How to measure for your retaining wall project
- Measure the wall footprint. Measure the total length of the wall face in feet, then measure (or plan) the exposed height from finished grade to the top of the last full course. Do not include the buried starter course in the height — it sits below grade and is covered by the base gravel.
- Pick a block size. Choose the block face size that matches the product you plan to buy. The 12 × 4-inch option is the common small garden-wall block; the 16 × 8-inch option suits larger structural walls. The block size drives both how many units you need and how many courses stack to your target height.
- Read the results and add waste. You get total blocks, courses, cap blocks (one per linear foot of wall), base gravel for the 2-ft-wide leveling pad, and drainage gravel for the 12-inch zone behind the wall. Add 5–10% to the block count for end cuts, damaged units, and the buried starter course.
How the retaining wall calculator works
The block count is simply courses × blocks-per-course. Courses = wall height (in inches) ÷ block height, rounded up; blocks per course = wall length (in inches) ÷ block width, rounded up. For a 20-ft × 3-ft wall in 12 × 4-in blocks: courses = (3 × 12) ÷ 4 = 9; blocks per course = (20 × 12) ÷ 12 = 20; total blocks = 9 × 20 = 180. Base gravel uses a 2-ft-wide leveling pad: (length × 2 ft × base depth in ft) ÷ 27 = (20 × 2 × 0.5) ÷ 27 ≈ 0.74 cubic yards at the default 6-inch depth. Drainage gravel fills a 12-inch zone behind the full wall face: (length × height × 1 ft) ÷ 27 = (20 × 3 × 1) ÷ 27 ≈ 2.22 cubic yards.
Which type are you estimating?
Standard concrete garden-wall block (small, no pins)
The flat, rectangular 12 × 4-inch block sold at big-box stores. No lips or pins — courses are simply stacked with a slight setback. Best for landscape borders, raised beds, and short decorative walls up to about 2 ft. Gravity and mass hold these in place.
Enter: Select "12 in × 4 in" block; keep height ≤ 2 ft for unpinned stacking
Segmental retaining wall (SRW) block — interlocking lip or pin system
The most common structural system: a block with a rear lip or pinned connection that locks each course to the one below and creates a built-in backward batter (lean into the slope). Suitable for walls up to 3–4 ft without geogrid; taller walls need geogrid reinforcement layers every 2–3 courses. Popular brands include Allan Block, Versa-Lok, and Anchor.
Enter: Select "16 in × 6 in" or "18 in × 6 in" block; add geogrid spec for walls over 4 ft
Natural stone (fieldstone or dry-stack)
Irregular stone requires no mortar but demands careful fitting. Estimate coverage at roughly 1 ton per 35–40 sq ft of wall face at 12-inch depth, but material thickness varies widely. The block calculator above will overcount — use it for a rough floor estimate and adjust for actual stone depth and void space.
Enter: Use the calculator for a rough course count; verify with your stone supplier
Timber (railroad tie or landscape timber) wall
Pressure-treated 6 × 6 or 8 × 8 timbers stacked horizontally. Each timber course is 5.5 or 7.5 inches tall; use the 16 × 6-in block option to approximate course count. Timbers are typically pinned with rebar or timber screws. Best for informal garden walls; wood decays over 15–25 years.
Enter: Select "16 in × 6 in" for approximate courses; verify timber length matches wall length
Wall height decisions — DIY vs. engineered
Walls up to 3–4 ft are typically DIY with a compacted gravel base and drainage pipe. Walls 4 ft and taller carry significantly more soil pressure and usually require an engineered design, geogrid reinforcement layers, and a building permit. Always check local codes before breaking ground.
Enter: Under 4 ft: use this calculator; over 4 ft: consult a structural engineer
Tips & ways to save
- Bury the first course of blocks at least half a block height (2–4 inches) below finished grade — this "starter course" locks the base and hides the footing from view.
- Pitch the leveling pad and each block course very slightly backward (about 1 inch per foot of wall height) — this "batter" lean keeps soil pressure from pushing the wall face outward.
- Run a perforated drainpipe at the base of the drainage gravel zone, sloped to daylight at each end of the wall, to prevent hydrostatic pressure from building behind the wall and cracking blocks.
- Backfill in 6-inch compacted lifts rather than all at once — uncompacted loose soil behind a new wall can generate enough pressure to push it over before the backfill settles.
- Walls over 3–4 ft often need geogrid reinforcement — a mesh fabric that extends back into the hillside to anchor the wall — and most jurisdictions require a permit. Check with your local building department before starting.
Blocks by wall size (12 × 4 in garden-wall block)
| Wall (length × height) | Courses | Blocks | Cap blocks |
|---|---|---|---|
| 10 × 2 ft | 6 | 60 | 10 |
| 20 × 2 ft | 6 | 120 | 20 |
| 20 × 3 ft | 9 | 180 | 20 |
| 30 × 3 ft | 9 | 270 | 30 |
| 40 × 4 ft | 12 | 480 | 40 |
For the common 12-in-wide × 4-in-high garden-wall block; larger blocks need fewer units (a 16×8-in block covers about 2.7× the face). Bury the bottom course below grade and add 5–10% for cuts. Walls over 3–4 ft usually need engineering and a permit.
Frequently asked questions
How many retaining wall blocks do I need?
How much gravel do I need behind a retaining wall?
How high can I build a retaining wall myself?
Do I need drainage behind a retaining wall?
How deep should the gravel base be under a retaining wall?
What is the "batter" on a retaining wall and how much do I need?
Sources
Related calculators
Reviewed by the BackyardCalc editorial team. Figures are computed from the formula above and checked against manufacturer yields.