Retaining Wall Calculator: How to Calculate Blocks for a Retaining Wall

Planning a retaining wall and trying to figure out how many blocks to buy is one of those tasks that sounds simple until you actually sit down to do it. Too many and you’re wasting money on materials that sit in your garage. Too few and you’re making an extra trip to the supply store mid-project, which nobody wants. 

A retaining wall calculator takes the guesswork out of it. Instead of estimating, you’re working with real numbers based on your actual wall dimensions and block size. This guide walks you through how to use one, how to do the math yourself, and what else to factor in so you’re not caught off guard at the supply yard.

How Many Blocks Do You Need For A Retaining Wall?

The number of blocks you need comes down to two things: how big your wall is and how big your blocks are. 

To estimate the total number of retaining wall blocks, divide the wall’s width in inches by the block width to get the number of columns, and divide the wall’s height in inches by the block height to get the number of rows. Multiply the columns by the rows to find the total block count. 

Most people also forget to account for the first course going partially below grade, which affects your height measurement. Most experts suggest embedding the first course of blocks below grade about 10% of the wall height to support it correctly. For example, if the desired wall height is 6 feet, you should embed it below grade by 7 to 8 inches. Factor that in before you start counting rows or you’ll end up with a wall that comes in a bit short. 

Calculating The Right Number Of Blocks For A Retaining Wall

Here’s the step-by-step process laid out plainly. 
Step one: measure your wall length and height. Both in feet for the wall area calculation. 

Step two: multiply length by height to get the wall’s total square footage. 

Step three: take your block dimensions (in inches), multiply length by height, and divide by 144 to convert to square feet per block.

Step four: divide wall square footage by block square footage.

Step five: add your waste factor.

For example, if your retaining wall is 10 feet long by 3 feet tall, it’s 30 square feet. If your blocks are 16 inches long and 10 inches high, they’re equivalent to 1.11 square feet per block. Therefore you’d need about 28 blocks for your wall, before waste. 

It is recommended to buy at least 10 to 15% more materials than the exact number to account for any mistakes that may occur and any unexpected repairs. 

How To Measure For A Retaining Wall

Measuring seems like the easy part, and mostly it is. But there are a couple of details worth getting right the first time.

Length is straightforward: measure along the face of where the wall will run. If it has bends, measure each segment separately.

Height needs a bit more thought. Be sure to account for embedding the first course of blocks below grade when measuring the desired wall height. Accounting for this from the start will ensure that the wall does not end up too short.

If your wall is going to be 36 inches above ground, you want to plan for 4 to 6 inches below grade for the base course. Your total block height for calculation purposes is the above-grade height plus whatever you’re burying. Get that number right and the rest of the calculation follows. 

Cap Blocks: Don’t Forget The Top Row

Cap blocks are easy to forget until you’re finishing the top of the wall and realise you don’t have them.

They’re calculated based on length rather than area since they only run along one row at the top. Divide the retaining wall length by the cap block length. For example, using a 16-inch retaining wall cap block on a wall that is 144 inches long gives you 9 cap blocks needed.

If your wall has corners or a curved section, add a few extra cap blocks to account for those angles. Cap blocks often need cutting at corners and those cut pieces sometimes aren’t reusable. 

How Much Gravel Do You Need For Backfill?

Blocks are only part of what you need to order. The gravel backfill behind the wall is just as important and just as easy to underestimate.

Backfill is something that cannot be forgotten when building a retaining wall. It is crucial for keeping the wall steady as well as making sure it does not get eroded by water. It is recommended to use a granular material such as gravel. The backfill area’s height and length typically correspond to the wall’s height and length, and for thickness, it should be at least several inches.

Drainage gravel should be a 12-inch layer of clean crushed stone behind the entire wall face. A gravel base should be 6 inches deep and in a trench at least 24 inches wide.

Run the numbers before you order and you won’t be short on gravel when you’re mid-build. 

When To Hire A Professional For Your Retaining Wall

The DIY route works well for smaller walls. Once you get into taller or more structurally demanding builds, professional help earns its cost quickly.

Walls over 4 feet require professional engineering. A retaining wall calculator is an engineering tool that determines quantities of blocks, backfill materials, and drainage components needed for structurally sound retaining walls, incorporating safety factors, soil pressures, and building code requirements.

If your wall is near a driveway, a slope, or any existing structure, get a professional opinion before you start. The materials themselves aren’t that expensive. What’s expensive is pulling out a wall that wasn’t built right and doing it over. 

Retaining Wall Math Made Simple

The calculation is simple when you break it into steps. Get the wall dimensions right, know your block size, apply the waste factor, and don’t forget the gravel. Those four things cover the basics of any block retaining wall project.

If your wall is on the larger or more complex side, a professional is always worth talking to. But for most residential builds, the math is manageable and the project is within reach.