Retaining Wall Block Calculator
Blocks, caps, base pad and drainage stone for a segmental garden wall — counted from your block's size, with the height where the code says a wall needs an engineer.
Measure it once
Patios and pool decks, wall runs and pools. Every module below reads these.
One straight run of wall. Model a stepped or turning wall as several runs.
Visible height above finished grade in front of the wall. Buried courses are added by the wall module.
What are you building?
Pick one or several. Each reads the measurements above.
Retaining wall
Material quantities only. Walls over 48″, or over 24″ with anything above them, must be engineered (IRC R404.4). This tool never designs a wall. Poured concrete and structural walls belong in the Concrete Calculator.
Block and cap sizes vary by manufacturer, so none is assumed — counts appear once you enter them.
Base, buried course and drainage — per Allan Block
Per Allan Block's residential installation guide — follow your own block manufacturer's instructions.
Allan Block specifies 'wall rock' — clean, angular crushed stone — for the base, the cores and the drainage zone. #57 stone is used as the default open-graded material from the Gravel data. Change it to your manufacturer's specification. Allan Block also fills the hollow cores with wall rock. Core volume depends on the block, so it is not included — add it from your block's product sheet if it applies.
Where this calculator stops
IRC R404.4. Retaining walls that are not laterally supported at the top and that retain in excess of 48 inches of unbalanced fill, or retaining walls exceeding 24 inches in height that resist lateral loads in addition to soil, shall be designed in accordance with accepted engineering practice.
| Wall | What you get |
|---|---|
| Up to 48″, nothing above adding load or up to 24″ with load above | Blocks, caps, base pad, drainage stone and adhesive |
| Over 48″ or over 24″ with load above | Block and cap counts only, with an engineering notice. Base, drainage and reinforcement come from the design. |
The code measures unbalanced (retained) fill. The exposed wall height is used here as the closest proxy the tool can measure; where grade behind the wall slopes up, the retained height is greater. The IRC is a model code. Your jurisdiction may adopt it with changes, and many require a permit — or an engineer — at lower heights. Check locally before building any retaining wall.
This is not a wall design tool. It never calculates geogrid layout, length or spacing, setback or batter design, surcharge loading, soil pressure, sliding, overturning or global stability, footing or leveling-pad design for an engineered wall, whether any wall height is safe. Foundation walls, stem walls and cast-in-place concrete retaining walls are the Concrete category's job and use the Concrete engine. This module is for dry-stacked segmental units only.
Block and cap counts
Segmental blocks are not a standard size — face length, height, depth and setback differ between manufacturers and even between products in one line. So the calculator asks for your block's face length and height instead of offering a list that would be a guess dressed as a specification.
A straight run of whole blocks has no waste. Corners, ends, curves and stepped tops need cuts, which is why the allowance starts at zero and offers 5% and 10% — typical practice, not published figures.
Base, buried course and drainage
| Item | Figure |
|---|---|
| Base trench width | 24″ |
| Compacted base pad | 6″ minimum |
| Buried block | 1″ per ft of height |
| Drainage stone behind the wall | 12″ minimum |
Per Allan Block's residential installation guide — follow your own block manufacturer's instructions. Allan Block specifies 'wall rock' — clean, angular crushed stone — for the base, the cores and the drainage zone. #57 stone is used as the default open-graded material from the Gravel data. Change it to your manufacturer's specification.
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length by the block face length for blocks per course, and the total height — exposed height plus the buried course — by the block height for the number of courses. Multiply the two. Enter your block's face length and height from its product sheet; the calculator rounds each up and adds any allowance you choose for corners and curves.
How much of the wall goes below ground?
Allan Block's residential guide buries 1 inch of block for every foot of wall height, on a trench 6 inches deeper than that for the base. The calculator uses that figure by default, labelled as Allan Block's, and lets you enter a different rate or a fixed buried depth from your own manufacturer.
How much gravel goes under and behind a retaining wall?
Following Allan Block's guide: a base pad 24 inches wide and 6 inches deep of compacted wall rock, and a zone at least 12 inches wide of wall rock behind the full height of the wall. The calculator converts both to tons of the stone you choose. Allan Block also fills the hollow cores with wall rock. Core volume depends on the block, so it is not included — add it from your block's product sheet if it applies.
When does a retaining wall need an engineer?
Under IRC section R404.4, when it retains more than 48 inches of unbalanced fill, or when it is more than 24 inches tall and resists loads in addition to soil — a driveway, parking, a slope, a fence or a structure above it. The IRC is a model code. Your jurisdiction may adopt it with changes, and many require a permit — or an engineer — at lower heights. Check locally before building any retaining wall.
What happens in the calculator above that height?
It still counts blocks and caps, because those are geometry and true of any wall. It stops giving base, drainage and adhesive quantities and shows an engineering notice instead, because an engineered wall's base, drainage and geogrid reinforcement come from its design — and a figure from a garden-wall guide would imply a design that has not been done.
Does this calculate geogrid?
No. Geogrid length, spacing and layout, setback, surcharge loading and wall stability are engineering, and this calculator never does them.
What about a poured concrete retaining wall?
Use the Concrete Calculator's wall shape for the volume. Cast-in-place and structural walls are designed differently and are not what this calculator counts.
How many caps do I need?
Wall length divided by the cap length, rounded up, for each wall run. Enter the caps-per-tube figure from your adhesive for a tube count.
Sources
- International Residential Code, Section R404.4 — Retaining walls — International Code Council. Walls retaining more than 48 in. of unbalanced fill, or more than 24 in. high and resisting lateral loads in addition to soil, must be designed in accordance with accepted engineering practice.
- Allan Block — residential installation guide — Allan Block Corporation (manufacturer). Base trench 24 in. wide; minimum 6 in. of compacted wall rock; trench depth 6 in. plus 1 in. of buried block per 1 ft of wall height; fill cores and a minimum 12 in. behind the wall with wall rock. One manufacturer's instructions. Other systems differ; the tool labels these figures as Allan Block's and makes every one editable.
Other calculator websites are never cited. See the methodology page.
Last reviewed September 16, 2026.
Related
- Hardscaping Calculator — walls, pavers and pools on one order sheet.
- Paver Calculator — the patio behind the wall.
- Concrete Calculator — poured concrete walls and footings.
- Gravel Calculator — stone for other jobs.