Retaining Wall Calculator

Introduction

A retaining wall holds back soil and keeps land in place. Whether you are building a wall for a garden, a hillside, or a property edge, you need to know the right amount of materials before you start. Our Retaining Wall Calculator helps you figure out how many blocks, stones, or concrete units you need for your project. Just enter the wall's height, length, and the size of your chosen material, and the calculator does the math for you. This saves you time, cuts down on waste, and helps you avoid buying too much or too little material. Getting your material estimates right from the start keeps your project on budget and on track.

How to Use Our Retaining Wall Calculator

Enter the details of your retaining wall below. This calculator will tell you how many blocks, how much gravel, and how much backfill material you need to build your wall.

Wall Length: Type in the total length of the retaining wall you plan to build. Measure from one end to the other in feet or meters. If you need help converting your measurements into area, our Square Footage Calculator can assist with that step.

Wall Height: Enter how tall you want your retaining wall to be. This is the distance from the base of the wall to the top.

Block Length: Enter the length of one single block. You can find this size on the product label or measure the block yourself.

Block Height: Enter the height of one single block. This helps the calculator figure out how many rows of blocks you will need.

Block Depth: Enter how deep one block is from front to back. This measurement is used to estimate the amount of gravel and backfill needed behind the wall.

Gravel Trench Depth: Enter how deep you plan to dig your base trench. A solid gravel base keeps your wall level and helps with drainage.

Backfill Depth: Enter how far behind the wall you want to fill with gravel or drainage stone. Good backfill keeps water from building up and pushing against your wall.

Retaining Wall Calculator – Estimate Materials and Costs

A retaining wall is a structure built to hold back soil on a slope or hillside. It keeps the earth in place and prevents it from sliding or eroding. Retaining walls are common in yards, driveways, gardens, and anywhere the ground changes height. They can be short decorative borders or tall walls that hold back thousands of pounds of soil.

Materials Used in a Retaining Wall

Most retaining walls are built using interlocking concrete blocks, sometimes called segmental retaining wall (SRW) units. These blocks stack on top of each other in rows called courses. Each course is one horizontal layer of blocks running the full length of the wall. The blocks come in different sizes, but common dimensions include 12 inches long by 4 inches high and 18 inches long by 6 inches high. If you are considering a traditional brick wall instead, our Brick Calculator can help you estimate those quantities.

Here are the key materials you need for a typical block retaining wall:

• Wall blocks – The main building units that make up the body of the wall. The number you need depends on the wall's length, height, and the size of each block.
• Cap blocks (coping) – A finishing row placed on top of the wall. Cap blocks give the wall a clean look and help keep water from seeping into the joints below. They are usually glued in place with construction adhesive.
• Connection pins – Small plastic or metal pins that fit between courses to lock the blocks together and create a slight setback (lean) into the hillside for added strength.
• Backfill gravel – Crushed stone placed behind the wall to allow water to drain away. Without proper drainage, water pressure (called hydrostatic pressure) builds up behind the wall and can cause it to lean, crack, or fail. You can use our Gravel Calculator to get a more detailed estimate for drainage gravel, or our Pea Gravel Calculator if you plan to use pea gravel for specific drainage layers.

How Block Quantities Are Calculated

To find the number of blocks, you divide the total wall length by the length of one block to get the blocks per row. Then you divide the wall height by the height of one block to get the number of courses. Multiply blocks per row by the number of courses, and you have the total block count. If you include a cap row, the top course uses cap blocks instead of standard blocks.

Why Backfill Gravel Matters

Drainage gravel is one of the most important parts of a retaining wall. A zone of gravel behind the wall, typically 12 inches deep, lets water flow down and away instead of pushing against the blocks. The volume of gravel is found by multiplying the wall length, wall height, and the depth of the gravel zone. For walls over 3 to 4 feet tall, a perforated drain pipe at the base of the gravel zone is also recommended to carry water to a safe outlet. If your project also involves filling areas with decorative or structural stone, our River Rock Calculator can help you estimate those materials separately.

Site Access and Cost Factors

The total cost of a retaining wall depends on more than just materials. Getting blocks, gravel, and equipment to the job site can add to the price. An easy site next to a driveway costs less than a steep backyard where materials have to be carried by hand or moved with a small machine. This is why site access difficulty is an important factor when budgeting for a retaining wall project.

Helpful Tips for Planning

• Always order 5 to 10 percent extra blocks to account for cuts, breakage, and waste.
• Walls taller than 4 feet often require engineering and permits. Check your local building codes before starting.
• A compacted gravel base, usually 6 inches deep, should be placed under the first course of blocks. This base is separate from the backfill gravel behind the wall.
• Use landscape fabric between the gravel and the surrounding soil to stop dirt from clogging the drainage zone over time.
• If your project involves pouring a concrete footing beneath the wall, our Concrete Calculator can help you determine how much concrete you need.
• For projects that include surrounding landscaping, our Mulch Calculator, Topsoil Calculator, and Sand Calculator are useful for estimating those additional materials.

Frequently Asked Questions

How many retaining wall blocks do I need per square foot?

It depends on the block size. For a standard 12×4 inch block, you need about 3 blocks per square foot of wall face. For a larger 18×6 inch block, you need about 1.3 blocks per square foot. The calculator figures this out for you based on the block size you pick.

What is a cap row and do I really need one?

A cap row is the top finishing course on your retaining wall. It gives the wall a clean, finished look and helps stop water from getting into the joints below. While not required, it is strongly recommended. Cap blocks are usually glued down with construction adhesive for a secure hold.

What are connection pins and how many do I need?

Connection pins are small plastic or metal pieces that fit between rows of blocks. They lock the blocks together and create a slight backward lean into the hillside for strength. The calculator estimates one pin per block per pin layer. The number of pin layers depends on how many courses your wall has.

How deep should the backfill gravel be behind my retaining wall?

A common backfill depth is 12 inches behind the wall. This gives water enough room to drain down and away from the blocks. For taller walls or areas with heavy rainfall, you may want to increase this depth. The calculator uses 12 inches as the default.

Can I switch between imperial and metric units?

Yes. Click the Imperial or Metric button at the top of the calculator. All your measurements and results will convert automatically. Imperial uses feet and inches, while metric uses meters and centimeters.

What does the site access difficulty setting do?

Site access difficulty applies a cost multiplier to your total estimate. Easy access (like a flat driveway) uses a 1.0× multiplier. Normal residential access uses 1.15×. Complicated access with steep terrain or hand-carrying materials uses 1.35×. This accounts for the extra labor and delivery costs of harder job sites.

How accurate is the cost estimate from this calculator?

The cost estimate is a good starting point for budgeting. It uses the material prices you enter. Actual costs can vary based on your location, the brand of blocks, delivery fees, and labor costs. Always get quotes from local suppliers for the most accurate pricing.

Does this calculator include labor costs?

No. This calculator estimates material costs only. Labor costs vary widely based on your area, wall height, and site conditions. As a rough guide, professional labor can add $10 to $25 per square foot of wall face on top of material costs.

Should I order extra blocks beyond what the calculator shows?

Yes. It is smart to order 5 to 10 percent more blocks than the calculator shows. This covers cuts at corners and ends, breakage during handling, and any blocks that may be damaged or defective.

What block size should I choose for my retaining wall?

For short walls under 2 feet, small 8×4 inch blocks work well. For walls between 2 and 4 feet, standard 12×4 or 12×6 inch blocks are popular. For taller walls, larger 16×6 or 18×6 inch blocks provide more weight and stability per course.

How do I measure my wall length if it curves?

For a curved wall, use a flexible tape measure or a string to follow the curve from one end to the other. Measure along the face of where the wall will sit. Enter that total length into the calculator.

Do I need a permit to build a retaining wall?

Many areas require a building permit for retaining walls taller than 4 feet. Some areas set the limit at 3 feet. Check with your local building department before you start. Tall walls may also need an engineer to design them.

What is the difference between wall blocks and cap blocks in the cost breakdown?

Wall blocks are the standard blocks that make up the main body of the wall. Cap blocks are the decorative finishing blocks placed on the very top row. Cap blocks usually cost more per unit because they have a smooth or textured finish designed to be the visible top of the wall.

How is the backfill gravel volume calculated?

The calculator multiplies the wall length by the wall height by the backfill depth to get the total volume. It then converts that volume into cubic yards (imperial) or cubic meters (metric) so you know how much gravel to order.

What type of gravel should I use for backfill?

Use clean, crushed stone, typically 3/4 inch in size. Avoid using soil, sand, or round river rock for backfill because they do not drain as well. Crushed stone has angular edges that lock together and let water pass through easily.