Retaining Wall Block Calculator

What this calculator does

Wall length, exposed height and block dimensions go in. Out comes the course count including the buried course, blocks per course, wall blocks and cap blocks separately, base stone and drainage stone in both cubic yards and tons, the number of geogrid rows a wall this tall typically needs, and a total at your prices.

It also tells you, unambiguously, when the wall you have described is past the height where you should be building it from a web page.

Blocks: two divisions and a multiplication

Blocks per course is the wall length divided by the block face length. Courses is the wall height divided by the block height. Multiply them.

A 30-foot wall, 36 inches exposed, using the very common 12-inch-long by 6-inch-tall segmental block: 30 blocks per course, 6 courses to reach 36 inches, plus one buried course is 7. That is 210 blocks, of which the top 30 are caps.

Two details that trip people up. Blocks per course rounds up, because a partial block at the end of a run still costs a full block — you cut it. Setback means each course sits slightly behind the one below, so a wall leans back into the hill; that is by design, it does not change the count, and it is why the wall face area is measured on the incline rather than vertically.

The buried course, which is not optional

The first course goes below finished grade. Not level with it. Below it.

This is the single most skipped step in amateur retaining wall construction and the single most common reason walls fail. The soil in front of the wall — the toe — is what resists the wall sliding forward. Set the first course on top of grade and there is nothing holding the bottom, so the whole wall rotates forward around its base over a few freeze-thaw cycles.

One buried course is the usual minimum. On a slope, or a taller wall, bury more: a common rule is one inch of embedment for every eight inches of wall height, with one full course as the floor.

Base stone: the layer you cannot fix later

Under that buried course goes compacted crushed stone — six inches minimum for a low wall, in a trench wide enough that the block bears with several inches of stone on each side. Twenty-four inches of trench for a twelve-inch-deep block is a reasonable default.

Compact it in lifts, and level it obsessively. Every error in the base course is multiplied by every course above it: a quarter-inch of slope at the bottom of a seven-course wall becomes a very visible lean at the top, and there is no correcting it once you are three courses up.

Drainage, and the physics of why walls fall over

Here is the part worth internalising.

A retaining wall is not primarily fighting the weight of soil. It is fighting water. Dry soil pushes on a wall with a force roughly proportional to its weight and its angle of repose. Saturated soil pushes with that force plus full hydrostatic pressure — and hydrostatic pressure at the base of a four-foot column of water is substantial and completely indifferent to how heavy your blocks are.

The fix is to never let the water build up:

1. A chimney of clean, open-graded drainage stone behind the full height of the wall — twelve inches wide is a common default.
2. A perforated drain pipe at the base of that chimney, sloped, and daylighted somewhere it can actually discharge. A pipe that dead-ends in the fill is decoration.
3. Filter fabric between the drainage stone and the retained soil, so fines do not migrate in and clog it over a few seasons.

The gravel figures here cover items 1 and 2’s bedding. Fabric and pipe are not counted and are cheap relative to the consequence.

The four-foot line

Nearly every jurisdiction in the US treats roughly four feet of exposed height as the threshold where a retaining wall stops being landscaping and starts being a structure — permit, stamped engineering, inspection.

Above that line, gravity alone does not hold a segmental wall. It needs geogrid: polymer mesh laid between courses and extending back into the retained fill, which recruits the soil mass itself into resisting the load. The grid length, its vertical spacing and the fill it is embedded in all come from the manufacturer’s design tables for your specific block and your specific soil. This calculator estimates the number of rows at the common every-second- course interval purely so the cost estimate is not wildly low — it is not a design.

If your wall is over four feet, get it engineered. A failed retaining wall does not fail gently, and it usually takes whatever was above it along.

What this leaves out

• Engineering. Above four feet, and sometimes below it in poor soil.
• Geogrid material cost. Rows are counted; the material is not priced.
• Drain pipe, filter fabric, and the outlet. All required.
• Excavation and spoil removal. The excavation calculator handles the cut volume and how many truckloads it becomes.
• Compaction equipment. A plate compactor is not optional for the base.
• Tiered walls. Two short walls are not equivalent to one tall one unless the setback between them is large enough that the upper does not surcharge the lower — a question for an engineer.

Once the wall is up, the paver patio calculator covers the flat surface it is usually holding back.