Retaining wall calculator
Retaining wall
calculator
Enter the length and the height you want showing, and get the whole wall: courses including the one buried below grade, caps, the stone for the levelling pad, the drainage backfill behind it and how many geogrid layers the height calls for.
Enter measurements ↓Optional
What will it cost?
Every material the calculator worked out gets its own price. Each field starts at a typical US figure so you have something to work from — change any of them to what your own supplier charges.
These prices are approximate. They are national US retail figures, and what you actually pay varies with your region, your supplier, the season and how much you buy at once. Treat them as a starting point, not a quote — always price the job from your own supplier's current figures before you order.
Your prices
More in this series
Put these figures to work
Every wall here is counted the same way underneath. Start wherever your job starts; the figures carry across.
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- Concrete Block Sizes and Weights Nominal and actual CMU dimensions, weights and course heights
- Brick Dimensions and Course Heights US brick sizes, bricks per square foot, course heights and wall ties
- Paver Sizes and Weights Paver sizes and thicknesses, weight each and what a pallet weighs
Based on your result
What you'll need
Quantities update after every calculation. More than half the material on a retaining wall is stone you never see — the levelling pad and the drainage behind the face.
- Wall blocks
- Segmental units, including the buried course
- Calculate above
- Cap blocks
- The finished top course, usually glued down
- —
- Crushed stone
- Levelling pad plus drainage backfill
- —
- Geotextile fabric
- Wraps the drainage stone so soil cannot clog it
- —
- Perforated drain pipe
- At the back of the trench, vented to daylight
- —
- Geogrid
- Soil reinforcement, laid between courses
- —
The maths
How this retaining wall calculator works
The height you enter is the height you want to see. The wall itself is taller, because the bottom course is buried, and it leans backwards, because every course steps back on the lip of the one below. Both of those change the block count.
buried = 6 in + 1 in per ft of wall
blocks = (exposed + buried) × per course
base stone = length × 2 × block depth × 6 in
drainage = length × 1 ft × wall height
- 6 in + 1
- per ft of wall, buried
- 3/4 in
- setback per course
- 6 in
- compacted levelling pad
- 12 in
- drainage stone behind
- 3 ft
- above this, geogrid
- 4 ft
- above this, engineering
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01
Bury the base course
Six inches below grade plus another inch for every foot of wall — a 6 ft wall wants a full foot buried, which is two courses. That buried section is what stops the base sliding forward under load.
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02
Let the wall lean back
Each course steps back about three quarters of an inch on the lip of the block below. Over seven courses that is more than five inches of lean into the slope, and it is doing real work.
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03
The stone matters more than the blocks
Six inches of compacted stone under the wall and a foot of free-draining stone behind it. Water pressure, not soil weight, is what pushes most failed walls over.
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04
Above three feet, reinforce the soil
Geogrid layers run back into the fill every other course. Above four feet of exposed height you are into engineered territory and a permit in most of the US.
Good to know
Retaining wall questions, answered
Building the same wall in standard CMU instead of segmental block? Use the concrete block calculator.
Divide the exposed height by the block height, round up, then add the buried course — usually one. Multiply by the wall length divided by the block length. A 30 foot wall showing 3 feet in standard 12×8 block is 6 courses of 30, so 180 blocks plus caps.
Six inches plus one more for every foot of wall height. A 3 ft wall wants 9 in buried, a 6 ft wall wants 12 in — rounded up to whole courses, that is two courses on anything much over 2 ft. It sits on compacted stone, never on soil.
On any wall over four feet, on every reinforced wall, and anywhere the soil drains badly. A 4 in perforated pipe goes at the lowest point at the back of the trench and is vented to daylight roughly every 50 ft — a pipe with nowhere to go is just a reservoir.
The same clean drainage stone that goes behind the wall. The cores are part of the drainage path, not dead space, and they are counted separately in the result above so you can see how much of the order they account for.
About 60 per cent of the wall height, and never less than four feet. It is laid between courses and runs into the compacted fill behind, so the wall and the soil behind it act as one mass instead of a face holding back a bank.
The lip on the back of each block sets it back about three quarters of an inch from the course below. That batter tilts the face into the slope, so the weight of the wall works against the soil pushing it out rather than with it.
A foot of free-draining crushed stone the full height of the wall, with a perforated pipe at the bottom daylighted to somewhere. Water building up behind a wall is the single most common reason they fail.
Generally above three feet of exposed height, laid back into the fill every other course. It reinforces the soil mass behind the wall so the whole block acts together instead of the face taking the load alone.
Usually above four feet of exposed height, and often lower if the wall carries a surcharge — a driveway, a slope, a structure above it. Above that threshold most US jurisdictions want an engineered, stamped design. This calculator sizes materials; it is not a design.
Yes. Caps have no lip to lock them in and will walk off under frost and foot traffic. A bead of concrete adhesive on the top course is standard, and it is the last thing you do.