Retaining Wall Calculator

Created by Maria Kluziak
Reviewed by Bogna Szyk and Jack Bowater
Last updated: Feb 02, 2023

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.

1. CALCULATING THE NUMBER OF RETAINING WALL BLOCKS

  • 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.

2. CALCULATING GRAVEL FOR BACKFILL

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 12 in12\ \mathrm{in} (30 cm30\ \mathrm{cm}) to assure proper drainage, as you can learn at our french drain calculator.

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

3. CALCULATING THE COSTS

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%10\% - 15%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 we hereby present you with all the necessary equations!

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

nrow=hwallhblockn_{\mathrm{row}} = \frac{h_{\mathrm{wall}}}{h_{\mathrm{block}}}

And:

ncolumns=lwalllblockn_{\mathrm{columns}} = \frac{l_{\mathrm{wall}}}{l_{\mathrm{block}}}

Both quantities get rounded up if they are not full numbers. In these equations, we found the following quantities:

  • hwallh_{\mathrm{wall}} and hblockh_{\mathrm{block}} — The heights of wall and block;
  • lwalll_{\mathrm{wall}} and lblockl_{\mathrm{block}} — The length of wall and block; and
  • nrown_{\mathrm{row}} and nrown_{\mathrm{row}} — The number of rows and columns in the retaining wall.

If there isn't 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.

nblocks=ncolumns×nrowsn_{\mathrm{blocks}} = n_{\mathrm{columns}} \times n_{\mathrm{rows}}

If there is a cap row, the formula is almost the same, with the exception that you subtract one row:

nblocks=ncolumns×(nrows1)n_{\mathrm{blocks}} = n_{\mathrm{columns}} \times (n_{\mathrm{rows}} - 1)

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

VBF gravel=tBF×lBF×dBFV_{\mathrm{BF\ gravel}} = t_{\mathrm{BF}}\times l_{\mathrm{BF}}\times d_{\mathrm{BF}}

🙋 The backfill's length (lBFl_{\mathrm{BF}}) and depth (dBFd_{\mathrm{BF}}) correspond to the wall's length and height, respectively. As already stated above, the thickness (tBFt_{\mathrm{BF}}) should be no less than 12 in12\ \mathrm{in} (30 cm30\ \mathrm{cm}).

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

wBF=Dgravel×VBFw_{\mathrm{BF}} = D_{\mathrm{gravel}}\times V_{\mathrm{BF}}

Where:

  • wBFw_{\mathrm{BF}} — The weight of the backfill;
  • DgravelD_{\mathrm{gravel}} — The density of gravel; and
  • VBFV_{\mathrm{BF}} — The volume of the backfill.

🙋 Gravel's density is 84.03 lb/ft384.03\ \mathrm{lb/ft^3}, or 1346 kg/m31346\ \mathrm{kg/m^3} 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,
  • Hs 3030 feet long,
  • Hs 1010 feet high,
  • Hs made out of 10 in×15 in10\ \mathrm{in} \times 15\ \mathrm{in} blocks,
  • Has a standard 12 in12\ \mathrm{in} thick backfill.

Let's say the prices in our area are:

  • $5\text{\textdollar}5 per a retaining wall block,
  • $2\text{\textdollar}2 per a pound of gravel.

In our example, the calculations go as follows. First, we found the number of rows of the wall.

nrows=hwallhblock=30 ft10 in=360 in10 in=36\begin{split} n_{\mathrm{rows}}& = \frac{h_{\mathrm{wall}}}{h_{\mathrm{block}}}= \frac{30\ \mathrm{ft}}{10\ \mathrm{in}}\\ & = \frac{360\ \mathrm{in}}{10\ \mathrm{in}} = 36 \end{split}

Then the number of columns:

ncolumn=lwalllblock=10 ft15 in=120 in15 in=8\begin{split} n_{\mathrm{column}}& = \frac{l_{\mathrm{wall}}}{l_{\mathrm{block}}}= \frac{10\ \mathrm{ft}}{15\ \mathrm{in}}\\ & = \frac{120\ \mathrm{in}}{15\ \mathrm{in}} = 8 \end{split}

The number of blocks in our retaining wall is:

nblocks=36×8=288n_{\mathrm{blocks}} = 36\times 8 = 288

The total cost of the blocks is:

costblocks=5 $×288=1, ⁣440 $\mathrm{cost}_{\mathrm{blocks}} = 5\ \text{\textdollar} \times 288 = 1,\!440\ \text{\textdollar}

The volume of the backfill is:

VBF=12 in×360 in×120 in=518, ⁣400 in3=300 ft3=11.11 yd3\begin{split} V_{\mathrm{BF}} &= 12\ \mathrm{in} \times 360\ \mathrm{in} \times 120\ \mathrm{in}\\ & = 518,\!400\ \mathrm{in^3} = 300\ \mathrm{ft^3} \\ &= 11.11\ \mathrm{yd^3} \end{split}

The weight of the backfill gravel is:

wBF gravel ⁣= ⁣84.03 lb/ft3 ⁣ ⁣× ⁣300 ft3=25, ⁣208lb\begin{split}w_{\mathrm{BF\ gravel}}& \!=\! 84.03\ \mathrm{lb/ft^3}\! \!\times\! 300\ \mathrm{ft^3}\\ &= 25,\!208 \mathrm{lb} \end{split}

The cost of the backfill gravel is:

costBF gravel=2 $×25, ⁣208 lb=10, ⁣373.06 $\begin{split}\mathrm{cost}_{\mathrm{BF\ gravel}}& = 2\ \text{\textdollar} \times 25,\!208\ \mathrm{lb}\\ &= 10,\!373.06\ \text{\textdollar} \end{split}

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

🙋 Are you looking for tools related to the raw materials you need to use for a retaining wall? Omni created the gravel calculator and the concrete block calculator for the purpose!

FAQ

How do I calcualte the number of bricks I need in a retaining wall?

To calculate the number of bricks you'll need for your retaining wall:

  1. Define the length and height of your retaining wall.
  2. Measure the height and length of an individual brick.
  3. Multiply the height and length of the wall to find its surface area.
  4. Find the surface area of a single brick: multiply its length and height.
  5. Divide the area of the wall by the area of a brick, and round up to the next integer to find the exact number of bricks you'll need.

To be safe, buy at least 10% more bricks than the exact number!

What is the purpose of backfill in a retaining wall?

The backfill primarily aims to provide adequate drainage to your retaining wall. If soil is in direct contact with the bricks, water can pool, and the materials can degrade quickly over time due to improper conditions. A layer of gravel (usually more than 1 ft thick) allows the water to drain away from the wall. Be sure to include the backfill in your plans for a retaining wall!

How much gravel do I need for 1 ft of backfill of a 30×3 wall?

For the backfill of a 30×3 feet retaining wall you will need a volume of 90 ft³, for a weight of 7,563 lb. To find these results:

  1. Assume you are using the same dimensions of the retaining wall for the backfill.
  2. Compute the volume of the backfill by multiplying height (3 ft), length (30 ft), and width (1 ft):

V = 3 × 30 × 1 = 90 ft³
3. Calculate the weight of the backfill gravel by multiplying the volume by the density (84.03 lb/ft³):
W = 90 ft³ × 84.03 lb/ft³ = 7,563 lb.

How do I include the cap row in my retaining wall calculations?

To include the cap row, an extra row of material you can place atop your retaining wall to give it a complete finish, remove one row from your retaining wall calculations. In the calculations for the number of bricks, consider the number of rows decreased by one. Remember that the presence of a cap row doesn't affect the estimates for the backfill!

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