Retaining Wall Block Calculator for a Garden Wall

Calculate blocks, base and backfill

Four numbers come out of one measurement pair. Length along the face, and the height you want showing when it is finished.

But the height you enter is not the height that gets built. A course goes below grade. So a wall showing 3 ft in Pavestone RockWall Small is really 3.33 ft of block, which is 10 courses rather than 9. That single course is where most block orders come up short.

Read that table as a sequence rather than a shopping list. The base goes in and gets compacted before a single block is set, and the backfill goes in course by course as the wall rises. Order it so the stone arrives before you need it, not after the blocks are already stacked.

How many blocks do you need?

Courses × blocks per course. Blocks per course is wall length in inches ÷ the face width of one block, rounded up. Courses is total wall height ÷ block height, also rounded up.

Block face size changes the count far more than it changes the cost. A Pavestone RockWall Large covers 2.2 times the face of a Pavestone RockWall Small, so the same wall takes roughly that many fewer pieces. Fewer, heavier lifts against more, lighter ones.

Buy a few spare. Blocks arrive chipped, and corners break when you split one for an end. Come back for six more and you often get a different pallet in a slightly different color. Five percent over is the usual allowance.

Ends and corners are the other thing the sum misses. A wall that turns, or that steps down into a slope, needs cap units and sometimes a corner block, and those are sold separately. Count them off the drawing rather than hoping the spares cover it.

Base gravel depth and quantity, and why it is wider than the wall

6 inches of compacted crusher run, in a trench 12 in wider than the block. Walls over 4 ft go to 8 in.

The extra width is not padding. A wall pushes down and slightly forward, so the load spreads into the ground at an angle rather than straight down. Cut the base to the exact width of the block and there is nothing under that spread. The front edge settles first, and the wall tips forward.

Compact it in two passes rather than one, and get it flat with a level rather than by eye. Every error in the base multiplies as the wall goes up. A quarter inch out on the bottom course is visible by the fifth, and by then there is no fixing it without taking the wall back down.

Backfill gravel behind the wall, and how wide the zone has to be

12 inches of clean washed stone. It goes directly against the back of the blocks and runs the full height of the wall. On the 20 ft example that is 2.5 cubic yards, or about 3.5 tons.

Clean is the operative word. Retaining wall backfill has to be washed stone, with no fines or dust in it. Fines migrate down through the gaps and settle into the voids. So the zone seals up. And a sealed drainage zone is worse than none at all, because now it holds water right against the wall.

Fill it as you build. Set a course, shovel stone in behind it, compact that lift, set the next course. Compacting in 6 in lifts is the standard, and it matters because loose backfill settles later and takes the top of the wall with it.

A layer of landscape fabric between the stone and the soil behind it is worth the twenty minutes. Without it, soil works its way into the stone over a few seasons and does the same sealing job the fines would have done.

Drainage: what goes where

Bottom to top, the order is base, pipe, stone, fabric, soil. Block retaining wall drainage is really just that sequence, and getting it right is most of what separates a wall that lasts from one that bulges.

The pipe is 4 in perforated, laid at the bottom of the stone zone with the holes facing down. Holes down looks wrong and is correct. Water rises into the pipe from below, and holes up would let it fill with sediment falling through the stone.

But it has to go somewhere. A pipe that runs the length of the wall and stops is a trough, not a drain. Fall it slightly to one end and daylight it out the side. That is why the tool adds ten feet to the run.

Now cap the top with soil rather than more stone. A few inches of compacted soil over the drainage zone stops surface water pouring straight down into it every time it rains. The pipe then only handles what comes through the slope.

Most people miss that last step and finish the wall with stone right up to grade, because it looks tidier. It also turns the whole backfill zone into a funnel that collects every drop landing on it.

Curved and circular walls, and how the count changes

Measure a curve along its face, not across it. Run a tape or a string along the line the wall will follow and use that length. The block count works out the same way from there.

Blocks made for these walls have a slight taper, wider at the front than the back, so a course closes into a curve without cutting. Check the maker’s minimum radius. It is usually near 3 ft on the small units. Tighter than that and you are cutting every block on the inside of the bend.

Convex and concave curves behave differently as you go up. Curving away from the soil, each course sits on a slightly longer line and the count creeps up. Curving into it, the line shortens and blocks come out. Neither moves much on a garden wall. But on a tight circle around a tree or a fire pit, walk the string at the top course as well as the bottom.

For a full circle, the length is the circumference. Diameter × 3.14 gives it, and add one block, because the last one in a ring almost never lands flush.

Block weight and delivery, and what a pallet actually is

Retaining wall block weight runs 22 to 40 lb across the common sizes. That sounds manageable until you multiply it. The 20 ft example is 210 blocks, which is 4,620 lb, or 2.3 tons of concrete moved by hand.

Here is the part that reads backwards. A pallet of the small blocks weighs 3,168 lb, and a pallet of the large ones only 1,920 lb. There are 144 small blocks on there against 48 large, so the lighter block makes the heavier pallet.

Either way it arrives on a truck with a forklift, and it gets set down once. Work out where before it turns up. Moving a pallet of blocks across a lawn happens one armful at a time.

Order the stone for the same day or earlier. The base has to be in and compacted before any block is laid, so a delivery that arrives after the pallets leaves you stacking blocks twice.

And plan the gravel for retaining wall drainage as a second drop, separate from the base. They are different materials going to different places, and a yard that sends one loose load of each is doing you a favor. One mixed pile means sorting it with a shovel.

What a block retaining wall costs

$3 to $7 a block for the garden sizes, and $35 to $55 a ton for the stone. The 20 ft example works out near $1,032 in materials, before delivery and before the pipe.

The stone column is the interesting one. It runs roughly a fifth to a quarter of the material bill, and it is the line people cut when the quote comes in high. It is also the line that decides whether the wall is still straight in ten years.

Installed, expect $25 to $60 a square foot of wall face depending on access and region. Labor is most of that. Every block gets handled several times, and the base takes longer than the stacking does.

So where does the money actually go? Not the blocks. On a typical garden wall the blocks are under half the material bill and well under a third of the installed price. Access drives the rest, because a wall a wheelbarrow can reach costs a fraction of one that needs everything carried down steps.

Related calculators

• Crusher run gravel calculator for the base and backfill stone priced on its own, across eight grades.
• French drain calculator where the drainage run is a project rather than a strip behind a wall.
• Recycled aggregate as a cheaper base under a wall nobody will ever see the bottom of.
• All the project calculators, for jobs that need several materials in the right order.