Construction

Retaining Wall Calculator

Calculate exactly how many blocks, gravel, backfill, and drainage pipe you need for your retaining wall project, plus a full material cost estimate.

Quick Answer

A standard 20 ft long, 4 ft high retaining wall requires approximately 90 blocks, 0.7 cubic yards of base gravel, 1.8 cubic yards of backfill, and 24 feet of drainage pipe. Total material cost typically runs $400-$700 depending on block style. Always add 10% extra for breakage and cuts.

Wall Length

1 ft200 ft

Wall Height

1 ft10 ft

Block Size

Material Estimate

Wall Blocks

105

15 per row × 7 rows

Cap Blocks

Top finishing row

Total Weight

3,990 lbs

2.0 tons

Materials Breakdown

Material	Quantity	Est. Cost
Wall Blocks (Standard (8" x 6" x 16"))	105 blocks	$262.50
Cap Blocks	15 blocks	$45.00
Base Gravel (6" bed)	0.49 cu yd	$22.22
Backfill (crushed stone)	2.96 cu yd	$103.70
Drainage Pipe (perforated)	24 ft	$36.00
Landscape Fabric	120 sq ft	$30.00
Total Estimated Cost		$499

Cost Breakdown

Wall Blocks$262.50 (53%)

Cap Blocks$45.00 (9%)

Base Gravel$22.22 (4%)

Backfill$103.70 (21%)

Drainage Pipe$36.00 (7%)

Landscape Fabric$30.00 (6%)

Pro Tips

Order 10% extra blocks for cuts, breakage, and pattern matching.
Walls over 4 feet tall typically require a building permit and may need a structural engineer.
Install drainage pipe with 1% slope (1 inch per 8 feet) toward the outlet.
Compact each layer of gravel with a plate compactor for stability.
Use construction adhesive between the cap blocks and the top course.

About This Tool

The Retaining Wall Calculator helps homeowners, landscapers, and contractors estimate all the materials needed for a retaining wall project. Whether you are building a small garden wall or a large hillside retaining structure, this tool provides accurate block counts, gravel volumes, and cost estimates so you can plan your budget before breaking ground.

How the Calculator Works

The calculator takes three key inputs: wall length, wall height, and block size. From these dimensions, it computes how many blocks you need per row, how many rows high the wall will be, and the total number of wall blocks and cap blocks required. It also calculates the volume of base gravel for the foundation trench (assuming a standard 6-inch deep bed at twice the block width), the backfill volume behind the wall (assuming 12 inches of drainage aggregate), and the length of perforated drainage pipe needed to channel water away from the structure.

Choosing the Right Block Size

Standard concrete retaining wall blocks (8 by 6 by 16 inches) are the most common choice for residential projects. They are affordable, widely available at home improvement stores, and manageable for DIY installation at around 35 pounds each. Large blocks (12 by 8 by 18 inches) are heavier but cover more area per block, making them better suited for taller walls or commercial applications. Split-face blocks have a textured front that mimics natural stone, adding visual appeal to garden walls and landscape features. Allan Block style units are engineered with a built-in lip system that provides automatic setback for gravity wall construction.

Why Proper Drainage Matters

The number one cause of retaining wall failure is water pressure building up behind the wall. Hydrostatic pressure can push a wall forward, crack blocks, and eventually cause a collapse. That is why every retaining wall needs three drainage components: a gravel base for the foundation that allows water to flow under the wall, backfill aggregate behind the wall that prevents soil from holding water against the blocks, and a perforated drainage pipe at the base that channels water to a safe outlet. Landscape fabric between the backfill and native soil prevents fine particles from clogging the drainage aggregate over time.

Permits and Engineering Requirements

Most municipalities require a building permit for retaining walls over 4 feet tall (measured from the bottom of the footing to the top of the wall). Walls taller than 4 feet often require engineering by a licensed professional who will specify the footing depth, block type, geogrid reinforcement spacing, and drainage requirements for your specific soil conditions and surcharge loads. Even if your wall is under the permit threshold, it is wise to check local building codes as requirements vary by jurisdiction. Walls near property lines, public sidewalks, or utility easements may have additional setback requirements.

Cost Factors to Consider

The material cost estimate provided by this calculator covers blocks, gravel, backfill aggregate, drainage pipe, and landscape fabric at typical retail prices. However, your actual project cost will depend on several additional factors: delivery charges for heavy materials (blocks and gravel are typically delivered by truck), equipment rental if you need a plate compactor or mini excavator, geogrid reinforcement for taller walls, and labor if you hire a contractor. Professional installation typically adds $10 to $25 per square face foot of wall. For a DIY project, the materials shown in this calculator represent the majority of your total cost, though you should budget an additional 10 to 15 percent for miscellaneous supplies like adhesive, rebar pins, and filter fabric.

Frequently Asked Questions

How many retaining wall blocks do I need?

The number of blocks depends on your wall length, height, and the block dimensions. Divide the wall length by the block length to get blocks per row, then multiply by the number of rows. For example, a 20-foot wall using 16-inch blocks needs 15 blocks per row. A 4-foot tall wall with 6-inch blocks needs about 7 rows (6 wall rows plus 1 cap row), so roughly 90 wall blocks plus 15 cap blocks. Always add 10% extra for cuts and breakage.

How deep should the gravel base be for a retaining wall?

The standard recommendation is a 6-inch compacted gravel base beneath the first course of blocks. The trench should be at least twice as wide as the block depth to ensure proper weight distribution. For walls over 4 feet tall, many engineers specify an 8 to 12-inch gravel base. Use angular crushed gravel (not rounded river rock) because it compacts better and provides more structural support.

Do I need a drainage pipe behind my retaining wall?

Yes, drainage is essential for any retaining wall. Without proper drainage, water pressure builds up behind the wall (hydrostatic pressure) and can cause the wall to lean, crack, or fail entirely. Install a 4-inch perforated PVC or corrugated drain pipe at the base of the wall, wrapped in filter fabric, with a minimum 1% slope toward a daylight outlet or catch basin. This channels groundwater away before pressure builds.

How tall can I build a retaining wall without a permit?

In most jurisdictions, retaining walls under 4 feet tall (measured from the bottom of the footing to the top of the wall) do not require a building permit. However, local codes vary significantly. Some areas set the limit at 3 feet, and walls near property lines or in flood zones may have stricter requirements. Always check with your local building department before starting construction. Walls over 4 feet typically require engineered drawings and a permit.

What type of backfill should I use behind a retaining wall?

Use clean, angular crushed stone (typically 3/4-inch aggregate) for backfill directly behind the wall. This material drains freely and does not retain water like soil does. The drainage zone should extend at least 12 inches behind the wall from top to bottom. Place landscape fabric between the crushed stone and the native soil to prevent fine particles from migrating into the drainage aggregate and clogging it over time. Never use clay or organic soil as backfill.

How much does it cost to build a retaining wall per linear foot?

Material costs for a DIY retaining wall typically range from $15 to $30 per square face foot, depending on block style and wall height. A 20-foot long, 4-foot tall wall (80 square face feet) costs roughly $400 to $700 in materials. Professional installation adds $10 to $25 per square face foot for labor, plus equipment and delivery. Total installed cost usually ranges from $25 to $55 per square face foot, or $2,000 to $4,400 for our 20x4 example wall.

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