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
retaining wall calculator reports the base and the backfill alongside the
block count rather than stopping at the courses.
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.
The rest of the groundwork
A retaining wall is mostly a hole with stone in it. The
gravel driveway calculator
works out base stone by the ton the way a quarry sells it, the
french drain calculator
covers the drainage that runs behind a wall, and the
topsoil calculator handles
the backfill going in above it. Building in block or brick instead? The
concrete block calculator
and the brick calculator both
include the mortar.
Common questions
Can I build two short walls instead of one tall one?
That is terracing, and it is the usual way to get a tall slope retained without an engineered design — two three foot walls each sit under the four foot threshold that one six foot wall does not. Set the terrace count and the calculator splits the rise between the walls and buries and caps each in its own right, which is why two three foot walls take more block than one six foot wall rather than the same. Whether the terrace actually helps is a soil question though, not an arithmetic one: set too close together, the upper wall sits inside the lower one’s zone of influence and the pair behave as one tall wall. The setback that avoids that comes from an engineer or the block manufacturer.
Do I count the buried course?
Yes, and this page now does it for you. Allan Block bury the bottom of the wall — one inch for every foot of finished height, on top of the base itself — so it cannot be pushed out at its foot. Enter the height you want showing and the calculator adds the burial, works the courses out from the total, and tells you how many of the blocks you are buying will never be seen. Counting courses from the exposed height alone leaves you a course short on essentially any wall.
What is batter, and does it change what I order?
Batter is how far the wall leans back into the bank, and segmental block is made in three, six and twelve degree versions — it is a property of the pallet rather than something you choose on site. Say which yours is and the ticket gives the setback per course and in total, which is what decides how much ground the top of the wall gives back. A twelve degree block on a five course wall sets back about eight and a half inches, and people are routinely surprised by that once it is built.
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.
