Retaining Wall Block Calculator
How many blocks for a retaining wall
Building a retaining wall? Enter the wall length and finished height to estimate the number of standard retaining-wall blocks needed.
How many retaining wall blocks?
This retaining wall calculator doubles as a retaining wall estimator for the base and backfill stone, so use it alongside the retaining wall gravel calculator figures below.
Enter the wall length and finished height as your two dimensions. A standard block face is 0.5 ft² — 12 inches wide by 6 inches high — so you need 2 blocks per square foot of wall face.
| Wall | Face area | Blocks | Courses | Per course |
|---|---|---|---|---|
| 10 × 2 ft | 20 ft² | 42 | 4 | 10 |
| 20 × 3 ft | 60 ft² | 126 | 6 | 20 |
| 20 × 4 ft | 80 ft² | 168 | 8 | 20 |
| 30 × 3 ft | 90 ft² | 189 | 6 | 30 |
| 40 × 4 ft | 160 ft² | 336 | 8 | 40 |
Enter the finished height above grade, then add one extra course for burial (see below). Block faces vary by manufacturer — 8 inch and 18 inch wide units exist alongside the standard 12 — so check yours and divide the face area by its actual coverage if it differs.
The base course decides everything
Nothing else on this page matters as much as the first course. It is set in a compacted gravel trench, levelled block by block in both directions, and buried about one block-height below grade — roughly 6 inches for a standard block, or a tenth of the wall height for taller walls, whichever is more.
| Item | Quantity |
|---|---|
| Trench gravel (6 in deep, 2 ft wide) | 0.81 yd³, 1.21 tons |
| Drainage backfill behind a 3 ft wall (12 in deep) | 2.44 yd³, 3.63 tons |
| Buried course | 1 full course, 20 blocks |
Burying the first course is what stops the wall sliding forward under load, and levelling it is what keeps every course above it straight. A base course that is out by a quarter inch is out by an inch and a half at the top of a six-course wall, and there is no fixing it later. Take the time here; it is the slowest and most valuable part of the job. Price the stone with the crushed stone calculator.
Drainage is why walls fail
Retaining walls almost never fail because the blocks were too weak. They fail because of hydrostatic pressure — water saturating the soil behind the wall and pushing. Wet soil is heavy and it pushes hard, and a wall built without drainage is holding back far more force than its designer intended.
- Backfill with free-draining angular gravel, at least 12 inches deep behind the whole wall face, so water reaches the base instead of building up behind it.
- Lay a perforated drain pipe at the base of that gravel, sloped to daylight at one end. Wrap it or the gravel in filter fabric so silt does not clog it.
- Separate the gravel from the soil with geotextile fabric. Without it, fines migrate into your drainage stone and it stops draining within a few seasons.
- Never backfill with the clay you dug out and call it done. That is the classic failure.
- Grade the top away from the wall so surface water does not run straight in behind it.
Setback, batter and reinforcement
Most segmental blocks have a lip or pin system that sets each course back slightly from the one below, giving the wall a batter — a backward lean into the slope, typically about 1 inch per foot of height. That lean is structural, not cosmetic: it puts the wall's mass against the load rather than balanced on top of it. Do not fight it or try to build plumb.
Above about 3 to 4 feet, gravity alone stops being enough and the wall needs geogrid — layers of structural mesh laid between courses and extending back into the retained soil, which effectively turn the soil itself into part of the structure. That is an engineered design, not a rule of thumb, and most jurisdictions require a permit and a stamped drawing at that height.
Two situations always need an engineer regardless of height: a wall surcharged by a driveway, pool, structure or slope above it, and any wall retaining saturated or poorly drained soil.
Curves, corners and caps
- Curves work because the blocks taper front to back. An inside curve tightens the joints and an outside curve opens them, so a circular retaining wall or a tight radius needs a few extra blocks — add 10% rather than 5%.
- Corners need corner units or cut blocks, and both faces must be measured; a 90-degree return is two separate wall lengths.
- Cap blocks are a separate item sold by the linear foot, not included in the count above. Glue them down with masonry adhesive.
- Cut with a masonry blade or split with a chisel, and wear respiratory protection — silica dust from cutting concrete block is genuinely hazardous.
How this retaining wall block calculator works out its numbers
Blocks are wall face area × (1 + waste) ÷ 0.5 ft², rounded up, with a 5% default waste allowance. Face area is length × finished height.
- 0.5 ft² is a standard 12 × 6 in block face, giving 2 blocks per square foot. Manufacturers also sell 8 in and 18 in wide units; check yours and adjust the area if it differs.
- Enter the finished height above grade. The buried base course is additional, so add one course of blocks beyond the count shown.
- Base and backfill quantities in the table use 110 lb/ft³ compacted crushed stone, the same constant as the paver base and crushed stone calculators, with 10% for compaction.
- Gravity wall geometry only. Nothing here models soil type, surcharge loads, geogrid layout or global stability. Walls over 3 to 4 ft, or any wall with a load above it, need an engineered design.
- Not included: cap blocks, masonry adhesive, drain pipe, geotextile fabric and excavation.
Reviewed September 2026.
Figures reviewed . Every worked example on this page is checked against the calculator above.
Retaining Wall Block Calculator: frequently asked questions
How many retaining wall blocks per square foot?
Two, for a standard 12 × 6 inch block face of 0.5 ft². A 20 ft long, 3 ft high wall is 60 ft² of face, so about 126 blocks including 5% waste — six courses of 20.
How many blocks for a 20 foot retaining wall?
About 126 blocks for a 3 ft high wall and 168 for a 4 ft one, both including 5% waste. Add one extra course beyond the finished height for the buried base course.
How deep should a retaining wall base be?
Bury the first course about one block height, roughly 6 inches, or a tenth of the wall height for taller walls, whichever is more. Set it in 6 inches of compacted gravel and level every block in both directions.
Do I need gravel behind a retaining wall?
Yes, always. At least 12 inches of free-draining angular gravel behind the full wall face, with a perforated drain pipe at the base sloped to daylight. Hydrostatic pressure from saturated soil is the main reason walls bulge and topple.
How tall can a retaining wall be without a permit?
Commonly 3 to 4 feet, though it varies by jurisdiction. Above that you need geogrid reinforcement and usually an engineered, stamped design. Confirm the limit with your building department before buying block.
What is batter on a retaining wall?
The backward lean into the slope, typically about 1 inch per foot of height, created by the setback lip on each block. It is structural rather than cosmetic, putting the wall's mass against the load. Do not try to build plumb.
How many extra blocks for a curved wall?
Add about 10% rather than the usual 5%. Blocks taper front to back so they form curves naturally, but inside curves tighten the joints and outside curves open them, and both produce more offcuts.
When does a retaining wall need an engineer?
Above 3 to 4 feet, and at any height if the wall is surcharged by a driveway, pool, structure or slope above it, or if it retains saturated or poorly drained soil. Those loads are outside what gravity block design covers.