Allan Block Wall Calculator: How to Estimate

An Allan Block wall calculator estimates the number of Allan Block retaining wall units you need for a project. You enter the wall’s length, height, and the type of block (or its face dimensions), and the calculator returns the total block count, plus cubic yards of base material and drainage aggregate. This guide explains the math behind the estimate, walks through a sample project, and covers the key factors that affect your final order.

Formula

The core formula for the number of blocks is:

Blocks = (Wall Length × Wall Height × 144) / (Block Face Area)

Where:

• Wall Length (l) – measured in feet along the top of the wall.
• Wall Height (h) – measured in feet from the base (buried level) to the top of the wall.
• 144 – converts square feet to square inches (12 × 12).
• Block Face Area – the face of one block in square inches. For a standard Allan Block (6″ tall × 18″ long), the face area is 108 in² (6 × 18). For a large block (8″ × 24″), it’s 192 in².

The calculator also estimates base and drainage aggregate:

• Base aggregate – typically a 6-inch deep layer extending 1.5 feet beyond the wall face. Volume (yd³) = (Length × 1.5 × 0.5) / 27.
• Drainage aggregate – crushed stone placed behind the wall, usually 1.5 feet wide. Volume (yd³) = (Length × Height × 1.5) / 27.

All dimensions are in feet; divide by 27 to convert cubic feet to cubic yards.

Worked Example

Let’s use the default values: a wall 40 feet long and 4 feet high, using standard blocks (6″ × 18″).

Step 1: Calculate the wall face area in square feet: 40 ft × 4 ft = 160 ft².

Step 2: Convert to square inches: 160 × 144 = 23,040 in².

Step 3: Divide by the block face area: 23,040 ÷ 108 = 213.33 blocks.

Step 4: Round up to the nearest whole block: 214 blocks.

Base aggregate: (40 × 1.5 × 0.5) / 27 = 30 / 27 = 1.11 yd³.

Drainage aggregate: (40 × 4 × 1.5) / 27 = 240 / 27 = 8.89 yd³.

So for a 40′ × 4′ wall, you need 214 standard blocks, about 1.1 yd³ of base material, and about 8.9 yd³ of drainage aggregate.

When to Use This Calculator

Use this calculator during the planning phase of a retaining wall project—before ordering materials. It helps you:

• Estimate the number of blocks to order (always order 5–10% extra for cuts and breakage).
• Determine the volume of base and drainage aggregate needed.
• Compare costs between standard and large blocks.
• Plan for delivery and storage space.

How to Read the Results

The block count is the minimum number of full blocks needed. Because blocks are often cut at ends or corners, you’ll likely need more. The aggregate volumes are in cubic yards—the unit used when ordering from a supplier. If you’re buying bagged aggregate, convert cubic yards to bags (typically 1 yd³ ≈ 27 bags of 1 ft³).

Common Mistakes

• Mixing units: Always convert inches to feet or vice versa consistently. Forgetting to convert inches to feet is the most common error.
• Forgetting the buried base: The wall height should include the buried portion (usually 10% of the exposed height). If you only measure visible height, you’ll undercount blocks.
• Rounding down: Always round up to the nearest whole block. Partial blocks are not sold.
• Ignoring cuts: Corners, curves, and ends require cut blocks. Add 5–10% to your total.

Limitations of the Formula

This formula assumes a straight, vertical wall with no steps, curves, or special caps. For curved walls, the number of blocks increases because the inside radius requires narrower joints. For stepped walls, each step adds height and length that must be measured separately. The aggregate estimate assumes a uniform 1.5-foot-wide drainage zone; if your soil requires more or less, adjust accordingly.

Choosing the Right Block Size

Allan Block offers several sizes. Standard blocks (6″ × 18″) are lighter and easier to handle, ideal for walls up to 3–4 feet high. Large blocks (8″ × 24″) are heavier, provide more mass, and are often used for taller walls. The calculator lets you switch between sizes to see how the block count changes.

Common Problems and Misconceptions

A frequent misconception is that the block count is the same as the number of courses times the number of blocks per course. That works only if the wall height is an exact multiple of the block height. In practice, you may need to cut blocks for the top course. Also, some people forget that the base course is buried, so the wall height includes that buried portion.

Another issue is that the calculator gives the face area, but blocks have flanges that interlock, so the actual coverage per block is slightly less than the face dimensions. Always consult the manufacturer’s specifications for the exact coverage.

Frequently Asked Questions

How many Allan Blocks do I need for a 20-foot long, 3-foot high wall?

Using standard blocks (6″ × 18″), the wall face is 20 × 3 = 60 ft², which is 8,640 in². Dividing by 108 in² per block gives 80 blocks. Add 5–10% for cuts, so order about 84–88 blocks.

Do I need to include the buried base course in the height?

Yes. The wall height should include the buried portion. Typically, the first course is buried about 10% of the exposed height. For a 4-foot exposed wall, add about 4–5 inches, making the total height about 4.4 feet.

Can I use this calculator for curved walls?

Not directly. The formula assumes a straight wall. For a curved wall, the number of blocks increases because the inside radius requires narrower joints. A good rule is to add 10–15% more blocks for curves.

What is the difference between standard and large blocks?

Standard blocks are 6″ × 18″ and weigh about 50–60 lbs each. Large blocks are 8″ × 24″ and weigh about 80–90 lbs. Large blocks cover more area per block, so you need fewer, but they are heavier to handle.

How much aggregate do I need for the base and drainage?

The calculator gives separate volumes for base and drainage aggregate. For a 40′ × 4′ wall, you’ll need about 1.1 yd³ of base and 8.9 yd³ of drainage. These are estimates; your local soil conditions may require more or less.

Should I order extra blocks?

Yes, always order 5–10% extra to account for cuts, breakage, and mistakes. This is especially important if you’re ordering from a supplier that charges for delivery—making a second order can be costly.

Related

• Weighted Average Calculator (Three Values)
• AAC Block Calculator for Wall: How to Use It
• Percent of Total Calculator (What Percent Is It?)

in l | Wall Length | ft | 40 | 0 | 100000
in h | Wall Height | ft | 4 | 0 | 100000
in bt | Block Type | - | 2 | 1 | 4
sel bs | Block Size | Standard (6" x 18")=1, Large (8" x 24")=2
out blocks | Blocks Needed | blocks | ceil(round(l * h * 144 / (18 * 6 * bt), 6)) | 0
out base | Base Aggregate | yd³ | l * 1.5 * 0.5 / 27 | 2
out drain | Drainage Aggregate | yd³ | l * h * 1.5 / 27 | 2