Every online retaining wall calculator gives you a block count. Then they send you to the store, and you buy 126 blocks. You get them home, you start digging, and somewhere on day two you realize you need drainage stone behind the wall — several cubic yards of it that you forgot entirely. This is how retaining walls fail. Not from the block count being wrong, but from skipping the drainage aggregate step. Water building up behind an undrained retaining wall creates hydrostatic pressure that can exceed 60 times the lateral soil pressure the wall was designed for. Our calculator is the only one that shows block count and drainage stone together because you need to order both on the same truck.
The second thing every other calculator misses is the engineering flag. In most US jurisdictions, retaining walls over 4 feet of exposed height require a building permit and a review of the wall design for adequate drainage, base depth, and either deadman reinforcement or geogrid layers. A retaining wall over 4 feet that fails can damage property, injure people, and expose the owner to significant liability. Our calculator automatically flags this threshold and shows the geogrid layer count for walls that need it — information that competitors simply do not provide.
Block count: wall length (ft) times total height including burial (ft) divided by block face area (0.75 sq ft for standard 6×18-in block) times 1.05 waste. Base burial is the maximum of 6 inches or 1 inch per foot of exposed height, whichever is greater. Drainage stone: wall length times exposed height times 1 foot (the required drainage column width) divided by 27 for cubic yards. Base course stone: wall length times 1 foot (block depth) times 0.5 foot (6-in base layer) divided by 27. Geogrid layers: for walls over 4 ft, one layer per 2 ft of height starting at 2 ft from the top, each extending 6 ft back into compacted fill. Engineering flag triggers at 4 ft exposed height.
01
The Drainage Aggregate Is Not Optional — It Is the Wall
The most important element of a retaining wall is not the blocks — it is the drainage system. Water saturating the soil behind a wall creates hydrostatic pressure that can push the wall forward with enormous force. The required drainage system consists of two components that work together: a 12-inch-wide column of No. 57 crushed stone (3/4-inch clean crushed stone, not pea gravel or bank run gravel) placed directly behind the block face for the full height of the wall, and a 4-inch-diameter perforated drainpipe laid in crushed stone at the base of the first block course. This drainpipe must daylight (exit the wall) at both ends at a downhill point where water can flow away from the structure. Our calculator shows the exact cubic yards of drainage stone for your wall — and it is a larger number than most people expect.
02
Step Your Wall — Never Build One Straight Vertical Face Above 4 Feet
The US standard for segmental retaining walls (like the blocks sold at Lowe’s and Home Depot) is that walls over 4 feet of exposed height require either geogrid reinforcement (layers of polymer mesh embedded in the compacted fill at 2-foot intervals behind the wall), deadman anchors, or the wall must be broken into a stepped terrace design with each step set back enough to create two separate walls each under 4 feet. The stepped terrace approach — two 2-foot walls with a flat compacted bench between them — is often the most practical for DIY builders because it eliminates the geogrid requirement entirely. For a 6-foot slope, you can build two 3-foot walls set back 6 to 8 feet from each other and avoid both the geogrid and the engineering review in most jurisdictions.
03
Bury the First Course Correctly or the Wall Will Lean Forward
The base course of a segmental retaining wall must be buried in level, compacted 3/4-inch crushed stone. The minimum burial depth is 6 inches below finished grade, measured from the bottom of the first block. Many DIYers do not dig deep enough, resulting in the base course sitting on inadequately compacted soil that settles unevenly over the first winter, causing the wall to tip forward. Before laying the first block, excavate to at least 6 inches of depth, compact the subgrade with a plate compactor (available for rent at home improvement stores for approximately $60 per day), spread a 6-inch layer of compacted crushed stone, check for level in both directions with a long level, and then place the first block course on top. The first course is everything — get it level and you can build a good wall. Set it out of level and every subsequent course will amplify the error.
How many retaining wall blocks do I need?+
For standard 6-inch-tall by 18-inch-wide segmental retaining wall blocks: multiply wall length (ft) times total height including base burial (ft), divide by 0.75 (the face area of one block in sq ft), then multiply by 1.05 for waste. For a 20-ft wall at 4-ft exposed height with 6-in burial: 20 x 4.5 divided by 0.75 x 1.05 = 126 blocks. Our calculator handles the burial depth automatically based on your wall height. For walls with corners, add approximately 5% additional blocks for the corner blocks.
How deep should a retaining wall be buried?+
The base course of a retaining wall should be buried a minimum of 6 inches below finished grade for walls up to 6 feet tall. For taller walls, the burial depth increases by 1 inch for every foot of exposed wall height beyond 6 feet. The base course sits on 6 inches of compacted crushed stone, not on soil directly. The 6-inch minimum burial is critical for preventing the base course from sliding or tipping forward under the lateral earth pressure of the retained soil. In cold climates, some contractors add an additional 2 to 4 inches of burial depth to keep the base course below the frost action zone, which reduces the heaving force on the base course during freeze-thaw cycles.
Do retaining walls need drainage?+
Yes — drainage is not optional for any retaining wall. Water saturating the soil behind a wall creates hydrostatic pressure that can be 60 or more times greater than the lateral soil pressure the wall was designed to resist. Every retaining wall needs a minimum 12-inch-wide column of clean crushed stone (No. 57 stone, 3/4-inch) placed directly behind the block face for the full wall height, and a 4-inch-diameter perforated drainpipe at the base of the wall that exits at both ends at a downhill point. Without this drainage, a wall that looks structurally sound can fail after a single heavy rain event. Our calculator shows the cubic yards of drainage stone required for your wall dimensions.
When do retaining walls need engineering?+
In most US jurisdictions, retaining walls over 4 feet of exposed height require a building permit and an engineering review by a licensed structural or civil engineer. This threshold is specified in the International Residential Code (IRC) and adopted with local amendments in most US states. Walls in seismically active zones (California, the Pacific Northwest, parts of the Mountain West), walls retaining soil on slopes steeper than 3:1, walls adjacent to structures, and walls within public rights-of-way may have lower thresholds or additional requirements. Always contact your local building department before constructing any retaining wall over 3 feet tall to confirm the permit requirements for your specific jurisdiction.
What is geogrid for retaining walls?+
Geogrid is a polymer mesh material embedded in the compacted fill behind a retaining wall at regular vertical intervals to add structural reinforcement. Geogrid works by creating a composite reinforced-earth structure behind the wall — the grid layers tie the retained soil to the wall blocks, dramatically increasing the lateral resistance of the entire assembly. Without geogrid, the blocks alone must resist all the lateral earth pressure, which limits wall height. With geogrid, the wall and the reinforced soil zone behind it act together, allowing wall heights of 6 to 8 feet or more. Standard residential geogrid (Tensar BX1200, Mirafi, or equivalent) is placed in the compacted fill at every 2 feet of wall height for walls over 4 feet tall, extending horizontally 6 feet or more back into the fill from the back face of the block. Our calculator shows the number of geogrid layers and total square footage for your wall.
How much drainage stone does a retaining wall need?+
The standard drainage stone requirement is a 12-inch-wide column of No. 57 crushed stone (3/4-inch clean stone) behind the wall for the full exposed height. Volume calculation: wall length (ft) times wall exposed height (ft) times 1 ft (the 12-inch column width) divided by 27 for cubic yards. For a 20-ft wall at 4-ft exposed height: 20 x 4 x 1 / 27 = 2.96 cubic yards. This is approximately 4.1 tons of stone, which typically requires one or two deliveries from a local stone yard depending on truck capacity. This material is often not displayed anywhere on retaining wall block packages or home improvement store calculators — which is why we built this tool.
What type of stone goes behind a retaining wall?+
The correct backfill material directly behind a retaining wall is No. 57 crushed stone (also called 3/4-inch clean crushed stone, washed stone, or drainage stone). This is a sharp-edged, uniformly-sized crushed stone that drains freely because it has no fine particles to clog the voids between stones. Do not use pea gravel (it is rounded and migrates), bank-run gravel or road base (it contains fines that can clog), topsoil, or decomposed granite as the drainage aggregate. The 12-inch column of No. 57 stone must be separated from the compacted fill soil behind it with a non-woven geotextile fabric (landscape fabric) to prevent fine soil particles from migrating into the stone column and clogging the drainage over time.
How far back should retaining wall blocks step back?+
Most segmental retaining wall blocks are designed with a built-in setback (batter) of 1/2 to 1 inch per course — meaning the wall leans slightly back into the retained soil as it gets taller. For standard big box store retaining blocks, this setback is built into the block geometry and happens automatically when blocks are stacked. Over a 4-foot-tall wall (9 courses at 6-in each), the cumulative setback is approximately 4.5 inches at the top, which is why the top course appears to be about 4 to 5 inches further into the slope than the base course. For taller walls with geogrid, the wall manufacturer’s installation guidelines specify the exact setback requirements for structural design. The setback (batter) increases the wall’s resistance to overturning — do not try to build a retaining wall perfectly vertical, as this reduces stability.
Can I build a retaining wall without mortar?+
Yes — the standard segmental retaining wall blocks sold at home improvement stores (like Allan Block, Pavestone, Versa-Lok, or generic landscape blocks) are dry-stacked with no mortar between courses. The blocks interlock by gravity, set-back angle, and pin connectors (or shear keys built into the block profile). The cap course can be adhered with concrete adhesive for a finished appearance and to prevent individual cap blocks from being displaced. Mortar is not required for standard residential retaining walls up to 4 feet in height with adequate drainage and base preparation. Taller walls and structural walls built from poured concrete or CMU block do use mortar, and those walls require engineering design. Our calculator is specifically designed for the standard dry-stacked segmental block walls most common in US residential construction.
How do I calculate the cost of a retaining wall?+
For a DIY retaining wall, the primary material costs are: blocks at approximately $3 to $6 each for standard 6×18-in segmental blocks ($4.00 is typical), No. 57 drainage stone at approximately $35 to $65 per cubic yard delivered, geotextile fabric at approximately $0.10 to $0.20 per square foot, and base course crushed stone (same price as drainage stone). For a 20×4 ft wall: blocks = 126 at $4.00 = $504; drainage stone = 2.96 cu yd at $50 = $148; base stone = 0.37 cu yd at $50 = $19. Total materials = approximately $671 before tax for a standard wall without geogrid. Our calculator adds the block cost when you enter the price per block. Geogrid, drainpipe, and cap adhesive add $100 to $300 depending on wall height and length.
What is the difference between a retaining wall and a garden wall?+
A retaining wall holds back soil — one side is buried against an earth grade change and the wall is under lateral soil pressure. A garden wall (also called a landscape wall or border wall) is a freestanding decorative wall on flat ground that does not retain soil on either side. Retaining walls require drainage behind them, adequate base burial below grade, and often geogrid or other reinforcement depending on height. Garden walls can be built with much simpler construction because they are not resisting lateral soil pressure — they need only a level foundation, adequate compaction, and standard mortar or dry-stacking. Our calculator is designed for retaining walls. For garden walls where both faces are at the same grade, you only need the block count portion of the calculation and no drainage aggregate is required.
How long does it take to build a retaining wall?+
For a standard 20×4 ft retaining wall (126 blocks), an experienced DIYer typically completes the project in 2 to 3 full working days. Day one: mark the layout, excavate the base trench to the required depth, compact, and lay the drainage stone base, set and level the first course of blocks. Day two: continue building courses, adding drainage stone behind each course as you go, install the geotextile fabric, and lay the perforated drainpipe. Day three: complete the upper courses, cap the wall, compact backfill in the soil zone (not the drainage stone zone), and clean up. The first course typically takes 30 to 60 minutes per linear foot due to the excavation, leveling, and checking required. Upper courses go much faster once the first course is level. Renting a plate compactor for the base and backfill compaction is strongly recommended.
How do I deal with a corner in a retaining wall?+
Corners in segmental retaining walls use alternating full-size and half-size blocks at each course to maintain the running bond pattern and tie the two wall sections together at 90 degrees. Most manufacturers sell corner blocks in their system, or you can cut full blocks in half with a masonry saw or angle grinder to create the corner pieces. At each course, the block orientation at the corner alternates — one course the long dimension runs one direction, the next course it runs the other. This creates an overlapping corner that is structurally adequate for standard residential retaining walls. Add approximately 10% to your block count for each inside or outside corner to account for the additional cuts and overlap required.
What causes retaining walls to fail?+
The three most common causes of retaining wall failure in US residential construction are: no drainage (water building up behind the wall creating hydrostatic pressure that pushes the wall forward), inadequate base preparation (the first course settling unevenly into poorly compacted soil), and insufficient height of buried base course (the wall tips forward when the first course has too little mass below grade to resist the overturning moment). A wall that was properly built without drainage can fail completely after a single heavy rain event or a spring snowmelt. Walls built on improperly compacted soil settle unevenly over the first winter, creating a wavy or leaning appearance that worsens each year. Our calculator includes the drainage aggregate calculation to help you avoid the first and most common failure mode.
Do I need a permit for a retaining wall?+
Most US jurisdictions require a building permit for retaining walls over 4 feet of exposed height. Some jurisdictions have lower thresholds (3 feet in some California counties) and some have higher (6 feet in parts of the Mountain West). The permit process for a retaining wall under 6 feet with adequate drainage is typically straightforward — often a counter permit with a simple site plan. However, skipping the permit and building without approval can result in a stop-work order if a neighbor complains, a requirement to remove and rebuild the wall, and significant liability exposure if the wall later fails and damages adjacent property. Always verify the local requirement with your building department before construction. Our calculator flags the 4-foot threshold that applies in most US jurisdictions.
What is the best base for a retaining wall?+
The best base for a segmental retaining wall is a 6-inch-deep layer of compacted No. 57 crushed stone (3/4-inch clean stone) at the bottom of the excavated base trench, topped with a level first course of blocks. The crushed stone base provides several critical functions: it distributes the weight of the wall uniformly along the base course, it drains water away from under the blocks so the base course does not frost-heave as severely, and it provides a material that can be raked and adjusted to create a perfectly level setting surface. Do not use sand as the base course material for retaining walls — sand migrates under load and the wall will settle unevenly. Do not set the first course directly on soil — soil compacts under load and cannot be leveled as precisely as a crushed stone base. The base course crushed stone is one of the values shown in our calculator.