Retaining Wall Calculator
Blocks, plus the base and drainage stone people forget.
Work out Retaining Wall. Blocks, plus the base and drainage stone people forget. Counts the extra post, the leftover box and the gap between boards.
Compacted gravel under the wall. Not optional.
Commonly about twice the block width.
Clean stone behind the full height of the wall.
Many jurisdictions require an engineer above about four feet, and lower where the wall retains a slope or carries a load above it. That is a local rule worth checking before you buy anything — a failed retaining wall is expensive and dangerous.
Blocks needed
100
5 courses × 20 per course
The block count is the easy part. The two quantities people leave out are both here and both large: 0.74 cubic yards of compacted base beneath the wall, and 2.22 behind it. A wall built straight onto soil settles and leans, and without drainage the water pressure behind the face is what actually brings retaining walls down.
How the Retaining Wall Calculator works
The block count is arithmetic. What gets left out of estimates is everything that is not block: the compacted base beneath the wall, and the drainage stone behind it. Neither is optional — a wall built straight onto soil settles unevenly and leans, and without drainage, water pressure builds behind the face, which is how retaining walls actually fail.
Also known as: retaining wall block calculator · how many blocks for a retaining wall · retaining wall base calculator · retaining wall gravel calculator
The calculation itself
Wall length divided by block length gives blocks per course. Wall height divided by block height gives courses. Multiply for the total, and add a course of caps.
Base volume is length times base width times base depth. Drainage volume is length times drainage width times the full wall height.
Those last two are the ones that get left off estimates, and they are not small.
In practice
A twenty-foot wall three feet high, from 12 by 8 inch blocks: 20 blocks per course over 5 courses, so 100 blocks plus 20 caps.
The base — twenty feet by two feet wide by six inches deep — is 20 cubic feet, about three quarters of a cubic yard.
The drainage stone behind it is twenty feet by one foot by three feet high: 60 cubic feet, or 2.2 cubic yards. That is three times the base and more stone than most people have budgeted for.
So a wall that reads as "a hundred blocks" is really a hundred blocks and three cubic yards of aggregate.
Why walls fail
Water, almost always. Saturated soil behind a wall exerts far more pressure than dry soil, and without a drainage layer and an outlet that pressure has nowhere to go but through the face.
The base is the other half. A wall built directly onto soil settles unevenly, and a leaning course near the bottom is magnified by everything above it. The base is compacted gravel for that reason, and it needs to be level.
Burying the first course matters too — commonly about a tenth of the wall height, and at least one full course. It is what stops the base kicking outward under load.
Where this stops being a materials question
Above about four feet, many jurisdictions require engineered design, and the threshold drops where the wall retains a slope, a driveway or a structure above it.
Those walls need geogrid reinforcement into the retained soil, and the geometry of that is genuinely an engineering calculation rather than a block count.
This page sizes materials. It does not tell you whether the wall stands up, and on anything tall or loaded that is the question that matters.
Frequently asked questions
How many blocks do I need for a retaining wall?
Wall length divided by block length gives blocks per course; wall height divided by block height gives the number of courses. A 20 foot wall 3 feet high from 12 by 8 inch blocks is 20 per course over 5 courses — 100 blocks, plus caps.
How much base material do I need?
A compacted base typically runs about twice the block width and six inches deep. For a 20 foot wall that is roughly 20 cubic feet, or three quarters of a cubic yard — and it is the single most important part of the build.
How much drainage stone goes behind the wall?
Commonly a foot of clean stone the full height of the wall. On a 20 foot wall 3 feet high that is 60 cubic feet, over two cubic yards, which is often more stone than people have budgeted for.
How high can I build without an engineer?
Many jurisdictions require engineering above about four feet, and lower if the wall retains a slope or carries a load above it. That is a local rule — check it, because a failed retaining wall is both expensive and dangerous.
Do I need to bury the first course?
Yes, usually about a tenth of the wall height, and at least one full course. It is what stops the base of the wall kicking out under load.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/retaining-wall-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Retaining Wall Calculator"></iframe>The only condition is that the credit line stays visible. It sits inside the frame, so you do not have to do anything to keep it.
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