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Use this free retaining wall calculator to plan your materials before you load up the truck. Enter your wall length and height, pick your block size, and you’ll get the number of blocks, caps, and gravel for the base and drainage, including the buried course most people forget.
How many retaining wall blocks do you need?
Enter your wall size and pick your block. You’ll get blocks, caps, and the gravel for the base and drainage.
Your wall
Your block
- Caps—
- Gravel base (24 in wide × 6 in deep)—
- Drainage gravel (12 in behind the wall)—
- Total gravel (#57 crushed stone)—
Walls 4 feet or taller usually need a building permit, an engineered design, and a drain pipe behind the wall. Check with your local building department before you start.
Includes 5% extra blocks for cuts and breakage. The first course is buried: at least one full block, or 1 inch per foot of wall height. Always follow your block maker’s instructions.
| Block size | 1 ft tall | 2 ft tall | 3 ft tall |
|---|
Enter your own wall size above for a full materials list.
How to Use the Retaining Wall Calculator
The retaining wall calculator works in 3 easy steps:
- Measure your wall. Enter the length in feet and the height you want showing above the ground when it’s finished. For a curved wall, run a garden hose along the curve and measure the hose.
- Pick your block size. Most home centers sell retaining wall blocks in a few standard sizes, from small 4-inch blocks to 8-inch blocks for bigger walls. The height and length are printed on the label or pallet tag. If yours aren’t listed, choose Custom.
- Choose your caps. Caps are the flat stones that finish the top of the wall. Pick the length that matches the caps you plan to buy, or choose No caps.
The result shows how many blocks to buy, how many caps, and how much gravel you need, in cubic yards and tons. The blocks include 5 percent extra for cuts and the occasional cracked block.
Why the First Course Goes Underground
The bottom row of retaining wall blocks gets buried. That buried course locks the wall in place so the soil behind it can’t push the bottom out. Allan Block, a major retaining wall maker, says to dig the base trench 24 inches wide and 6 inches deep, plus 1 more inch for every foot of wall height, so part of the wall sits below grade. Source: Allan Block — Basic Installation
The calculator buries at least one full course, or 1 inch per foot of wall height if that’s more. That’s why your total course count is always one or more higher than the courses you’ll see above ground.
How Much Gravel Does a Retaining Wall Need?
More than most people expect. A block wall needs gravel in two places:
- The base. Allan Block calls for at least 6 inches of compacted crushed stone in the bottom of a 24-inch-wide trench. This level, solid pad is what keeps the wall from sinking or leaning.
- Behind the wall. Allan Block also says to fill 12 inches behind the blocks with the same crushed stone, all the way up the wall. This lets water drain down and out instead of building up pressure behind your retaining wall blocks.
The retaining wall calculator figures both and adds them together. It uses #57 crushed stone at about 105 pounds per cubic foot, the same number our gravel calculator uses. Most landscape suppliers sell it by the ton, so the tons line is the one to give them when you order. Source: Allan Block — Basic Installation
Many installers also line the back of the gravel zone with landscape fabric so the soil behind the wall doesn’t wash into the stone and clog it over time.
When You Need a Drain Pipe or a Permit
Allan Block says a drain pipe is required for gravity walls over 4 feet tall and for any site with poor drainage. The pipe runs along the back of the base, at its lowest point, and should vent to daylight about every 50 feet. A perforated drain pipe with a filter sock keeps soil out of the pipe. Our guide on how to install a French drain covers laying perforated pipe.
Walls 4 feet and taller usually need a building permit and an engineered design, and some cities set the limit lower. Walls that hold up a driveway, a slope, or a structure need extra care at any height. Check with your local building department before you start a tall wall. The calculator shows a reminder once you enter a height of 4 feet or more.
Tools for Building a Block Retaining Wall
You don’t need much to build a short block wall, but these make the job go faster and keep it straight: A hand tamper to pack down the gravel base. Allan Block calls for compacting in layers no thicker than 8 inches. For a long wall, renting a plate compactor saves your back. A 24-inch box level to check every block, side to side and front to back. The first course matters most. If it’s level, the rest of the wall stacks straight. A rubber mallet to tap blocks into place without chipping them.
Start each new course so the seams don’t line up with the row below. Allan Block says to offset each course by at least a quarter of a block length. Source: Allan Block — Basic Installation
Before You Dig
Call 811 before you dig the trench. In Indiana, the law requires you to contact 811 at least two full working days before you dig, so the utility lines can be marked. Source: Indiana 811 — 5 Steps to Safe Digging
If you’re building a wall to stop soil from washing away, see our guide on how to fix erosion on a slope. If water is the bigger problem, start with how to grade a yard for drainage or what to do when your yard floods after rain.
Frequently Asked Questions About the Retaining Wall Calculator
How many retaining wall blocks do I need for a 20 foot wall?
A 20-foot wall that stands 2 feet tall, built with 6 × 16-inch blocks, takes about 79 blocks. That’s 15 blocks per course, 4 courses above ground plus 1 buried course, and 5 percent extra. Enter your own size in the retaining wall calculator above.
How many blocks are in a course?
Divide the wall length in inches by the length of one block, then round up. A 20-foot wall is 240 inches, so 16-inch blocks need 15 per course.
How much gravel do I need for a retaining wall base?
For a 24-inch-wide, 6-inch-deep base, plan on 1 cubic foot of gravel for every foot of wall. A 20-foot wall needs about 20 cubic feet, or 0.7 cubic yards, for the base alone, plus more for drainage behind the wall.
Do I need caps on a retaining wall?
Caps aren’t structural, but they finish the top of the wall, shed water, and give you a flat edge to sit on. Most people glue them down with concrete adhesive made for landscape blocks.
How tall can I build a retaining wall myself?
Most DIY block walls are kept under 4 feet. Taller walls usually need a permit, an engineered design, and a drain pipe. Check your block maker’s height limits and your local building rules before you build.