Retaining Wall Calculator

What the retaining wall calculator does

A block retaining wall is mostly the part you can see, the blocks, plus a fair bit you cannot, the gravel that holds it up and drains it. This estimates both. You give it the wall size and the block size, and it returns the number of blocks, the wall face area, and the cubic yards of base gravel and backfill gravel.

It covers the main bill of materials for a segmental block wall. Below is how it works and the things that matter as much as the block count.

How to use it

1. Enter the wall width and height, the run and the height of the wall, each with its unit.
2. Enter the block width and height in inches, the face dimensions of one block.
3. Press Calculate for the block and gravel estimate, or Reset to clear it.

How the block count is worked out

The blocks come from the area of the wall face divided by the area one block covers on that face:

Blocks = wall face area ÷ block face area

A common segmental block shows a face about 12 inches wide by 6 inches tall, which is half a square foot, so a wall takes roughly 2 blocks per square foot of face. The calculator rounds up to whole blocks. It is worth adding a little for breakage and cuts on top, around 10 percent, and more if the wall curves, since curves mean trimming blocks.

The base and the backfill

The two gravel figures are the hidden half of the wall. The base gravel is the compacted crushed stone the wall sits on, a level pad that keeps it from settling, figured here as a band roughly a foot wide and six inches deep running the length of the wall, with a bit added for compaction. A wider footing for a taller wall would use more, so scale it up if your base is broader.

The backfill gravel is the clean stone packed behind the blocks, taken here as a column about a foot deep behind the full face of the wall. This is the part that does the quiet work: it lets water drain away down behind the wall instead of building up and pushing it over. It is not optional, which is why the calculator sizes it alongside the blocks.

Caps and the rest of the order

A couple of things sit outside the figures here and are worth adding yourself. Cap blocks are the flat finishing blocks glued along the very top course; they close off the hollow cores and give the wall a finished edge. Caps are a different size from the wall blocks, so count them separately, roughly one per foot of wall length.

The other usual extras are a perforated drain pipe along the base of the backfill to carry water to a low point, and a layer of filter fabric between the gravel and the soil so the stone does not silt up over the years. Neither is in the block or gravel count, but both belong on the order for a wall that lasts.

A worked example: a 20 ft wall

Say the wall is 20 feet long and 3 feet high, built from blocks with a 12 by 6 inch face.

The face is 20 × 3 = 60 square feet. Each block covers half a square foot, so that is 120 blocks, and adding 10 percent for cuts brings it to about 132. The backfill works out to roughly 2.2 cubic yards of drainage stone behind the wall, plus the base pad beneath it. On top of that, count about 20 cap blocks for the 20 foot top.

So the order is the blocks, the two lots of gravel, and a row of caps, with a drain pipe and fabric to finish.

Height, drainage, and when to call an engineer

A short garden wall is a friendly weekend project, but retaining walls hold back earth, and earth is heavy and unforgiving when a wall is built wrong. Two things keep one standing. The first is drainage: trapped water behind a wall is the single most common reason they fail, which is why the backfill stone, the drain pipe, and grading water away all matter so much.

The second is height. Most areas treat a wall up to about 4 feet of exposed height as landscaping you can build yourself, but at or above that, and for any wall holding back a slope or carrying a load above it like a driveway, the rules generally call for an engineered design, often with geogrid reinforcement tying the wall back into the soil. So use this to plan and price a modest wall, and for anything tall, or anything holding back more than a flat garden bed, check your local code and have it designed by an engineer. That is the helpful path, not a hurdle.

Questions people ask

How many blocks do I need for a retaining wall?

Divide the wall face area by the area one block covers. For common 12 by 6 inch blocks that is about 2 a square foot, so a 60 square foot wall is around 120 blocks before waste.

How much gravel goes behind a retaining wall?

Plan on a column of clean drainage stone about a foot deep behind the full height of the wall. For a 20 by 3 foot wall that is roughly 2.2 cubic yards, plus a compacted base beneath the blocks.

Does it include cap blocks?

No, count those separately, about one per foot of wall length, since caps are a different size from the wall blocks. The calculator covers the wall blocks and gravel.

How tall can I build without an engineer?

Most places allow up to about 4 feet of exposed height as a do-it-yourself wall. Taller than that, or any wall holding back a slope or a load above, generally needs an engineered design. Check your local code.

References

A quick note on where these figures come from. The block count is geometry, the wall face divided by the block face. The base and backfill practice, a compacted gravel pad and a column of drainage stone behind the wall, and the roughly 4 foot threshold for engineered design follow the segmental retaining wall guidance of the Concrete Masonry and Hardscapes Association, formerly the National Concrete Masonry Association. The unit conversions follow the US National Institute of Standards and Technology guide.

1. Concrete Masonry and Hardscapes Association (CMHA, formerly NCMA), Design Manual for Segmental Retaining Walls. https://www.masonryandhardscapes.org
2. National Institute of Standards and Technology (NIST), Special Publication 811, Guide for the Use of the International System of Units (SI). https://www.nist.gov/pml/special-publication-811