Retaining Wall Calculator — Blocks, Base and Backfill
Counts the buried base course most people forget, then the blocks, caps, base gravel and drainage stone behind the wall.
Take this list to the store
We'll email the quantities above, what you typed in, and a link that reopens this calculator already filled in — so you can adjust it on your phone in the aisle.
How to work it out
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Measure the length and the exposed height
Length along the face of the wall. Height is what you want to see above ground — the calculator adds the buried course on top of that.
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Bury the first course
The bottom course sits below grade on a compacted gravel base, typically one full block deep. It is what stops the wall sliding forwards under the load behind it, and it is the course everybody forgets to order.
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Level the base obsessively
Six inches of compacted gravel, and the first course level in both directions to within about an eighth of an inch. Every course above copies the first, so an error at the bottom is magnified all the way up.
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Drain behind the wall
A foot of clean drainage stone against the back of the blocks, with a perforated pipe at the bottom running out to daylight. Water pressure is what pushes retaining walls over — far more than the soil itself.
Blocks by wall length
12 in. long blocks. Courses include one buried base course.
| Wall length | 2 courses | 3 courses | 4 courses | Per course |
|---|---|---|---|---|
| 10 ft | 20 | 30 | 40 | 10 |
| 20 ft | 40 | 60 | 80 | 20 |
| 30 ft | 60 | 90 | 120 | 30 |
| 40 ft | 80 | 120 | 160 | 40 |
| 50 ft | 100 | 150 | 200 | 50 |
| 75 ft | 150 | 225 | 300 | 75 |
| 100 ft | 200 | 300 | 400 | 100 |
Caps are one per block in the top course, so the same as the per-course figure.
When you need an engineer
| Exposed height | What is needed |
|---|---|
| Under 3 ft | Usually a DIY job. Check local rules anyway. |
| 3–4 ft | Many areas require a permit. Geogrid reinforcement is common. |
| Over 4 ft | Engineered design and a permit almost everywhere. |
| Any height with a slope above | Get advice — the load is far higher than it looks. |
| Any height carrying a driveway | Engineered. A vehicle surcharge changes everything. |
Four feet is the usual threshold in the US, but it varies. One phone call to your building department settles it.
How many blocks for a 20 ft wall, 2 ft high?
With 12 in. long, 6 in. high blocks: 20 courses of blocks across (20 ft × 12 ÷ 12), and 2 ft of exposed height is 4 courses — plus one buried base course, so 5.
That is 100 blocks, plus 20 caps. Order 5% over — blocks chip in handling and on the saw.
The buried course is not optional
A retaining wall holds back soil that is constantly trying to push it forwards. The buried bottom course, sitting in a trench on compacted gravel, is what resists that push at the base.
Build straight off the ground and the wall gradually walks outwards — slowly enough that you notice a year later, by which point the fix is taking the whole thing down.
Rule of thumb: bury one full course, or one inch for every eight inches of wall height, whichever is more.
Water is what knocks walls down
Saturated soil weighs far more than dry soil and exerts hydrostatic pressure on the back of the wall. Most retaining wall failures are drainage failures, not strength failures.
Twelve inches of clean angular drainage stone behind the blocks, a perforated pipe at the base running out to daylight at one end, and geotextile between the stone and the soil so fines cannot wash in and clog it.
Setback and geogrid
Most segmental blocks have a lip that sets each course back slightly from the one below, leaning the wall into the slope. Do not fight it — that batter is part of the design.
Above about three feet, or with a slope or driveway above, the wall needs geogrid: sheets of reinforcing mesh laid between courses running back into the retained soil. That is the point where this stops being a weekend project and starts needing a drawing.
Curves, corners and terraces
Curves — most segmental blocks are tapered slightly at the back so they form a curve naturally. Outside curves open the joints at the back and need no cutting; inside curves tighten them, and above a certain radius you have to trim the back corners. Add 10% blocks for a curved wall.
Corners — 90 degree corners need dedicated corner blocks, finished on two faces, laid alternating direction on each course so the corner interlocks rather than forming a straight vertical joint.
Terraces — two short walls beat one tall one, but only if they are far enough apart. The rule of thumb is that the upper wall must sit back from the lower by at least twice the lower wall's height. Closer than that and the upper wall's load lands on the lower one, and you have effectively built one tall wall with a weak middle.
Why the first course takes half the time
Experienced wall builders spend roughly half the total job on the base course, and it is the right allocation.
Every course above copies the one below. A base block an eighth of an inch out of level is an inch out by the sixth course, and there is no correcting it partway up — the blocks interlock in one position.
Dig the trench, compact six inches of gravel, then set each base block individually with a level in both directions, tapping it down into the gravel rather than packing underneath it. Check across to the neighboring block as well as along the wall.
Once the base course is right, the rest of the wall goes up surprisingly fast.
How this is worked out
No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.
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blocks per course = length × 12 ÷ block lengthRounded up — part blocks still cost a whole block. -
courses = exposed height ÷ block height, + 1The extra course is buried below grade. -
blocks = per course × coursesThe full wall. -
caps = blocks per courseOne cap per block along the top. -
base gravel = length × (block depth + 6 in.) × 6 in. ÷ 27A 6 in. compacted base, wider than the block. -
drainage stone = length × 12 in. × wall height ÷ 27A foot of clean stone behind the full height.
Common questions
How many blocks do I need for a retaining wall?
Divide the wall length in inches by the block length for blocks per course, then divide the exposed height by the block height and add one for the buried base course. Multiply the two.
How deep should the base course be buried?
One full block, or one inch for every eight inches of wall height — whichever is greater. It sits on six inches of compacted gravel below that.
Do I need drainage behind a retaining wall?
Yes, and it matters more than the blocks. Twelve inches of clean stone, a perforated pipe at the base running out to daylight, and geotextile between stone and soil. Most wall failures are water failures.
How high can I build without an engineer?
Usually up to four feet exposed, but it varies by jurisdiction and drops sharply if there is a slope or a driveway above. One call to your building department settles it before you buy anything.
What is geogrid and do I need it?
Reinforcing mesh laid between courses and extending back into the retained soil, tying the wall to the ground behind it. Generally needed above about three feet, and always where there is a slope or vehicle load above.
Should the wall lean back?
Yes. Most segmental blocks have a lip that automatically sets each course back about an inch, leaning the wall into the slope. That batter is part of the design — do not try to build it plumb.
How much base gravel do I need?
A trench six inches deep and about six inches wider than the block, filled with compacted crusher run. For a 20 ft wall that is roughly half a cubic yard.
Do I need cap blocks?
Not structurally, but they finish the top, stop water sitting in the block cores, and are usually glued down with construction adhesive. One cap per block in the top course.
Where these figures come from
Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.
- National Concrete Masonry Association — segmental retaining wall design, base and drainage detailing
- Allan Block and Versa-Lok installation guides — block dimensions, setback and buried course depth
- International Residential Code R404.4 — when a retaining wall requires engineered design
Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 11, 2026.