Retaining Wall Blocks and Base
A 20 ft wall that is 3 ft exposed (about 4 courses of 8-inch-tall blocks) needs roughly 120 face blocks before corners and ~5–10% waste, plus a compacted gravel trench often 6 inches deep and a few inches wider than the block. Taller walls need engineering and may require geogrid—follow the manufacturer height chart.
How block and base estimates work
For a straight wall with uniform rectangular face blocks:
Courses = ceil(wall height ÷ block face height).
Blocks per course = ceil(wall length ÷ block face length).
Blocks = courses × blocks per course (then add waste for cuts and breakage).
Base trench gravel volume ≈ wall length × trench width × trench depth (convert to yards by dividing cubic feet by 27).
YardTally’s retaining wall calculator defaults to common 18×6 in face blocks, 5% waste, and a gravel trench about 24 in wide by 6 in deep unless you change those inputs. Refine aggregate tonnage with the gravel calculator. Material choice echoes themes in gravel types for driveways and drainage layers used under paver base gravel and sand.
Segmental landscape walls often include buried base courses, geogrid at specified elevations, drainage pipe, and clean stone behind the blocks. Those components are specified by height and soil conditions—do not scale a 3 ft DIY patio wall detail to a 8 ft yard cut without professional design.
Worked example: 20 ft long × 3 ft exposed height
With 18 in long × 6 in tall face blocks:
- Courses = ceil(3 ÷ 0.5) = 6 courses.
- Blocks per course = ceil(20 ÷ 1.5) = 14.
- Blocks = 6 × 14 = 84; with 5% waste ≈ 89 blocks.
- Face area = 20 × 3 = 60 sq ft.
- Base gravel at 20 ft × 2 ft wide × 0.5 ft deep = 20 cu ft ≈ 0.74 cu yd.
These figures match the retaining-wall calculator defaults for a 20×3 wall. If your manufacturer requires an extra buried course, add that course to the block count even though it does not show in the exposed height.
Block planning table
| Wall L × H | Courses | Per course | Blocks |
|---|---|---|---|
| 10 × 2 ft | 4 | 7 | 28 |
| 20 × 2 ft | 4 | 14 | 56 |
| 20 × 3 ft | 6 | 14 | 84 |
| 30 × 3 ft | 6 | 20 | 120 |
Curves, corners, and capped walls change counts. Cap units are often ordered as a separate linear-foot item.
Drainage, geogrid, and when DIY stops
Water is the quiet load behind many wall failures. A perforated drainpipe at the base of the drainage stone, outlets to daylight or an approved system, and free-draining backfill keep hydrostatic pressure down. Compacted clay jammed against the back of hollow blocks is a common mistake—follow the block producer’s backfill specification, which often calls for clean stone in a drainage zone.
Geogrid extends the reinforced soil mass for taller segmental walls. Grid type, length, and vertical spacing are not guesswork; they come from the manufacturer’s engineered charts or a site-specific design. If your project exceeds the published maximum for a gravity (unreinforced) stack—or if a driveway, fence post footing, or slope sits above the wall—stop and get qualified design help.
Base preparation deserves as much care as the visible face. Excavate to stable material, place angular gravel, and compact until the first course sits level (or correctly stepped on slopes). A crooked base course telegraphs all the way to the cap. For gravel selection intuition, read gravel types for driveways; for related thin-layer patio bases (not wall backfill), see paver base gravel and sand depth.
Utilities, easements, and property lines also constrain walls. Call before you dig, and confirm that redirecting drainage will not send water onto a neighbor’s foundation.
Step-by-step
- Determine retained height, slope above/below, and any driveway or structure surcharge.
- Check whether the wall height triggers permit or engineering thresholds in your city.
- Select a block system and read its maximum unreinforced height tables.
- Estimate blocks and base gravel with the retaining wall calculator.
- Excavate trench; place and compact base gravel level (or stepped on slopes per manual).
- Set base course carefully—errors compound upward.
- Stack with pins/lips as designed; install drainpipe and drainage stone; place geogrid when specified; backfill in lifts.
Tips
- Keep underground utilities marked before digging.
- Compact behind the wall in thin lifts; heavy clay dumped in one lift settles for years.
- Outlet drains to daylight or an approved system—trapped water raises pressure.
- Order extra blocks from the same production lot for color match.
- For low decorative seats under ~2 ft, still use a proper base; skipping gravel is a frequent failure mode.
- Dry-stack a mockup course on the driveway to confirm pattern and cap overhang before trench day.
Common mistakes
- Building “as tall as the slope” without drainage or grid when the system requires them.
- Setting the first course on native dirt without compacted aggregate.
- Using rounded decorative rock as structural base.
- Ignoring global slope stability above the wall.
- Counting only exposed face and forgetting buried courses.
- Backfilling with saturated soil in one dump truck pass and calling it done.
Height thresholds, permits, and utilities
Many local codes treat retaining walls above a certain height as engineered structures that need a permit, and a measurement of around four feet from the bottom of the footing to the top of the wall is a common threshold in many places—though it varies, and surcharges from driveways, slopes, or buildings can trigger earlier review. Call your building department before design, and schedule a utility locate before digging. Mark property lines, since walls near boundaries can raise drainage and access issues with neighbors. Where the soil is poor or the wall holds back water, a professional designer is worth the cost.
Sources
FAQ
How many retaining wall blocks do I need?
Divide wall length by block length and height by block height, multiply, then add waste. Example: 20 ft × 3 ft with 18×6 in blocks → 84 blocks before waste.
How deep should the gravel base be?
Manufacturer manuals vary; many small landscape walls use several inches of compacted dense-grade gravel in a trench wider than the block. The calculator’s 6 in depth × 24 in width is a planning default—not a universal code.
When do I need an engineer?
As walls get taller, carry surcharge, or support structures, codes and manufacturers require engineered designs. When in doubt, ask the building department.
Can I reuse the paver base depths?
Related ideas (compacted angular gravel), different structure. Use wall-system details, not patio sand beds, behind retained soil.
Do caps count in the face-block total?
Usually order caps separately by linear feet of wall.
Related calculators
- Retaining Wall Block Calculator — SRW blocks, courses, and base gravel.
- Gravel Calculator — Driveway and path gravel in yards and tons.