The wall is the easy part
Counting blocks is arithmetic. What decides whether the wall is still
standing in ten years is underneath and behind it, which is why this
calculator reports the base and the backfill alongside the block count.
per course = ceil(wall length in inches ÷ block length)
courses = ceil(wall height in inches ÷ block height)
blocks = per course × courses × (1 + waste)
base = 6 in compacted gravel + 1 in levelling sand
backfill = 12 in of drainage gravel, full height
Bury the first course
The bottom course goes fully below finished grade, sitting in the
compacted base. It is what stops the foot of the wall walking forward
under the pressure behind it. A wall built up from grade looks the same on
the day it is finished and behaves very differently after a wet winter.
Water is the thing that pushes walls over
Saturated soil exerts far more pressure than drained soil. The foot of
gravel behind the wall gives water a path down and out rather than a
reservoir behind the blocks — pair it with a perforated pipe at the base,
which the French drain
calculator sizes, and the wall is doing the job it was built for.
The base gravel itself is the same crushed stone the
gravel calculator
prices by the ton.
What this will not tell you
- Geogrid spacing. It depends on your soil, the surcharge
and the block system. Manufacturers publish tables; use theirs.
- Whether your wall needs engineering. It flags the
common four foot threshold, but your local code decides.
- Anything about surcharge. A driveway, a slope or a
building above the wall changes the design entirely.
A 30 ft wall of 12 × 8 in block, by height
Base is 6 in of gravel plus 1 in of sand under the buried first course. Backfill is 12 in of drainage gravel for the full height. Walls over about 4 ft normally need engineering.
Common questions
How many blocks do I need for a retaining wall?
Blocks per course is the wall length in inches divided by the block length; courses is the height divided by the block height. A 30 foot wall of 12 inch blocks is 30 per course, and 36 inches of height in 8 inch blocks is five courses — 150 blocks before waste. Block sizes vary far more between manufacturers than brick or CMU, so measure yours.
How deep should the base be?
Six inches of compacted gravel with an inch of levelling sand on top, and the first course buried in it entirely. That buried course is not optional — it is what stops the base of the wall sliding forward under the load behind it, and a wall that starts at grade has nothing holding its foot.
How much gravel goes behind a retaining wall?
A foot of clean drainage gravel for the full height, running the length of the wall. This is the part most DIY walls skip and it is why most DIY walls fail: water trapped behind a wall exerts far more pressure than the soil does, and gravel plus a drain gives it somewhere to go instead.
When does a retaining wall need engineering?
Commonly above about four feet, and many jurisdictions write that into code. Above that height the forces stop being something a rule of thumb handles, and the wall needs designing for your actual soil and surcharge. This calculator flags the height but will not design around it.
Do I need geogrid, and how far apart?
Possibly, and this tool deliberately will not tell you the spacing. Geogrid requirements depend on your soil type, the surcharge behind the wall and the specific block system, and the manufacturers publish their own tables for exactly that reason. A spacing figure invented by a calculator is worse than no figure at all on something holding back a hillside.
Do I need cap blocks?
They finish the top course and shed water off it, and they are usually glued rather than laid. You need one per block in the top course, which is what this calculates when you include them. Some systems use a purpose-made cap that is a different length from the wall block, so check before ordering.
Sources
- Default waste allowance
- 5%
Base and backfill now follow Allan Block's own residential installation manual rather than the implementation plan: a 24 in wide trench with at least 6 in of compacted wall rock, and 12 in of wall rock behind the block. The plan's 1 in of levelling sand was wrong for a segmental wall — these are bedded on compacted aggregate, and Allan Block permits coarse sand only up to half an inch and only for minor adjustment, which is what is stored here. Geogrid layout stays a site-specific engineering design: layer count, spacing, length and strength all vary with the wall, so no geogrid quantity is computed and none should be. VERSA-LOK puts the height at which most segmental units need it at roughly three to four feet, sooner with slopes, surcharge or poor soil.
Unit conversions are exact by definition and need no source.
How we choose and cite these figures