Retaining Wall Block Calculator
Calculate how many blocks you need for your retaining wall, based on the wall's length and height and the size of the blocks you'll use.
How to use the calculator
- Enter how long and tall the wall should be.
- Enter the length and height of the blocks you plan to use.
- See how many blocks you need in total, with 5% extra for waste and cutting.
How it's calculated
The number of blocks per row is rounded up to the nearest whole block (you can rarely use a fraction of a block lengthwise), and the number of rows is rounded up to the nearest whole row. A small 5% allowance is included for cutting, fitting, and breakage.
Drainage behind the wall decides whether it stays standing
Most retaining walls that fail do not give way because the blocks were too small, but because water collected behind them. Saturated soil weighs far more than dry soil and exerts a hydrostatic pressure against the wall that can multiply the load it was designed for. A drainage layer of crushed stone is therefore normally placed behind the wall, with a perforated pipe at the base to lead water away rather than letting it build up.
In cold climates frost adds to this. Water freezing in the backfill expands and pushes the wall outwards, and repeated freeze-thaw cycles through winter can gradually walk a wall out of position over a few seasons. Well-drained backfill free of fines is the answer to this as well, since it is the water that freezes — not the crushed stone. Walls above a certain height should also be engineered rather than merely quantified.
Frequently asked questions
Why is 5% waste added?
Because some blocks need to be cut to fit at edges or corners, and some may break during transport or installation. 5% is a common industry rule of thumb.
Do I need a foundation under the retaining wall?
For low walls (under about 1 meter) well-compacted gravel is often enough, but taller walls should have a poured foundation and good drainage behind the wall. Check local building codes and consider a professional for taller walls.
Can I use this for a curved wall?
The calculator computes for a straight wall. For a curved wall, measure the actual curve length and use that as the "wall length".