Allan Block is one of the most popular retaining wall systems in North America — a dry-stack segmental block that interlocks without mortar or pins. The system uses natural setback (each course steps slightly back) to create a stable battered wall that handles soil pressure, freeze-thaw cycles, and drainage better than most DIY alternatives.

This guide covers how to calculate blocks, what materials you need, realistic 2026 pricing, and step-by-step installation. Use the calculator below for your exact block count and gravel estimates, or read on for the full guide.

Allan Block Calculator

Enter your wall length and height to get block count, gravel backfill needed, and estimated project cost. Select the block size that matches your Allan Block product.

Allan Block Sizes and Coverage

Allan Block makes several block lines. The three most common for residential projects are:

AB Classic is the most widely available and the default for most residential projects. AB Europa is better for taller walls where fewer courses reduce labor time. AB Fieldstone has the same dimensions as AB Classic but a rougher, more natural texture.

How Many Allan Blocks Do I Need?

The formula is straightforward — you are calculating wall face area and multiplying by the block coverage rate:

Wall Length (ft) × Wall Height (ft) = Wall Face Area (sq ft) Wall Face Area × Blocks per Sq Ft = Number of Blocks

Add 10% overage for cuts, waste, and broken blocks. Do not count cap blocks in this calculation — they are ordered separately.

Example: AB Classic, 20 ft × 3 ft Wall

Wall face area: 20 × 3 = 60 sq ft. At 3.0 blocks per sq ft: 180 blocks. With 10% overage: 198 blocks (order 200). You will also need about 20 cap blocks (1 per linear foot for standard caps).

Block Count Quick Reference

All counts include 10% overage for cuts and waste.

What Other Materials Do You Need?

Allan Block walls require more than just blocks. Here is the full material list for a typical gravity wall under 4 feet:

• Base aggregate: 6 inches of compactable gravel (Class 5, ¾-inch crushed, or crusher run) under the first course. Extend 6 inches beyond the front of the wall. A 20 ft wall needs roughly 0.5 cubic yards.
• Gravel backfill: ¾-inch clean crushed stone directly behind the block, extending 12 inches back from the wall. Use approximately 0.75–1.0 cubic yards per 20 sq ft of wall face. Do not use pea gravel or native soil — they retain water and increase pressure on the wall.
• Cap blocks: One cap per linear foot of wall. AB Cap blocks are designed to fit flush on top of AB Classic. Budget 1.1 caps per foot to account for corner pieces.
• Landscape fabric: Optional but recommended behind the gravel backfill to prevent soil migration into the drainage zone. Use non-woven geotextile fabric.
• Construction adhesive: Applied under cap blocks only. One tube covers approximately 20 linear feet of caps.

Allan Block Cost (2026)

Prices vary by region and retailer. Big-box stores (Home Depot, Menards, Lowe's) typically stock AB Classic at lower prices than specialty masonry suppliers, but specialty suppliers carry more block styles.

AB Europa blocks cost significantly more ($8–$15 each) but cover more wall face per block, which partially offsets the price difference on taller walls.

Step-by-Step Installation

Allan Block installs without mortar or special tools. Here is the process for a gravity wall under 4 feet:

1. Mark and excavate: Dig a trench 12 inches deep and 18 inches wide (front to back). The trench needs to be level along its entire length — use a transit level or string line to check. Remove all organic material from the base.
2. Lay the base: Add 6 inches of compactable gravel to the trench. Compact in 2-inch lifts with a plate compactor. Check for level. The first course should be partially buried — at least one block height below grade for walls over 2 feet tall.
3. Set the first course: Place blocks with the hollow core facing up and the front lip facing forward. Check for level front-to-back and side-to-side on every block. Adjust by adding or removing compacted base material, not by shimming.
4. Backfill and compact: After every 1–2 courses, add ¾-inch crushed stone behind the blocks to the top of the current course. Compact the backfill — not the blocks themselves. Do not use native soil as backfill.
5. Stack successive courses: Each course automatically sets back approximately ¾ inch due to the block geometry. Stagger the vertical joints (like bricklaying) by starting alternate courses with a half block. No pins or mortar are needed for walls under 4 feet.
6. Install drainage: On longer walls or slopes with high water table risk, run a perforated drain pipe at the base of the gravel backfill, sloped to daylight at one end.
7. Cap the wall: Apply a bead of construction adhesive to the top course and press cap blocks firmly into place. Cut corners at 45° angles with an angle grinder or masonry saw.
8. Backfill and grade: Fill behind the landscape fabric with native soil, sloping away from the wall at 1–2% to direct surface water away from the structure.

When Do You Need Geogrid?

Standard gravity Allan Block walls work without geogrid up to approximately 3–4 feet tall (depending on soil conditions). For walls taller than 3 feet on soft or clay-heavy soils, or walls supporting surcharge loads (driveways, buildings), geogrid reinforcement is required.

Allan Block publishes free engineering design tables on their website that specify geogrid placement depth and extension length for walls up to 10 feet. For anything over 4 feet, check local building permit requirements before starting.

Common Mistakes to Avoid

• Skipping base compaction: The most common failure point. Uncompacted base causes the first course to shift, which throws off every course above it. Compact in 2-inch lifts — one pass with a plate compactor is not enough.
• Using soil as backfill: Native soil holds water and expands with frost, creating enormous pressure on the wall. Always use ¾-inch clean crushed stone directly behind the blocks.
• Building too tall without geogrid: The block geometry handles gravity walls up to about 3 feet. Beyond that on poor soils, the wall will lean forward over time.
• Ignoring surface drainage: Even a perfect wall fails if surface water is directed into the backfill zone. Grade soil away from the wall and install a drain if needed.
• Not burying the first course: The base course should be at or below grade. A visible first course that is sitting on top of the ground will eventually kick out.

Related Calculators and Guides

• Retaining Wall Calculator — blocks, gravel, and cost for any wall system
• Gravel Calculator — calculate backfill and base gravel by cubic yard or ton
• How to Build a Retaining Wall — material comparison, permitting, and full installation guide
• Gravel Base Calculator — how much base gravel for any hardscape project