Retaining Wall Calculator

By Maria Kluziak
Last updated: Jul 06, 2021

If your garden needs a retaining wall, and you have no idea how to set about getting it done, fear not - the retaining wall calculator is here to help! Whether you're building your retaining wall for aesthetic reasons, or to even out the landscape, this calculator will help you plan and carry out the construction process. Save time you would otherwise spend meticulously calculating the number of retaining wall blocks on paper, and kill two birds with one stone by having your retaining wall cost estimated simultaneously. On top of that, the retaining wall calculator will show you how much gravel you will need for backfill, so there's no need to worry about that either! Use this time-saving tool, and your retaining wall project will be as easy as pie!

Retaining wall calculator: illustration

How to use the retaining wall calculator?

The retaining wall calculator may seem complicated at first, but don't worry! Just follow this simple set of instructions, and you will get your calculations done in no time.


  • Start by deciding whether your retaining wall will have a cap row (pick "Yes" or "No" at the very top of the calculator). A cap row is the top-most row of the retaining wall, reaching higher than the wall's actual height, which is often used for decoration.
  • Input the height and length of the retaining wall.
  • Input the height and length of one of your chosen blocks.
  • You will be given the total number of retaining wall blocks needed. If you've chosen to have a cap row, the number of blocks required for the cap will show as well.


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 do use a granular material such as gravel, which is what we chose for the retaining wall calculator.

  • Declare the backfill area's height, length and thickness. Height and length typically correspond to the wall's height and length. As for the thickness, it should be at least 12in/30cm to assure proper drainage.

  • The calculator will return both the volume and weight of gravel you should buy.


Our retaining wall calculator also allows you to calculate retaining wall costs. To do so, all you need to do is:

  • Input the price of a single retaining wall block (and that of a cap block, if you chose that option at the top of the calculator)
  • Input the price of gravel per unit weight - any unit will do.
  • The calculator will provide with your total retaining wall cost.

Additionally, we recommend that you buy an additional 10 - 15% of all the materials needed to account for any mistakes that may occur, as well as any unexpected repairs. Better safe than sorry - why waste gasoline driving too and from the store when you can be prepared in advance? Feel free to use our percentage calculator to figure out exactly how much more you ought to buy.

Retaining wall formulas

The beauty of our retaining wall calculator is the fact that it does all the work for you. We understand, however, that you might be in the mood to do the maths yourself, and hereby present you with all the necessary equations!

The number of retaining wall blocks is calculated using the following formulas:

number of rows = wall height / block height (rounded up if not a full number)

number of columns = wall length / block length (rounded up if not a full number)

  • NOTE: If there is a cap row, the number of cap blocks equals the number of columns, so you would use the number of columns formula to calculate it.

number of blocks (not including cap row blocks) = number of rows * number of columns

  • NOTE: If there is a cap row, the formula is almost the same, with the exception that you subtract one row: number of blocks = number of columns * (number of rows - 1)

To calculate the volume of gravel needed for the backfill, we use the following equation:

backfill gravel volume = backfill thickness * backfill length * backfill depth

  • NOTE: The backfill's length and depth correspond to the wall's length and height respectively. As already stated above, the thickness should be no less than 12in/30cm.

To convert the gravel's volume into a particular weight of gravel, we need the following formula:

backfill weight = gravel's density * backfill volume

  • NOTE: Gravel's density is 84.03 lb/ft³, or 1346 kg/m³ in the metric system. You can also use our size to weight calculator for more help with the conversion.

To calculate the retaining wall cost, you need to multiply the results of the previous calculations by the price of that material.

Calculating retaining wall costs and materials - example

Let's say we're building a retaining wall, that:

  • has no cap row,
  • is 30 feet long,
  • is 10 feet high,
  • is made out of 10in x 15in blocks,
  • has a standard 12in thick backfill.

Let's say the prices in our area are:

  • 5$ per a retaining wall block,
  • 2$ per a pound of gravel.

In our example, the calculations go as follows:

wall height / block height = 30ft / 10in = 360in / 10in = 36

wall length / block length = 10ft / 15in = 120in / 15in = 8

number of retaining wall blocks = 36 * 8 = 288

total cost of blocks = 5$ * 288 = 1,440$

backfill volume = 12in * 360in * 120in = 518,400in³ = 300ft³ = 11.11yd³

backfill gravel weight = 84.03lb/ft³ * 300ft³ = 25,208lb

total cost of backfill gravel = 2$ * 25,208lb = 10,373.06$

We hope that this example helped make using our retaining wall calculator a little easier.

Maria Kluziak
Will there be a cap row?
Wall dimensions
Block dimensions
Materials needed
Number of wall blocks
Number of cap blocks
Gravel for backfill
Backfill area thickness
Backfill area length
Backfill area height
Volume of gravel for backfill
cu yd
Backfill weight
Single wall block price
Single cap block price
Gravel price (per unit weight)
$ per
Total cost of wall blocks
Total cost of cap blocks
Total cost of gravel for backfill
Total expenses
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