Retaining Wall Block Calculator
Work out how many retaining wall blocks a wall needs, plus the compacted gravel base trench underneath. Enter the wall length, height and block size.
Blocks needed
Base trench assumed 6 in deep and 4 in wider than the block depth. Taller or load-bearing walls need engineered reinforcement.
How to calculate retaining wall blocks
A segmental retaining wall is built in courses, stacked rows of blocks running the length of the wall. Divide the wall length (converted to inches) by the block's face width to get how many blocks fit in one course, rounding up. Divide the wall height (in inches) by the block's face height to get the number of courses, also rounding up. Multiply blocks-per-course by courses for the base count, then add a waste allowance for corner cuts, half-blocks at wall ends, and breakage. The formula is blocks = ceil(wall length ÷ block width) × ceil(wall height ÷ block height) × (1 + waste %).
The base trench matters as much as the blocks
A retaining wall is only as stable as what it sits on. The standard approach is a compacted gravel base trench roughly 6 inches deep, wider than the block's depth by a couple of inches on each side, buried so the first course sits partly below the finished grade. This calculator estimates that trench's gravel volume using your block depth plus 4 inches of extra width, at 6 inches deep along the full wall length. Compact the gravel in lifts before setting the first course, since an uncompacted base is one of the most common causes of a leaning wall.
When you need more than block and gravel
This calculator covers the block count and base gravel for a straightforward wall. Taller walls, generally anything over about 3 to 4 feet, or walls holding back a slope, saturated soil or a surcharge load like a driveway above, usually need geogrid reinforcement layered into the compacted backfill at intervals, a perforated drain pipe behind the wall tied to daylight or a drain, and often an engineer's review. Treat those as a separate design step, not something this tool estimates.
Common block sizes
Segmental retaining wall blocks vary by manufacturer, but many popular systems use a face around 17.75 x 7 inches with a 12 inch depth, the default in the calculator above. Always check your chosen product's spec sheet for its exact face dimensions, since even a small difference changes the course count over a long wall. Once the wall height and base are planned, the backfill and any cap stones are usually ordered separately from the supplier; soil fill behind the wall can be estimated with the fill dirt calculator.