Calculator
Retaining Wall Block Calculator
A retaining wall built with exactly enough block and nothing else is a wall waiting to fail — not from too little material in the visible face, but from water. A segmental wall with no course buried below grade has nothing stopping its base from kicking forward under load, and a wall with no drainage stone behind it is holding back a column of saturated soil at full hydrostatic pressure every time it rains. Neither of those failure modes shows up if all you counted was the exposed face.
This calculator counts five things a retaining wall actually needs together: wall block, a separate cap course, base stone for the trench the first course sits on, drainage stone behind the wall, and the pipe that carries water out of it. Leave any one of the last three off the order and the block count above them stops mattering.
Calculate your quantity
Height you will see above finished grade.
One full course below grade is the usual minimum.
Wall blocks
140 blocks
Cap blocks
20 caps
- Courses total
- 7 courses
- Courses buried
- 1 courses
- Blocks per course
- 20 blocks
- Base stone
- 0.65 cu yd
- Base stone weight
- 0.91 tons
- Drainage stone
- 2.59 cu yd
- Drain pipe
- 30 ft
- Filter fabric
- 130 sq ft
- Bury one full course plus the base stone so the wall cannot kick out at the bottom.
Shopping summary
- • 140 wall blocks + 20 cap blocks
- • 0.65 cu yd base stone, 2.59 cu yd drainage stone
- • 30 ft of perforated drain pipe, 130 sq ft filter fabric
- • 2 tubes of masonry adhesive for the cap course
This is an estimate — confirm structural work with a professional.
How this calculation works
Total wall height is exposed height plus buried height, and only the buried part is optional to skip — which is exactly why it shouldn't be. Exposed courses come from the height you'll actually see divided by the block's height, rounded up. Buried courses are set below finished grade before the first exposed course begins, so the wall's toe is locked into the ground rather than resting on top of it. The very top course is different again: it's cap block, a solid unit that closes the wall visually and sits above the load-bearing stack rather than as part of it, so it's counted and reported separately from the rest.
The trench underneath gets sized from the block's own depth, not a flat number: block depth plus roughly 6 inches of working room on each side gives the trench width, and that trench is filled to the base depth with compacted crushed stone before the first block goes down. Behind the finished wall, a full-height column of free-draining stone — sized by the drainage width you set and the wall's total height, buried plus exposed — gives water somewhere to go besides against the back of the wall, and a perforated pipe at the base of that column carries it out to daylight.
- This page counts material for a segmental block wall assembled to a manufacturer's stacking pattern. It does not check soil bearing capacity, surcharge loads from a driveway or slope above the wall, or whether geogrid reinforcement is needed at height — those are a geotechnical or engineering question, not a material count.
- Most jurisdictions require an engineer's stamped design and a permit once a retaining wall's exposed height passes a modest threshold, commonly around 4 feet — confirm your local requirement before you dig, not after the block is delivered.
- Base stone compaction matters as much as its depth; loose, uncompacted fill under the first course settles unevenly and takes the whole wall's alignment with it.
- Corners, curves, and steps in grade each need their own piece count from the block manufacturer's layout guide — this page's block-per-course figure assumes a straight run.
The formula
wallBlocks = blocksPerCourse × (exposedCourses + buriedCourses); baseStone = wallLength × (blockDepth + 12 in) ÷ 12 × (baseDepth ÷ 12); drainageStone = wallLength × (drainWidth ÷ 12) × totalWallHeight
- exposedCourses
- Rows of block visible above finished grade: exposed height divided by block height, rounded up to the next whole course.
- buriedCourses
- Rows set below finished grade before the first exposed course, locking the wall's base into the ground so it cannot kick out at the bottom under load from the soil it's retaining.
- capBlocks
- The single top course only — a solid unit, usually a different product from the wall block below it, that closes the wall's visible top rather than carrying load through the stack.
- base trench width
- Block depth plus roughly 12 inches total (about 6 inches clear on each side), giving room to set and level the first course on compacted crushed stone rather than directly on native soil.
- drainage stone
- A free-draining crushed stone column running the full buried-plus-exposed height of the wall, sized by the width you set behind the wall face — the mechanism that keeps water from building up hydrostatic pressure against the back of the wall.
Base stone and drainage stone are reported separately because they do different jobs and are sized differently: base stone is a shallow, wide trench under the wall's footprint, while drainage stone is a narrow, full-height column behind its face. Ordering one and assuming it covers the other under-delivers whichever one got skipped.
Where these numbers come from
- Trench width = block depth + about 12 in
- Trade convention drawn from typical segmental retaining wall installation guidance, which commonly calls for a few inches of working room on each side of the block for base stone and leveling. Exact figures vary by manufacturer and soil condition, so treat this as a working allowance rather than a fixed spec.
- 1.4 tons per cubic yard for compacted base stone
- Trade convention density for 3/4 in crushed stone base material, consistent with the density used across this site's other gravel and base-stone figures. Actual weight varies with stone type and compaction.
- At least one full course buried below grade
- Trade convention from general segmental block installation guidance — burying the base course is standard practice for keeping a wall's toe from kicking out. Taller or steeper walls commonly require more than one buried course; this is a floor, not a complete design.
- Roughly 4 ft exposed height as an engineering/permit threshold
- Commonly required rather than a specific cited figure — many jurisdictions require an engineered design and a permit above a set retaining wall height, and that height is set locally and varies. Confirm the number that applies in your area before treating any height as a safe DIY limit.
Worked examples
A 30 foot garden wall retaining a 2.5 foot grade change
| Wall length | 30 ft |
|---|---|
| Exposed height | 2.5 ft |
| Block length | 12 in |
| Block height | 6 in |
| Block depth | 9 in |
| Buried courses | 1 |
| Base stone depth | 6 in |
| Drainage stone width | 12 in |
| Wall blocks | 180 blocks |
|---|---|
| Cap blocks | 30 caps |
| Courses total | 6 courses |
| Courses buried | 1 courses |
| Blocks per course | 30 blocks |
| Base stone | 0.97 cu yd |
| Base stone weight | 1.36 tons |
| Drainage stone | 3.33 cu yd |
| Drain pipe | 40 ft |
| Filter fabric | 180 sq ft |
A 2.5 foot exposed height at 6 inch block courses rounds up to 5 courses, plus the one buried course makes 6 total — so a wall that reads as '2.5 feet tall' is actually a 3 foot stack of block once the buried course and the rounding are both counted. Thirty 6 inch blocks per course times 6 courses is 180 wall blocks, with the 30 cap blocks on top of that as a separate line, not included in the 180.
Just under a cubic yard of base stone and about 3.3 cubic yards of drainage stone go with those 180 blocks, and both are trip-worthy quantities — this is not a job where you tuck a bag of gravel behind the wall and call the drainage handled. If a bulk delivery is coming for the base and drainage stone together, the bulk delivery load calculator will tell you whether it fits in one truckload or needs to be split.
An 18 foot wall retaining 4.5 feet, at the engineering threshold
| Wall length | 18 ft |
|---|---|
| Exposed height | 4.5 ft |
| Block length | 16 in |
| Block height | 8 in |
| Block depth | 12 in |
| Buried courses | 2 |
| Base stone depth | 8 in |
| Drainage stone width | 18 in |
| Wall blocks | 126 blocks |
|---|---|
| Cap blocks | 14 caps |
| Courses total | 9 courses |
| Courses buried | 2 courses |
| Blocks per course | 14 blocks |
| Base stone | 0.89 cu yd |
| Base stone weight | 1.24 tons |
| Drainage stone | 6 cu yd |
| Drain pipe | 28 ft |
| Filter fabric | 171 sq ft |
This wall's shorter run needs fewer blocks per course than the first example — 14 against 30 — but nine total courses against six, because 4.5 feet of exposed height at 8 inch block rounds up to 7 courses before the 2 buried courses are even added. The result is 126 wall blocks on an 18 foot run, more than the 30 foot garden wall used despite being roughly half the length, purely because this wall is taller and buried deeper.
At 4.5 feet exposed, this design is past the point where a homeowner project should proceed on this page's numbers alone — get a second opinion from an engineer or your local building department before ordering, not after the block arrives. Six cubic yards of drainage stone behind an 18 foot run this tall is also worth checking against your access: if a truck can't reach the back of the wall, that stone is getting moved by wheelbarrow, and that changes the labor estimate more than the material one.
Common mistakes
- Counting cap blocks as part of the wall block total instead of a separate top course. They're a different, usually solid unit and ordering them together under-counts one or over-counts the other.
- Skipping the buried course to save block on a short wall. It's the buried course that keeps the wall's base from kicking forward under load — the shortest walls are not exempt from needing one.
- Setting base stone directly on native soil instead of an excavated, compacted trench. An uncompacted or unlevel base is the single most common reason a segmental wall leans within its first year.
- Leaving out drainage stone and pipe, or treating them as an upgrade rather than part of the wall. Water pressure behind an undrained wall is one of the most common causes of a segmental wall bulging or failing outright.
- Building past the local permit-and-engineering height threshold on the strength of a material calculator. This page counts block and stone; it does not check whether the wall as designed can actually hold back the soil behind it.
- Applying this straight-run block-per-course figure to a wall with a corner, a curve, or a step in grade. Those layouts need piece counts from the block manufacturer's own guide, not a straight-line count.
Shopping summary
Wall block, cap block, base stone, drainage stone, drain pipe and filter fabric are six separate line items on this page's output, and all six belong on the same order — leaving any of the last four off the truck is how a wall with 'enough block' still fails within a year or two.
Digging the trench for the base stone produces spoil that has to go somewhere; the excavation spoil calculator accounts for the fact that dug soil takes up more volume loose than it did in the ground, which matters when you're sizing a haul-away. For drainage detailing beyond what a retaining wall's stone column handles on its own — a wet yard or a downspout that needs to be routed away from the wall entirely — the french drain calculator covers the pipe-and-stone trench for that separately.
FAQ
Why are cap blocks counted separately instead of just adding them to the wall block total?
Because they're a different product doing a different job — a solid unit that finishes the top course rather than a hollow or lipped unit carrying the stack below it. Suppliers price and package them separately, so keeping the two counts apart on this page matches how you'll actually order.
Why does a wall described as '3 feet tall' come out needing more than 3 feet worth of block?
Because '3 feet' almost always means exposed height, and this page adds the buried course underneath it — commonly one full course, more on a taller or steeper wall. A 3 foot exposed wall with one 6 inch buried course is really a 3 foot 6 inch stack of block, and rounding each course up to the block height adds a little more on top of that.
How much protection does one buried course actually provide?
It's a minimum, not a complete foundation design — burying the base course keeps the wall's toe from simply sliding forward under load, which is the most basic failure mode for a gravity segmental wall. Taller walls, steep sites, or poor soil commonly need more than one buried course and a properly engineered base, which this page's default doesn't attempt to size.
What actually triggers the requirement for an engineer and a permit?
It's set locally, not by this page, and it usually depends on exposed height, what's above the wall (a driveway, a slope, a structure), and sometimes proximity to a property line. Treat any number here as a rule of thumb and call your local building department before you commit to a height — the cost of asking first is far less than the cost of a wall that has to come back out.
Does this calculator size geogrid reinforcement for a taller wall?
No. Geogrid — horizontal reinforcement laid between courses and extended back into the retained soil — is a structural design decision that depends on wall height, soil type, and the load behind the wall, and it's exactly the kind of thing that pushes a wall into needing an engineer. This page counts block and drainage material for a gravity wall design; it does not size reinforcement.
My wall has a corner or a curve — does the block-per-course number still work?
The straight-run count here is a starting point, not the finished takeoff. Corners typically use a dedicated corner unit and curves use the block's built-in radius allowance (or need a smaller unit), and both change the blocks-per-course math from a simple length-divided-by-block-length. Check your block manufacturer's layout guide for corner and curve piece counts before finalizing an order on a wall that isn't a straight line.
Where to go next
The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.