EstimatorKitFree retaining wall calculator
Block, base and stone for a segmental wall
Enter a wall length and the height showing above grade and this returns everything a segmental wall is built from: courses of block, cap units, compacted base, the clean-stone chimney behind it, the pipe at the bottom of that chimney, cap adhesive, and geogrid at whatever elevations your design names. Wall height for code purposes is measured from the bottom of the base and not from the ground you are standing on, and the model threshold is 48 inches of unbalanced fill measured that way — above it, a retaining wall is an engineered, permitted design. This page counts material; it does not assess a wall.
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- 7 line items
- Setback and batter
- Grid entered, never proposed
The mid-size segmental unit. Six-inch courses give a finer setback ladder and land on more grid elevations. Overwrite any dimension with your unit’s own.
Exposed height is what shows above finished grade. The buried courses are added below, and the two together are the height the code text measures.
Base defaults to the block depth plus 12 in so the pad runs past the front face and the back of the unit. Twelve inches of clean stone behind the wall is the usual chimney.
Adhesive bead and geogrid from your design
Both of these come off a stamped drawing. Leave them blank and no grid is counted; filling them in converts a schedule you already have into square yards, and nothing here proposes one.
7 courses of 30, plus caps
210 block + 27 caps
- Base aggregate
- 2.29 tons
- Drainage stone
- 6.48 tons
- Drain pipe
- 50 lf
- Cap adhesive
- 3 tubes
- Spoil to move
- 3.4 cu yd
Total wall height, bottom of base to top of block: 3' 6"
The model code sets its threshold at 48 in of unbalanced fill, measured from the bottom of the footing or base — not from the grade you are standing on. Above that line a retaining wall is an engineered design: a permit, a soils assessment, and a drawing signed by an engineer. Measured that way, this wall is below that line. Jurisdictions commonly set a lower number of their own, and the one that governs is where the wall is being built.
A surcharge puts a wall of any height on the same footing: a driveway or parking area above it, ground that keeps rising back from the top, a pool, or a footing inside the wedge of soil behind the wall.
This page counts block, stone, pipe, grid and adhesive. It does not evaluate a wall, and no number on it is a finding that one is safe.
Geometry
6 courses show above grade and 1 is buried below it. At 0.75 in of setback per course the face leans back 7.13° from vertical, so the top course sits 4.5 in behind the bottom one — measure the finished wall from the base, or the string line will be wrong by that much.
The trench goes 12 in deep and 22 in wide, which leaves 3.4 cu yd of spoil to move once it has bulked — excavated soil bulks about a quarter over its in-place volume once it is dug, and it moves with soil type — a heavy clay swells more than a sandy loam, and neither one goes back at bank volume.
The two stones are not the same stone
The pad under the wall is dense-graded and gets compacted, so it is ordered at 7.5 in loose to finish at 6 in — 20% of the depth goes away under the plate. Weight uses 2,700 lb per cubic yard, loose delivered weight, typical for crushed limestone base.
The 12 in chimney behind the block is washed and open-graded, priced at 2,500 lb per cubic yard — loose delivered weight of washed 3/4 in open-graded stone, the drainage fill behind a segmental wall, varying with parent rock, and whether the yard sells it washed or dusty. Hollow units take the same stone in their cores; add the per-unit core volume from the manufacturer’s sheet if yours are hollow, because that quantity is a property of the block and not of the wall.
Adhesive: a 10 fl oz tube is 18.05 cu in, which run as a 3/8" bead is 13.6 linear feet. One bead along 40 ft of cap is 3 tubes. Halving the bead diameter quadruples the run, which is why the coverage printed on a tube is only true for the bead the label assumed.
No geogrid is counted, because none was entered. Grid elevations and embedment lengths are an output of a wall design and an input here — this page will convert a schedule into square yards and will not propose one.
How to take off a segmental retaining wall
Two dimensions, one unit, and one schedule that has to come from a drawing.
Give the length and the height that shows above grade
Exposed height is what a tape reads from finished grade to the top of the cap, and it is not the height the code text measures. The calculator adds the buried courses underneath it using the convention install guides give — one inch of embedment for every eight inches of finished wall — and then reports both numbers separately, because the one that decides how the wall is permitted is measured from the bottom of the base.
Set the unit and its setback off the manufacturer's sheet
Three presets cover the common shapes, from the 4 by 12 by 8 inch landscape unit up to the 8 by 18 by 12 inch segmental block, and every dimension is editable because yours will differ. Setback is the lip or pin offset per course: it is what leans the face back into the slope, and at 3/4 inch on a 6 inch unit that is a batter of 7.1 degrees and four and a half inches of travel over seven courses.
Read the six materials, and enter the grid from your drawing
Block, caps, compacted base, drainage stone, drain pipe and cap adhesive all come back from the dimensions above. Geogrid does not: its elevations and its embedment length are an output of a wall design, so they are fields here rather than suggestions. Enter the interval and length your drawing specifies and the page converts them to square yards; leave them empty and no grid is counted.
Technical specifications
| Block presets | 4 × 12 × 8 in landscape, 6 × 16 × 10 in standard and 8 × 18 × 12 in large segmental — every dimension editable |
|---|---|
| Embedment convention | 1 in of buried wall per 8 in of finished height, minimum one full course; solved as exposed height ÷ 7, since the buried part is part of the height |
| Base default | 6 in compacted and block depth + 12 in wide, ordered at 7.5 in loose to allow the 20% a plate compactor takes out |
| Drainage default | A 12 in chimney of washed 3/4 in stone at 2,500 lb/yd³, run the full height of the block |
| Adhesive coverage | Derived, not quoted: a 10 fl oz tube is 18.05 in³, which is 13.6 ft at a 3/8 in bead and 30.6 ft at 1/4 in |
| Height threshold stated | 48 in of unbalanced fill from the bottom of the base; a surcharge applies the same requirement at any height |
| Verdicts rendered | None. The page reports quantities and states the threshold — a design and a jurisdiction decide the wall |
| Data leaving the browser | None; the wall you describe is never sent anywhere or stored |
Frequently asked questions
How tall can a retaining wall be before it needs a permit and an engineer?
The model code sets its threshold at 48 inches of unbalanced fill, measured from the bottom of the footing or base to the top of the wall. Above that line a retaining wall is an engineered design: a permit, a soils assessment and a drawing signed by an engineer. Two details decide more cases than the number itself. The first is where you measure from — a wall that reads three feet six standing in the yard is over the line on paper once the buried courses and the base are counted. The second is surcharge: a driveway or parking area above the wall, ground that keeps rising back from the top, a pool, or a footing inside the wedge of soil behind it all put a wall of any height into the engineered category. Many jurisdictions set a number lower than the model code, and the one that governs is the one where the wall is being built.
How deep and how wide does the base of a retaining wall go?
Six inches of compacted dense-graded aggregate is the usual leveling pad, set in a trench wide enough to run past both faces of the block — this page defaults that to the block depth plus twelve inches. Order it at the loose depth rather than the finished one: aggregate loses about a fifth of its thickness under a plate compactor, so a pad that has to finish at six inches goes in at seven and a half. The trench itself is deeper than the pad, because the buried courses sit on top of it, and the calculator reports that total excavation depth and the spoil it produces separately.
How much gravel goes behind a retaining wall?
A chimney of clean, washed, open-graded stone twelve inches wide, running the full height of the block. Its job is to give water somewhere to go before it can push on the back of the wall, which is the load that takes most failed walls down rather than the weight of the soil. It has to be a washed stone with no fines in it — a dense-graded product placed here will silt the voids closed and turn the drainage layer into a wet clay poultice. Hollow units also want the same stone in their cores, at a volume the manufacturer publishes per unit, and the top of the chimney gets capped with soil or fabric so surface silt cannot wash down into it.
What is geogrid and how far back does it go?
Geogrid is a stiff polymer mesh laid between courses and running back into the compacted fill, which turns the soil behind the wall into a reinforced mass rather than a load pushing on it. Where it goes and how far back is the output of a wall design and not a rule of thumb: the elevations, the embedment length, the grid strength and the compaction the fill has to reach are all specified together by the engineer who ran the stability analysis. This page will convert an interval and a length into square yards to buy; it will not propose either, and a schedule taken from an internet table is not a design.
What is setback, and how far does it lean the wall back?
Setback is the horizontal offset each course sits back from the one below it, fixed by the unit's lip, pin holes or tongue rather than by the installer. Divide it by the course height and take the arctangent and you have the batter: three quarters of an inch on a six-inch unit is 7.1 degrees off vertical, while an eighth of an inch is under a degree and reads as a straight wall. The practical consequence is that the top of the wall is not above the bottom of it — at that setback a seven-course wall finishes four and a half inches behind its base — so a string line pulled at the top and a footprint measured at the base describe two different lines.
Does a segmental retaining wall need a drain pipe?
Yes, wherever the wall retains soil that can hold water, which is nearly everywhere. A four-inch perforated pipe sits at the bottom of the stone chimney, behind and slightly below the base course, and it has to go somewhere — daylighted at the low end of the run, or into a drain that is not itself going to back up. A chimney with no outlet is a reservoir. That outlet run is why the pipe field on this page adds a length to daylight rather than just matching the wall, and the total comes back as ten-foot sticks of rigid pipe or as coils of corrugated.
How many blocks does a retaining wall take?
Divide the wall length by the unit's face length to get the blocks in one course, then multiply by the total courses — including the ones below grade, which people leave out and then find themselves a pallet short. A forty-foot wall in a sixteen-inch unit is thirty blocks a course; at six inches high with three feet showing, that is six courses above grade plus one buried, so 210 blocks and 27 caps. Caps run off their own length rather than the block's, and on a curved wall the outside radius needs more of them than a straight run of the same length does.
About segmental walls, and the part of them nobody sees
A dry-stacked segmental wall holds ground back with mass and friction rather than with mortar, which is why the material list has seven lines on it and only two of them are block. The leveling pad under the first course is what keeps the whole run flat and takes the bearing; the courses buried below finished grade are what stop the base sliding forward; the washed stone behind the units is what keeps water from collecting into a load; the perforated pipe at the bottom of that stone is what gives the water an exit; and geogrid, where a design calls for it, converts the fill behind the wall from something pushing on the block into a reinforced mass that the block is merely the face of. Skip any one of them and the wall does not fail immediately — it fails in the third or fourth wet spring, which is what makes this the wrong place to economise.
Water is the reason for most of that list. Saturated soil behind a wall exerts hydrostatic pressure on top of the earth pressure that was already there, and it does so at the worst possible moment, which is when the ground is heaviest and the fill is weakest. The drainage chimney has to be an open-graded washed stone with no fines in it: put a dense-graded base product back there and its own dust migrates into the voids, closes them, and leaves you with a wet clay poultice against the block. That is two different products from two different bins on the same job, and the crushed stone calculator prices them by grade if you want to check what your yard is selling you. The compacted pad itself is ordinary base aggregate and is ordered the way any compacted layer is — deeper than it finishes — which the gravel calculator handles for a driveway and this page handles here. Some designs specify a rounded washed stone instead of an angular one behind the block, which is the domain of pea gravel, though angular is the more common call because it locks rather than rolls.
The one thing this page deliberately will not do is tell you a wall is fine. Whether a given wall stands depends on the soil it is founded on and the soil it is holding, the slope above and below it, what is parked on top of it, how well the fill was compacted and in what lifts, and where the water goes — none of which a calculator can see. So the threshold is stated as the code text states it, at 48 inches of unbalanced fill measured from the bottom of the base, and the surcharge clause is stated alongside it, because a driveway above a three-foot wall makes that wall an engineered design just as surely as five feet of height would. Above the line, or under a surcharge, what you need is a geotechnical opinion and a stamped drawing, and this page becomes what it should be: the thing that turns that drawing into a delivery. Below the line it is still a wall holding back earth, and the quantities are the easy part. For the mortared cousin of this problem — a garden wall in units bonded with mortar rather than stacked dry — the brick calculator counts the units and the mortar calculator prices the joints, with Type M or Type S being the types that belong in contact with earth. A poured concrete stem wall is a different structure again and starts at the concrete calculator.
What this page knows about your site
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — measurements, quantities, the prices your supplier quoted you — is uploaded, logged or kept, which is also why the calculators carry on working on a site with no signal.
No wall you describe here is submitted, filed or seen by anyone. That also means nothing on this page constitutes a review of it — the arithmetic is checkable, but only a person who has looked at the site can say what the wall needs.