Retaining Wall Calculator
Find how many wall blocks, cap stones and bags of gravel you need for a garden retaining wall.
How to use this retaining wall calculator
Measure the length of the wall along the ground in feet, and how tall it will stand above the finished ground. Pick the block size closest to yours, using the size of the face you see on the front of the wall. The calculator adds one extra row that gets buried below ground, which most block makers recommend, and 5% extra blocks for cuts and breakage.
It also estimates the crushed gravel for a 6-inch-deep, 2-foot-wide leveling trench, plus a 12-inch-wide strip of drainage gravel behind the wall.
The math behind it
- Rows = wall height in inches ÷ block height, rounded up, + 1 buried row.
- Blocks per row = wall length in inches ÷ block width, rounded up.
- Caps = blocks per row (one cap per block on top).
- Gravel = length × (0.5 ft × 2 ft trench + height × 1 ft backfill), in cubic feet.
Example: a 20 ft wall standing 2 ft tall with 12 × 4 inch blocks needs 7 rows (6 + 1 buried) of 20 blocks. That's 140 blocks, or 147 with 5% extra, plus 20 caps and about 2.2 cubic yards of gravel.
Building tips
- Get the first row perfectly level. Every mistake there gets bigger as the wall goes up.
- Stagger the joints like brickwork, so each block sits over the seam below.
- Run a perforated drain pipe at the base of the gravel behind taller walls so water can escape.
- Glue caps down with masonry adhesive.
Often used together
Quick answers
How many retaining wall blocks do I need per square foot?
Divide 144 by the face size of one block. A 12 × 4 inch block works out to 3 blocks per square foot of wall face, and a 16 × 6 inch block to 1.5.
How deep should the first row of blocks be buried?
A common rule is to bury one full row, or about 1 inch for every 8 inches of wall height, whichever is more. Always follow the block maker's guide.
Do I need gravel behind a retaining wall?
Yes. A strip of crushed gravel at least 12 inches wide behind the wall lets water drain away. Without it, water pressure builds up and can push the wall over.