Retaining Wall Calculator

Retaining walls fail for one reason far more often than any other, and it is not the blocks. It is water. Saturated soil behind a wall pushes with roughly twice the force of the same soil dry, and a wall built to hold dry soil will lean, bulge and eventually topple once the drainage stops working or was never there.

So a quantity estimate for a retaining wall has to cover three things that are invisible in the finished job: the buried course, the base aggregate under it, and the drainage stone behind it.

The buried first course

The bottom course goes below finished grade. The common rule is one inch of burial for every eight inches of exposed wall height, with a minimum of one full course whatever the height. A 3 foot wall therefore buries about 4½ inches, and since a standard block is 6 inches tall, one full course goes under.

That buried course is what resists the wall sliding forward at its base. It sits on 6 inches of compacted crushed aggregate, in a trench dug wider than the block, usually 6 inches clear on each side. Levelling that first course is most of the work of the whole wall: every error in it is repeated and magnified in every course above.

Drainage, which is the actual structure

Behind the blocks goes a zone of clean crushed stone, typically 12 inches deep and running the full height of the wall, with a perforated pipe at the bottom daylighting to somewhere water can leave. Filter fabric separates the stone from the soil so that fines do not migrate in and clog it.

The reason this matters is worth stating in numbers. Dry soil exerts a lateral pressure of roughly 30 to 45 pounds per square foot per foot of depth. Saturated soil roughly doubles it, because water adds hydrostatic pressure on top of the soil weight. A wall that is comfortable holding dry soil is not comfortable holding wet soil, and the failure comes without warning after a heavy autumn.

The pipe has to go somewhere. A drain that runs to a dead end fills up and the wall is back to holding water.

Setback and batter

Segmental blocks are made with a lip or a pin system that sets each course back slightly from the one below, typically about an inch. That backward lean, called batter, leans the wall into the slope it is holding, and it is doing structural work rather than decoration. Building a segmental wall dead plumb by grinding off the lip removes it.

When you need an engineer

Most jurisdictions require a design by a licensed engineer above about 4 feet of exposed height, and some set the threshold at 3. Even below the threshold, an engineer's input is worth having when any of the following is true:

• The wall holds a slope rather than level ground behind it, which adds surcharge.
• A driveway, patio or parking area sits within a distance of the wall roughly equal to its height.
• The soil is clay, or the water table is high.
• The wall is terraced above another wall, in which case the upper wall loads the lower one.

Walls above about 4 feet, and many below, need geogrid: layers of synthetic mesh laid between courses and extending back into the retained soil, which turn the soil itself into part of the structure. Geogrid length and spacing come from a design, not from a rule of thumb, and this calculator does not estimate it.

What the numbers above cover

Blocks, caps, base aggregate and drainage stone, plus pipe and filter fabric. Not included: geogrid, construction adhesive for the cap course, corner and step blocks, and the excavation itself, which on anything more than a garden wall is usually the largest single line in the quote.

Before excavating, have buried services located. In the United States that is a free call to 811 and is legally required in most states.

Cap blocks

The top course is finished with cap units, glued down with a masonry construction adhesive rather than laid dry. Caps are usually a different size from the wall blocks, so the count is one course worth rather than a proportion of the total, and on a curved wall they often need cutting to close the radius.