Terraced Retaining Wall Calculator
Splitting a slope into tiers only works if the tiers are far enough apart to act as separate walls. This sizes the bench between them from the published two-times-the-lower-wall rule, tells you how much hillside depth that costs you, and puts the material and price side by side against building one tall wall instead.
| One wall | 2 tiers | |
|---|---|---|
| Tallest single wall | 6.00 ft | 3.00 ft |
| Exposed face | 240 sq ft | 240 sq ft |
| Blocks with waste | 441 | 504 |
| Caps | 32 | 63 |
| Leveling pad gravel | 1.6 tons | 3.1 tons |
| Block, cap & pad cost | $2,467 | $3,038 |
| Hillside depth used | 1.00 ft | 8.00 ft |
| Height trigger for a designed wall | Over the 4 ft line | Under the 4 ft line |
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Two short walls are not automatically two short walls
The reason people terrace a slope is almost always the same: one 6 ft wall has to be engineered and permitted, two 3 ft walls might not. That logic is sound, but only under a condition most articles never state. If the upper wall sits close behind the lower one, its weight and the soil it holds press down on the block behind and above the lower wall — the upper tier becomes a surcharge, and the two are analysed together as one taller structure. You get the terraced look, the extra material and none of the relief you terraced for.
The condition is a horizontal distance. CMHA — the industry body for segmental retaining walls — puts it this way: the minimum distance between terraces for each wall to act independently must be at least equal to twice the height of the lower wall, written D > 2H₁. Allan Block states the same rule from the other direction: the zone of influence extends a horizontal distance equal to two times the wall height, and the distance between the walls must be at least twice the height of the lower wall. Both add that the upper wall should be no taller than the lower one. This calculator is built around that single measurement, because it is the one that decides whether terracing works.
How each number is worked out
- Tier height: the grade change divided by the number of tiers. Equal tiers are used deliberately — it is the split that satisfies "upper wall no taller than the lower wall" automatically, and it makes every bench the same width.
- Minimum bench: twice the height of the lower tier, straight from the published rule. With equal tiers every gap has the same requirement.
- Hillside depth needed: the benches plus the walls themselves — the number of gaps times the bench, plus one block depth for each tier's footprint. Three tiers have two benches, so the depth grows with tier count even though each tier gets shorter.
- Tallest independent tier that fits: the same equation solved the other way. Given the depth you actually have, it is what is left after the wall footprints, divided between the benches — and multiplied by the tier count it gives the largest grade change your yard can hold in independent tiers.
- Governing wall height: the height the design is judged on. If the tiers clear the separation it is one tier; if they do not, it is the whole grade change, because that is how the system has to be analysed.
- Courses and blocks: each tier's exposed height plus its buried course, divided by the course height and rounded up, times the blocks per course across the wall length, then the waste percentage. Counting is done per tier and summed — a tier is a whole wall, buried course and all.
- Leveling pad: one pad per tier, the block depth plus the pad overhang, at the pad depth, converted to weight at the gravel density you set.
- Single-wall column: the identical takeoff run once at the full grade change over the same wall length, so the only difference between the columns is the tiering itself.
What terracing actually costs and saves
The comparison table usually surprises people in two places. Tiers use more block, not less. Every tier needs its own buried bottom course and its own leveling pad, so splitting a wall into n tiers adds n−1 buried courses and n−1 pads that a single wall never pays for. At the default 6 ft over 40 ft of length that is about 14% more block and twice the pad gravel.
And tiers eat yard. One wall consumes its own footprint and nothing else. Two independent tiers off the same grade change consume the footprints plus a bench as wide as twice the lower tier — on a 6 ft grade change that is roughly 8 ft of hillside gone, measured horizontally. If the useful part of the yard is what sits above or below that slope, the depth line in the table is a real cost even though it has no price on it.
What sits on the other side of the ledger is engineering, reinforcement and permitting. A single wall over the 4 ft line needs a designed structure and, per the International Building Code, a permit for earth retaining structures over 4 ft in total height; it will very likely need geogrid, which the tiers may not. That is the trade this page is for: more block and more yard, against a design that may stay within what a homeowner can build. Neither column is automatically the answer.
Where the rules come from
The separation rule is CMHA's — the Concrete Masonry & Hardscapes Association, which publishes the design manual most segmental wall systems are engineered against. Their answer on wall heights states that the minimum distance between terraces for each wall to act independently must be at least equal to twice the height of the lower wall, and that where the spacing is tighter the tiers are analysed as a single taller structure. The same document is the source for the height thresholds used on this page: unreinforced segmental walls typically top out at 3 to 4 ft (1.0–1.2 m), less with poor soil or a surcharge, while reinforced walls have no theoretical maximum when properly designed; and the International Building Code requires a permit for earth retaining structures over 4 ft (1.2 m) in total height.
Allan Block's installation guidance is the second source and agrees: the distance between the retaining walls must be at least twice the height of the lower wall, the upper wall should be less than or equal to the height of the lower wall, and when the walls sit closer the lower wall must be engineered for the added weight and pressure. Allan Block is also explicit about the limit of the rule — even when the spacing is met, global stability of the overall slope may still require analysis, along with slopes above or below the walls, groundwater, surcharges and poor soils. That caveat is not fine print; it is why this page will not tell you that you are clear to build.
Block dimensions and prices use the same nominal figures as the rest of the retaining-wall tools here, so the numbers stay comparable across pages. Gravel is converted at about 1.4 tons per cubic yard (≈1,680 kg/m³) — ask your supplier for their figure and round the order up.
This tool handles the tier geometry and the block across all tiers. For the rest of a wall — drain rock, pipe, fabric and reinforcement — run a single tier through the full block takeoff, size reinforcement in the geogrid calculator and drainage in the drainage calculator. The bench between tiers has to be dug out level, which the excavation calculator will size, and if a tier curves, lay it out in the curve calculator.
Terrace notes that save rework
- Drain each tier separately. Every tier needs its own drainage stone and its own outlet. Water collected behind an upper wall that discharges onto the bench simply loads the wall below it — pipe it past the lower tier.
- The bench is not storage. A bench used for a shed, a parking pad or stacked material is a surcharge on the lower wall, and the calculation on this page assumes nothing is up there.
- Build from the bottom up. The lower wall and its base go in first; the bench is cut back afterwards. Excavating the upper bench first leaves the lower wall's cut face unsupported.
- More tiers is not a loophole. Splitting the grade into more, shorter tiers lowers each wall but raises the total hillside depth needed, so it usually fails on space before it helps on height.
- Measure the depth before you pick the tier count. The tallest-independent-tier figure above tells you what your yard can actually hold — decide the tier count from that number, not from the height you wish each wall was.
Frequently asked questions
How far apart do terraced retaining walls have to be?
The published rule across the segmental wall industry is that the horizontal distance between the two walls must be at least twice the height of the lower wall. CMHA writes it as D > 2H1 for each wall to act independently, and Allan Block states the same thing as a zone of influence extending a horizontal distance equal to two times the wall height. Inside that distance the upper wall is surcharge on the lower one, and the pair has to be analysed together as a single taller structure.
Does terracing let me avoid an engineer?
Only if the tiers genuinely act independently. Two 3 ft tiers separated by 6 ft of bench are two 3 ft walls. The same two tiers on a 4 ft bench are one 6 ft wall as far as the design is concerned, and a 6 ft wall needs a designed and permitted structure. Terracing without the separation buys you the terraced look and none of the engineering relief, which is the single most expensive misunderstanding on this page.
Is a terraced wall cheaper than one tall wall?
In block, no — it is usually more. Every tier needs its own buried bottom course and its own leveling pad, so several short tiers use more material than one wall of the same total height. What terracing can save is the engineering, the geogrid and sometimes the permit, and it breaks one big vertical face into walls a homeowner can actually build. The comparison table on this page shows both columns so you can see the trade rather than assume it.
How much yard depth does terracing need?
More than most people plan for. Two tiers off a 6 ft grade change need about 6 ft of bench plus the two wall footprints — roughly 8 ft of hillside measured horizontally. Splitting the same grade change into three tiers makes each tier shorter but needs more total depth, not less, because you have added another bench. If the hillside is not that deep the tiers cannot be independent, whatever their individual height.
Should the upper tier be shorter than the lower one?
Allan Block's guidance is that the height of the upper retaining wall should be less than or equal to the height of the lower wall. Splitting the grade change into equal tiers satisfies that automatically, which is what this calculator does. Stacking a taller wall on top of a shorter one puts the heavier surcharge exactly where the structure below has the least capacity to carry it.
Sources
The published rules and unit dimensions this page relies on, and the document each one is taken from. Manufacturers revise their literature — check your own product’s current spec sheet before you order.
- CMHA — What Height Restrictions Should I be Aware of With Segmental Retaining Wall Construction? — The separation rule D > 2H₁, the 3 to 4 ft (1.0–1.2 m) practical ceiling for unreinforced segmental walls, and the 4 ft (1.2 m) permit threshold used on this page.
- Allan Block — Terraced Retaining Walls — The zone of influence extending a horizontal distance equal to two times the wall height.
- Allan Block — In-Wall Planters and Terracing — That the upper wall should be no taller than the lower one, and that the distance between the walls must be at least twice the height of the lower wall.
- Allan Block — AB Collection retaining wall blocks — AB Classic preset dimensions — 18 in × 8 in × 12 in (460 × 200 × 300 mm).
- VERSA-LOK — Standard unit specifications — VERSA-LOK Standard preset dimensions — 16 in × 6 in × 12 in (406.4 × 152.4 × 304.8 mm).