# How do you calculate blocks for a retaining wall?

Building a retaining wall can be a great way to create usable space, prevent erosion, or enhance the aesthetic appeal of your yard. The key to building a structurally sound retaining wall is using the right number and size of blocks. Calculating how many blocks you need for your retaining wall project ensures you purchase the proper materials without overbuying.

## What is a retaining wall?

A retaining wall is a structure designed to resist the lateral pressure of soil when there is a desired change in ground elevation. Retaining walls keep soil in place and prevent it from assuming its natural angle of repose or slipping away under gravity. They are typically used to terrace a yard, create level areas for gardens, frame planting beds, or support a sloping embankment.

Retaining walls can be constructed from a variety of building materials, including poured concrete, blocks, timbers, and stones. The two main types of retaining walls are gravity walls and geogrid walls. Gravity walls rely on their sheer mass and weight to resist soil pressure. Geogrid walls use geogrid reinforcement and anchoring systems to stabilize the soil behind them.

## What factors determine how many blocks you need?

Calculating the number of blocks needed for a retaining wall involves considering several key factors:

– Wall height – The total height of the finished retaining wall structure. The higher the wall, the more courses of blocks required.

– Wall length – The linear length of the wall, measured in feet. Longer walls require more blocks.

– Block dimensions – The height, depth, and width of the particular blocks being used need to be factored in. Larger blocks contain more material and cover more square footage.

– Wall batter – Some retaining walls are built leaning slightly back into the hillside. The angle of batter changes the number of blocks needed per course.

– Cap blocks – If using cap blocks to finish the top of the wall, their unique dimensions must be included in the calculations.

– Drainage – Retaining walls often incorporate subsurface drainage materials like gravel and perforated pipe, requiring more blocks.

– Overlap – Blocks are overlapped by a certain amount from course to course. This overlap must be included in the calculations.

– Corners – Unique corner blocks are often used to achieve proper wall bonding at changes in direction.

– Design – More complex wall designs with curves or multiple levels require further block calculations.

## Calculate the number of blocks needed

To calculate the number of blocks required for a retaining wall:

1. Measure the total linear feet of the wall length.

2. Divide the total wall height by the block height to get the number of courses. For example, a 3-foot-high wall using 8-inch-high blocks will have 3 ÷ 0.67 = 4.5 courses. Round up to 5 full courses.

3. Determine the blocks needed for one course by dividing the total linear feet by the block length. For a 20-foot-long wall using 8-inch-deep blocks, you need 20 ÷ 0.67 = 30 blocks per course.

4. Multiply the blocks per course by the number of courses. In our example, 30 blocks per course x 5 courses = 150 total blocks needed.

5. Adjust for wall batter if angled back into the slope. The degree of batter changes the number of blocks required.

6. Factor in any additional blocks needed for corners, capstones, or design elements.

7. Add 5-10% extra for waste, breakage, and blocks cut to size.

## Use a block retaining wall calculator

Performing the retaining wall block calculations manually can be tricky. Many online block calculators are available to simplify the process.

Retaining wall block calculators ask you to input information like:

– Wall length
– Wall height
– Wall batter angle
– Block dimensions
– Overlapping depth

The calculator then runs the measurements and provides you with a report detailing:

– Number of standard blocks needed
– Number of capstones needed
– Number of corner blocks needed
– Total cubic yards of fill required

These user-friendly calculators eliminate the complex mathematical formulas and give you results instantly. They also account for all the little details like overlap, batter, and corners.

Many home improvement stores offer free online block calculators to help customers determine how many blocks they need for DIY retaining wall projects. Companies that manufacture and sell landscape blocks also typically provide calculators that work with their specific block products.

## Allow for compaction and wall embedment

When calculating blocks, it’s important to account for compaction and embedment.

Compaction refers to mechanically compressing the soil beneath the base level of the retaining wall. This creates a very dense, stable soil foundation to build upon.

As soil is compacted, it loses volume and shrinks. The base level of the wall drops lower from its original height. Extra courses of block may be required to achieve the total wall height after compaction.

Embedment is burying the bottom 1-2 courses of blocks below ground level. This anchors the base of the wall and prevents sliding or overturning.

Just like with compaction, embedding the first block layers means the visible wall above ground will be shorter than the full height calculated. Add 1-2 extra courses to account for the buried blocks.

After running through the block calculations and using a retaining wall calculator for your specific project, be sure to double check a few key measurements:

– Look at the wall site and confirm the total linear feet of wall length. Remeasure any corners or curves.

– Verify the maximum height from the bottom base level to the top of the wall. Recheck after compacting the base if possible.

– Examine the blocks and confirm their dimensions haven’t changed from the manufacturer’s specs used in your calculations.

– Review the overlap amount between courses. Inspect a sample section of stacked blocks.

Taking these simple steps to reconfirm your measurements can prevent serious miscalculations. Marking the base line locations with stakes and marking the wall height on a story pole is also wise.

Accurately determining retaining wall block needs takes the guesswork out of purchasing materials. Keep a copy of your block measurements and calculations during construction to track progress. Making adjustments may be required as work proceeds, but you’ll have a reliable reference point.

## Estimate fill material needed

In addition to blocks, retaining wall projects also require importing fill material known as aggregate base. This crushed stone layer fills the cavity behind the wall and extends beneath and in front of the base blocks.

Proper drainage and backfill are critical to retaining wall performance and longevity. Use these guidelines for estimating required fill:

– Minimum 12 inches of aggregate base behind the blocks
– Minimum 6 inches of base extending in front of the blocks
– Minimum 6 inches of fractured stone drainage fill behind the final earth backfill
– Allow for compaction when calculating fill volumes

To estimate cubic yards of fill needed:

1. Calculate the square footage of the area to be filled based on length x width x depth.
2. Convert to cubic yards by dividing by 27. For example, a retaining wall bed that is 6 inches deep, 4 feet wide, and 40 feet long requires about 11 cubic yards of aggregate fill ((0.5 x 4 x 40) / 27 = ~11 cubic yards).

When estimating drainage fill, remember retaining walls are built with slight batter. The cubic yardage increases slightly with each higher course. Allow 10-20% extra fill for compaction. Having excess materials on site is better than running short.

## Conclusion

Calculating the number of blocks needed for a retaining wall ensures you get it right the first time. Consider the total wall length, height, block dimensions, overlap, and design factors. Use a retaining wall block calculator for quick, reliable results. Confirm your measurements, allow for compaction and embedment, and estimate adequate fill materials. With careful planning and preparation, you can create a quality, lasting retaining wall that performs as designed. The end result will be a beautiful structure that elegantly solves your landscape needs.