Face length × face height × depth.
Surcharge changes everything.
Section 1
Follow curves, do not measure straight across.
Finished grade to top of wall.
Advanced options
6–8 in compacted. More in frost country.
12 in minimum of clean stone.
15–20% with curves or corners.
Commonly 48 in. Check locally.
This retaining wall calculator buries the base course the way it should be buried, counts the drainage stone that actually holds the wall up, and checks your height against the threshold where a licensed engineer stops being optional.
The short answer
A 20 ft × 3 ft wall in standard 12 × 6 in block needs 154 blocks, 22 caps and about 3.6 yd³ of gravel — seven courses, because one of them is buried below grade.
Watch the height. That wall measures 3 ft 6 in total, and many jurisdictions apply the 4 ft engineering threshold to total height, not the part you can see.
This calculator estimates materials. It does not assess whether your wall will stand — that depends on soil bearing capacity, internal friction, groundwater, frost depth and global stability, none of which a materials calculator can evaluate.
Anything over your local height threshold, anything with a driveway or slope above it, and anything on ground you’re unsure about needs a licensed engineer and a permit.
On this page
How to use this retaining wall calculator
Pick your block size, describe the soil you’re retaining, and say whether anything sits above the wall. Then enter each section — length following the curve, not straight across, and exposed height from finished grade to the top.
Add sections for a wall that steps or changes height. The calculator buries each one correctly for its own height and reports on the tallest.
The three inputs that change the answer most are height, soil type and surcharge. Block choice affects the count but not whether the wall stands.
The buried course
A retaining wall doesn’t start at ground level. The bottom course sits below finished grade, resting on compacted stone, and that’s what stops the base sliding forward under load.
Two rules circulate, and they disagree:
- Rule A: bury one full course. Most calculators just add one block height
- Rule B: bury one tenth of the finished height
The correct practice is the greater of the two, and they cross over at 5 ft:
| Exposed height | One tenth | One course | Bury |
|---|---|---|---|
| 2 ft | 2.4 in | 6 in | 6 in — one course |
| 3 ft | 3.6 in | 6 in | 6 in — one course |
| 4 ft | 4.8 in | 6 in | 6 in — one course |
| 5 ft | 6.0 in | 6 in | the crossover |
| 6 ft | 7.2 in | 6 in | 12 in — two courses |
| 8 ft | 9.6 in | 6 in | 12 in — two courses |
Below 5 ft, one course governs and “add a block” is fine. Above 5 ft it isn’t — a calculator that always adds exactly one course understates both the block count and the excavation on a tall wall.
In cold climates the base also needs to sit below the local frost line, which can be considerably deeper than either rule requires.
The threshold trap
Most jurisdictions allow unengineered segmental walls up to 4 ft (48 in). Above that you need a licensed engineer’s design, usually with geogrid, and often a stamped drawing for the permit.
It matters, and sources genuinely differ. Several state that the threshold applies to total height including the buried course:
| You measured | Buried | Total | On total height |
|---|---|---|---|
| 3 ft 0 in | 6 in | 42 in | Under |
| 3 ft 6 in | 6 in | 48 in | Right on it |
| 3 ft 8 in | 6 in | 50 in | Over — engineered |
| 4 ft 0 in | 6 in | 54 in | Over — engineered |
So a wall you measured at 3 ft 8 in and assumed was safely under can be over once the base course counts. One phone call to your building department settles it, and it’s worth making before you order.
The threshold field in the calculator is editable, because it isn’t universal — some jurisdictions use 3 ft, and some permit up to 6 ft with geogrid.
Why walls actually fail
Not the blocks. Per the NCMA segmental retaining wall design manual, trapped water is the leading cause of failure.
Saturated soil behind a wall generates hydrostatic pressure — the weight of standing water pushing outward, on top of the soil pressure the wall was designed for. It tips the wall forward from the top, and no amount of block strength resists it. You’ll see the bulge before it goes.
The structure
Clean stone
A 12 in chimney of washed angular stone directly behind the blocks, full height. Angular, so it locks; washed, so it doesn’t clog.
The separator
Filter fabric
Non-woven geotextile between stone and soil. Landscape fabric lasts a year or two; proper drainage-rated fabric lasts the wall’s life.
The exit
Perforated pipe
At the base of the stone, sloped at least ⅛ in per foot, discharging somewhere real. A pipe to nowhere is decoration.
Over a 40 ft wall, that ⅛ in per foot is 5 inches of fall from one end to the outlet. Worth planning before you set the base, because you can’t add it afterwards.
If you’re retaining clay, the honest advice is to replace the backfill rather than try to drain it. Clay holds water and expands when saturated, adding load the wall never saw dry. Excavate a full drainage zone and separate it from the clay with fabric.
Block counts
| Block | Face area | Per ft² of wall | Typical use |
|---|---|---|---|
| 12 × 4 in — garden | 0.33 ft² | 3.00 | Borders under 2 ft |
| 12 × 6 in — standard SRW | 0.50 ft² | 2.00 | The common choice to 4 ft |
| 16 × 6 in — large SRW | 0.67 ft² | 1.50 | Faster on long runs |
| 18 × 6 in — extra large | 0.75 ft² | 1.33 | Large projects |
| 16 × 8 in — heavy gravity | 0.89 ft² | 1.12 | Taller engineered walls |
The standard 12 × 6 block working out at exactly 2 per square foot of wall face is a useful thing to remember for quick checks.
Waste runs 10% on a straight wall with square ends, rising to 15–20% where there are curves, because blocks get cut to close the radius. Corners and steps add cutting too.
Base and drainage stone
Base = Length × (2 × block depth) × Base depth
Drainage = Length × Chimney width × Total height
Base trench is twice the block depth wide · drainage runs the full height
Worked example — 20 ft × 3 ft, 12 × 6 × 8 in block, average soil
Note the split: the drainage stone is five times the base volume. People budget for the base and forget the chimney, then discover mid-job that the largest single material order hasn’t been placed.
Base depth runs 6–8 in compacted, and deeper in frost country. The trench needs to be twice the block depth wide so the blocks sit on stone rather than half on stone and half on subgrade.
Surcharge
Gravity wall rules assume level ground behind the wall. Anything else is a surcharge, and it changes the problem:
- A slope rising behind — the retained soil extends upward, so the effective height is greater than the wall
- A driveway or parking area — live load, plus vehicle impact
- A building, pool or structure — dead load, and a foundation whose own pressure bulb may reach the wall
Pressure at the base rises sharply and the failure mode changes. A wall that would have been comfortably fine at 3 ft with level ground behind can need engineering at 2 ft with a driveway above.
The calculator will still give you a materials estimate so you can budget — but with a surcharge selected, treat that as planning information only. The design has to come from an engineer who knows your soil.
Geogrid
Geogrid is high-strength polymer mesh laid between courses and extending back into the backfill, tying the wall to a mass of soil behind it. Instead of the blocks resisting the soil, the blocks and a block of reinforced soil act together.
Most block systems require it above 3 to 4 ft of exposed height, depending on block weight, backfill and surcharge. Typical spacing is every second course, with lengths of 4–8 ft into the fill.
Those numbers are indicative only. Geogrid length and spacing are the output of an engineered design — get them wrong and the wall bulges outward and eventually fails, which is exactly the failure geogrid was supposed to prevent.
Batter
Each course steps back about ¼ in, so the wall leans into the slope it’s retaining. Most segmental blocks build this in with a lip or pin arrangement, so you get it automatically by stacking correctly.
Over a 4 ft wall that’s roughly 2 in of total lean. It looks like very little and it matters — a vertical or forward-leaning wall has no margin, and any settlement takes it past plumb.
Retaining wall mistakes to avoid
- Skipping the drainage stone. It’s the structure, not an extra.
- Adding one buried course above 5 ft. The one-tenth rule governs there.
- Measuring the threshold on exposed height. Many jurisdictions use total.
- Backfilling with the soil you dug out. Especially clay.
- A drain pipe with nowhere to go. It needs fall and an outlet.
- Using landscape fabric. Use drainage-rated non-woven geotextile.
- Ignoring a slope or driveway above. Surcharge changes the design.
- Compacting in one lift. Backfill in 6–8 in layers.
- Measuring a curved wall straight across. Follow the curve.
Frequently asked questions
How many blocks do I need for a retaining wall?
Wall face area divided by block face area, counting the buried course. Standard 12 × 6 in blocks give exactly 2 per square foot. A 20 ft × 3 ft wall is seven courses including one buried, so 140 blocks before waste — about 154 with a 10% allowance.
How deep should the first course be buried?
The greater of one full course and one tenth of the finished height. Below about 5 ft those come to the same thing, so one course is fine. Above 5 ft the one-tenth rule demands more — a 6 ft wall needs 7.2 in, which is two 6 in courses. In cold climates, also get below the frost line.
When does a retaining wall need an engineer?
Most jurisdictions set the threshold at 4 ft, though some use 3 ft and some allow 6 ft with geogrid. Critically, several measure it on total height including the buried course — so a wall 3 ft 8 in exposed is 4 ft 2 in total and may be over. Any surcharge above the wall lowers the threshold regardless.
Why do retaining walls fail?
Trapped water, more than anything else. Saturated soil creates hydrostatic pressure that pushes the wall forward, and block strength doesn’t resist it. Other causes are shallow bases in frost areas, skipping the backward batter, undersized block for the height, and building on unstable clay.
How much gravel do I need behind a retaining wall?
At least a 12 in wide chimney of clean angular stone running the full height, plus the compacted base. On a 20 ft × 3 ft wall that’s about 3 yd³ of drainage stone against 0.6 yd³ of base — the drainage is roughly five times the base, and it’s the one people forget to budget for.
Do I need a drain pipe behind a retaining wall?
Yes, on anything but the smallest garden border. A perforated pipe at the base of the drainage stone, wrapped in filter fabric, sloped at least ⅛ in per foot toward a real outlet. Over 40 ft that’s 5 in of fall, which has to be planned before the base goes in.
What is geogrid and do I need it?
High-strength polymer mesh laid between courses and extending into the backfill, tying the wall to a mass of reinforced soil. Most systems require it above 3 to 4 ft of exposed height, typically every second course extending 4–8 ft back. The exact lengths and spacing must come from an engineered design.
Can I backfill with the soil I dug out?
Not directly behind the wall. That zone needs clean angular drainage stone. Excavated soil can go behind the drainage zone if it’s granular, but clay should be replaced rather than reused — it holds water and expands when saturated, adding load the wall was never designed for.
How much does a retaining wall lean back?
About ¼ in per course, so roughly 2 in over a 4 ft wall. Most segmental blocks build this batter in through a lip or pin, so correct stacking produces it automatically. It gives the wall margin — a vertical wall has none, and any settlement takes it past plumb.
What is a surcharge on a retaining wall?
Any load above the wall beyond level ground — a rising slope, a driveway, a pool or a structure. It raises base pressure sharply and changes the failure mode. A wall that would be fine at 3 ft with level ground behind can need engineering at 2 ft with a driveway above.
How wide should the base trench be?
Twice the block depth, so an 8 in deep block needs a 16 in trench. That way the blocks sit fully on compacted stone rather than half on stone and half on subgrade. Depth runs 6–8 in of compacted base, and deeper where frost is a factor.
Can I use this retaining wall calculator in metric?
Dimensions are in feet and inches, since segmental block is sold that way in North America. Multiply feet by 0.3048 for metres and inches by 25.4 for millimetres. The one-tenth burial rule and the ¼ in per course batter are ratios, so they hold in any units.
Sources and method
- NCMA Design Manual for Segmental Retaining Walls — drainage as the primary failure mechanism, and general SRW practice
- Burial depth taken as the greater of one full course and one tenth of exposed height, which is standard segmental wall practice
- Height thresholds for engineered design vary by jurisdiction — 4 ft is common, some use 3 ft, and some permit 6 ft with geogrid. Local codes take precedence
- Block dimensions are nominal face sizes; confirm against your manufacturer’s spec sheet, which also states the maximum unreinforced height for that block
- Geogrid spacing and length figures are indicative only. They are the output of an engineered design, not a rule of thumb
- This estimates materials. It does not evaluate soil bearing capacity, internal friction angle, groundwater, frost depth, surcharge or global stability
Related calculators
How this retaining wall calculator works out its numbers
Each section is buried independently at the greater of one full course and one tenth of its exposed height, rounded up to a whole course since you can’t lay part of a block. Total height drives the course count, the drainage volume and the threshold check — not the exposed height you measured.
Blocks are courses times blocks per course, both rounded up, so partial courses and partial blocks are counted as whole units. Base volume uses a trench twice the block depth wide with a 5% compaction allowance. Drainage volume runs the full total height and is increased for poorer-draining soil, since clay needs a more generous stone zone to work at all.
The threshold check compares total height against your local figure, which is editable because it isn’t universal. Where the result sits within one course of the threshold, the calculator says so rather than quietly passing you.
None of this is a structural design. A retaining wall is a geotechnical structure, and whether it stands depends on the soil it sits on and the soil it holds back — bearing capacity, friction angle, groundwater, frost, and the stability of the whole slope, not just the wall. Failures are sudden and can injure people. Anything above your local threshold, anything carrying a surcharge, and anything on ground you’re not confident about needs a licensed engineer and a permit. Use this to plan and budget; use an engineer to decide whether to build.