Retaining Wall Calculator
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How the Retaining Wall Calculator works
This tool counts units of a product rather than cubic yards, so the answer depends on the piece size you actually buy. Change the selection and the figure changes with it. The suggested order adds the waste allowance shown beside it, as its own number rather than a multiplier you have to apply.
Using your measurements
7 courses × 15 blocks = 105 blocks
Assumption: Standard SRW · 16 W × 6 H in · 3 ft exposed wall · first course buried 6 in
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Retaining wall engineering & material estimating
Retaining wall calculator for interlocking blocks, cinder blocks, timber, and drainage stone
A retaining wall holds back soil, stabilizes slopes, prevents erosion, and creates functional flat terraces on uneven land. Whether you are building an interlocking concrete block garden border, a timber tie terrace, or a heavy-duty segmental retaining wall (SRW), accurate material estimation is critical. Every retaining wall requires more than just face blocks: it requires an excavated trench, a compacted crushed stone leveling pad, a buried foundation course for toe resistance, clean angular drainage aggregate backfill, and a perforated drainage pipe to prevent hydrostatic water pressure from overturning the structure.
To calculate how many retaining wall blocks you need, determine the wall face area in square feet: Wall Length (ft) × Total Height (ft, including the buried bottom course). Divide the total wall face area by the square footage of a single block face (e.g., a standard 16 × 6-inch block covers 0.67 sq ft). Always add 10% to 15% extra blocks for end cuts, curve step-downs, and corner fitting. For structural stability, the bottom course must be buried below grade by at least 1/10th of the wall height (minimum 4 to 6 inches) on top of a 6-inch compacted aggregate leveling pad.
Water is the number one cause of retaining wall failure. Saturated soil creates immense hydrostatic pressure behind the wall that can easily bulge, tilt, or blow out unreinforced masonry. To ensure decades of stability, every retaining wall taller than 2 feet must have at least 12 inches of free-draining 3/4-inch clean crushed rock (ASTM No. 57) placed directly behind the wall units, lined with non-woven geotextile filter fabric, and drained through a 4-inch perforated PVC or corrugated drain pipe sloped to daylight.
Retaining wall block dimensions, coverage & foundation specifications
Standard commercial retaining wall block sizes, unit face coverage, approximate weights, and structural base requirements.
| Retaining Wall | Retaining wall block dimensions, coverage & foundation specifications |
|---|---|
| Standard SRW Concrete Block (16 W × 6 H in) | 0.67 sq ft face area (1.50 blocks / sq ft) · 25–35 lb per block (~45–60 blocks per pallet) |
| Large Structural Block (18 W × 8 H in) | 1.00 sq ft face area (1.00 block / sq ft) · 55–80 lb per block (~30–40 blocks per pallet) |
| Garden Landscape Block (12 W × 4 H in) | 0.33 sq ft face area (3.00 blocks / sq ft) · 12–18 lb per block (Low borders under 2 ft) |
| Standard CMU Cinder Block (16 W × 8 H × 8 D in) | 0.89 sq ft face area (1.125 blocks / sq ft) · Requires poured concrete footer & rebar |
| 6 × 6 in Pressure-Treated Wood Timber | 0.46 sq ft face per linear foot · Requires galvanized lag screws & deadman anchors |
| Compacted Base Leveling Pad (Crushed Stone) | 6.0 in (150 mm) compacted depth · 3/4-inch minus dense-graded aggregate or road base |
| Drainage Aggregate Backfill Zone | 12 in minimum thickness behind wall · Clean 3/4-inch angular stone (ASTM No. 57) |
| Perforated Drain Pipe & Geotextile Fabric | 4-inch perforated drain tile along wall base · Non-woven filter fabric envelope |
Step-by-step retaining wall construction & installation process
Proven contractor installation sequence to ensure your retaining wall remains plumb, level, and structurally secure for decades.
Excavation & Trenching Step 1 | Dig a trench twice as wide as the block depth (approx. 18–24 inches wide) and deep enough to accommodate a 6-inch compacted gravel pad plus one buried base course (4 to 8 inches below finished grade). Ensure the trench base is firm and level. |
|---|---|
Compacted Base Pad Step 2 | Add 6 inches of 3/4-inch minus crushed aggregate road base. Compact thoroughly in 2-inch to 3-inch layers using a mechanical plate compactor or hand tamper until rock-hard and perfectly level side-to-side and front-to-back. |
Base Course Installation Step 3 | Set the first row of blocks centered on the gravel pad. Check every single block with a torpedo and 4-foot carpenter level in both directions. Tap with a rubber mallet to adjust. This foundation row governs the alignment of the entire wall. |
Stacking & Stepping on Slopes Step 4 | Stack succeeding courses in a staggered running-bond pattern (joints centered over lower units). Engage interlocking rear lips or fiberglass alignment pins. Step the base trench up the slope in increments equal to the block height when working on an incline. |
Drainage Pipe & Backfill Step 5 | Place a 4-inch perforated drain pipe with holes facing down behind the first course. Fill 12 inches directly behind the wall with clean 3/4-inch angular stone. Add biaxial geogrid reinforcement every 2 courses for walls above 3 to 4 feet. Cap with adhesive. |
Critical retaining wall construction mistakes that cause failure
Omitting the 12-inch drainage stone column and perforated pipe
Trapped groundwater is the leading cause of retaining wall collapse. Saturated backfill exerts massive lateral hydrostatic pressure that pushes walls forward. Always install a 4-inch perforated drain pipe wrapped in geotextile fabric and backfill with at least 12 inches of clean, angular crushed rock directly behind the block column.
Failing to bury the bottom foundation course
Placing the first row of blocks directly on ground level causes base slippage and toe failure. The entire first course (minimum 1/10th of total wall height, or 4 to 8 inches) must be fully buried below the finished lower grade on a 6-inch compacted aggregate leveling pad.
Skipping geogrid reinforcement on walls taller than 3 to 4 feet
Gravity walls rely strictly on block weight and setback. Once a segmental retaining wall exceeds 3.5 to 4 feet in exposed height (or holds a slope/driveway surcharge above it), soil reinforcement geogrid must be embedded between block courses extending several feet back into the compacted bank.
Using native clay or topsoil as direct backfill
Native soil, particularly clay or organic topsoil, expands when wet and contracts when dry, creating severe cyclic stress that destroys retaining walls. Backfill the immediate 12-inch zone with clean drain rock and use compactable granular fill for the structural backfill zone behind it.
Retaining walls, specifically
Frequently asked questions about retaining walls
What is a retaining wall?
A retaining wall is a rigid, engineered landscape structure designed to hold back soil and rock from a building, patio, driveway, or slope. It stabilizes sloped terrain, resists lateral earth and water pressure, prevents erosion, and creates flat, usable terraces and outdoor living spaces on properties with grade changes.
What are the four types of retaining walls?
The four primary types of retaining walls are: (1) Gravity retaining walls, which rely on the sheer weight and mass of stone, concrete blocks, or boulders to hold back earth; (2) Cantilever retaining walls, built from reinforced poured concrete or mortared masonry with an internal L-shaped or T-shaped structural footing; (3) Segmental / Sheet piling / Anchored retaining walls, which use interlocking dry-stack blocks reinforced with horizontal geogrid tiebacks or mechanical cable anchors embedded deep into the hillside; and (4) Counterfort / Buttressed retaining walls, which incorporate internal concrete webs or exterior support buttresses for extreme industrial heights.
What is the purpose of a retaining wall?
The primary purpose of a retaining wall is to hold soil in place, prevent landslides and downhill slope erosion, manage surface stormwater runoff, and convert steep, unbuildable slopes into functional, multi-tier flat terraces, garden beds, driveways, or walkout basement foundations.
What is the lifespan of a retaining wall?
A properly installed interlocking concrete block (SRW) or poured concrete retaining wall lasts between 50 and 100+ years. Natural stone retaining walls can easily endure over 100 years. In contrast, pressure-treated wood or railroad tie timber retaining walls typically last 15 to 25 years before rot and moisture decay require replacement.
What is the cheapest option for a retaining wall?
The cheapest retaining wall option is pressure-treated landscape timbers (such as 6x6 wood ties) or dry-stacked interlocking concrete garden wall blocks, costing $15 to $25 per square foot installed ($5 to $10 per sq ft for DIY materials). Dry-stack native fieldstone or interlocking segmental concrete blocks are the most cost-effective long-term options because they do not rot and require no mortared concrete footers.
How to build a retaining wall?
To build an interlocking block retaining wall: (1) Plan layout and excavate a trench 6 inches deeper than your base course plus 6 inches for the gravel pad. (2) Lay and compact 6 inches of 3/4-inch minus aggregate road base. (3) Set and level the first buried course of blocks side-to-side and front-to-back. (4) Stack subsequent courses in a staggered running bond, interlocking rear lips or pins. (5) Place a 4-inch perforated drain pipe behind the base course. (6) Backfill 12 inches with clean 3/4-inch angular drainage stone, add geogrid if over 3.5 ft, and secure cap stones with landscape masonry adhesive.
How to build a retaining wall on a slope?
To build a retaining wall on a slope, use a stepped trench method. Start at the lowest elevation of the slope and excavate a series of level step-downs, stepping upward in exact increments equal to the height of one block (e.g., 6 or 8 inches). Always install a minimum 6-inch compacted gravel leveling pad across each step. Place the lowest block first, then work into the hillside so each upper course overlaps and locks into the stepped foundation course.
How much does a retaining wall cost?
A residential retaining wall typically costs $20 to $60 per square foot of exposed face installed, or $35 to $90+ for natural stone or tall engineered walls; DIY materials run about $8 to $15 per square foot. A 20 ft long, 3 ft high block wall (60 sq ft of face) takes about 105 standard 16 × 6 in blocks including the buried course, 0.75 cu yd of leveling-pad gravel, and 2.2 cu yd of drainage stone. Enter your own block and stone prices in the cost panel above for a figure on your wall.
How to build a small retaining wall?
To build a small garden retaining wall (under 2 feet high): (1) Mark the perimeter with spray paint or garden hose. (2) Dig a trench 4 inches deep and 12 inches wide. (3) Pour 2 to 3 inches of crushed gravel base and hand-tamp flat. (4) Lay your first row of 12x4-inch garden blocks level. (5) Stack 2 to 4 courses in a running bond, interlocking the rear tabs. (6) Backfill with 6 inches of gravel directly behind the blocks, followed by garden soil, and glue the top cap units.
How to build a cinder block retaining wall?
To build a cinder block (concrete masonry unit / CMU) retaining wall: (1) Excavate and pour a structural reinforced concrete footing (minimum 8–12 inches deep and twice the wall thickness) below the local frost line. (2) Insert vertical #4 or #5 rebar dowels into the wet concrete spaced every 16 to 32 inches. (3) Lay CMUs in mortar with staggered joints over the cured footing. (4) Place horizontal rebar in bond-beam blocks every 2 courses. (5) Fill all hollow block cores with poured pea-gravel concrete grout. (6) Apply waterproof coating, install a drainage pipe, and backfill with clean stone.
How to cut retaining wall blocks?
To cut retaining wall blocks: (1) Mark the cut line on all four faces with a pencil and straightedge. (2) Score the line 1/4-inch to 1/2-inch deep using a circular saw or angle grinder equipped with a diamond masonry blade. (3) Place the block on flat ground, position a wide brick chisel (mason's bolster) into the scored groove, and strike firmly with a 3 lb sledgehammer to snap cleanly. Alternatively, use a diamond wet tile saw for perfectly smooth cuts or a mechanical guillotine paver/block splitter.
How to build a block retaining wall?
To build an interlocking segmental block retaining wall: Dig an 18-inch-wide trench, add 6 inches of 3/4-inch minus gravel, and compact until solid. Bury the bottom row of blocks 4 to 6 inches, leveling each block individually. Stack successive rows with staggered joints, pulling each block forward to engage its locking lip. Backfill with 12 inches of angular drainage stone and compact soil in 6-inch lifts as you build up. Secure the top course with heavy-duty exterior polyurethane concrete adhesive.
How to build a wood retaining wall?
To build a wood or timber retaining wall: (1) Excavate the slope and dig post holes 3 to 4 feet deep every 4 to 6 feet along the line. (2) Set pressure-treated 4x4 or 6x6 vertical posts in concrete below the frost line. (3) Fasten horizontal pressure-treated 2x8 or 6x6 timbers to the uphill side of the posts using 1/2-inch galvanized lag screws or structural timber screws. (4) Install perpendicular deadman tiebacks every 8 feet extending 4 to 6 feet into the hillside bank. (5) Line with landscape fabric, lay a 4-inch perforated drain pipe, and backfill with crushed stone.
How to build a concrete retaining wall?
To build a poured concrete retaining wall: (1) Excavate below the frost line and construct wooden forms for a wide reinforced concrete footing (slab foundation). (2) Install structural steel rebar grid (#4 or #5 rebar tied in a 12-inch grid). (3) Pour and cure the concrete footing. (4) Erect vertical formwork panels with internal steel tie rods. (5) Place horizontal rebar and vertical dowels, pour 3,000–4,000 PSI concrete into the wall forms, and vibrate to eliminate air voids. (6) Strip forms after 3 to 7 days, apply waterproofing sealant on the rear face, install weeping tile drainage pipe, and backfill with clean stone. The concrete calculator sizes the footing and the wall pour.