Retaining Wall Calculator
Size a segmental block retaining wall in seconds. Enter the wall length and height, set your block face, and get blocks, cap blocks, base gravel and drainage backfill.
| Detail | Result |
|---|---|
| Cap blocks | 15 |
| Base gravel | 0.74 cu yd |
| Drainage backfill | 2.22 cu yd |
| Wall face area | 60 sq ft |
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How Retaining Wall Calculator Works
Four quick steps happen behind the screen every time you change a number.
Counts the courses
Wall height is divided by block height and rounded up to a whole number of rows.
Counts blocks per row
Wall length is divided by block width and rounded up, so no gap is left.
Multiplies and adds cuts
Rows times blocks per row gives the total, plus a percentage for cuts and breakage.
Sizes the hidden layers
A trench of gravel below and a strip of drainage stone behind are turned into cubic yards.
How to Use Retaining Wall Calculator
Five quick steps take you from a tape measure to a usable result.
- Measure the wall runMark the wall line with string and measure its total length.
- Enter the finished heightUse the exposed height. Taller walls than about 4 feet need an engineer.
- Type your block sizeRead the face width and height from the block label. 16 by 8 inches is common.
- Set the base and backfillSix inches of compacted gravel and a foot of drainage stone behind the wall is a typical plan.
- Order blocks, caps and stoneAdd the cap blocks and the gravel to the same delivery.
Formula Used in Retaining Wall Calculator
These are the standard formulas behind every result, so you can check the numbers yourself.
Courses
The ⌈ ⌉ marks mean round up to a whole row.
Blocks
One course holds the same number of blocks.
Backfill
The result is in cubic yards of drainage stone.
Worked example
A 20 ft wall that is 3 ft high with 16 by 8 inch blocks has 5 rows. Each row needs 15 blocks, so 75 blocks plus 5 percent gives 79. The base needs 0.74 cubic yards and the backfill 2.22.
Retaining Wall Block Chart
Wall blocks for a 20 ft long wall at common heights, using 16 by 8 inch blocks.
| Size | Courses | Blocks | Backfill, cu yd |
|---|---|---|---|
| 1 ft high | 2 | 32 | 0.74 |
| 2 ft high | 3 | 48 | 1.48 |
| 3 ft high | 5 | 79 | 2.22 |
| 4 ft high | 6 | 95 | 2.96 |
| 5 ft high | 8 | 126 | 3.7 |
| 6 ft high | 9 | 142 | 4.44 |
Includes 5 percent extra. Walls over 4 feet usually need an engineer.
Wall Blocks by Length
Blocks for a 3 ft high wall at different lengths with 16 by 8 inch blocks.
| Size | Blocks | Cap blocks | Base, cu yd |
|---|---|---|---|
| 10 ft long | 42 | 8 | 0.37 |
| 20 ft long | 79 | 15 | 0.74 |
| 30 ft long | 121 | 23 | 1.11 |
| 40 ft long | 158 | 30 | 1.48 |
| 60 ft long | 237 | 45 | 2.22 |
| 100 ft long | 394 | 75 | 3.7 |
Cap blocks use the same width as the wall blocks.
Common Retaining Wall Mistakes
Four slips that throw off the result, and how to avoid them.
Building too tall
Walls taller than about 4 feet carry heavy soil loads. Ask a local engineer before you build one.
No drainage behind the wall
Water trapped behind a wall is what pushes it over. Keep a strip of clean stone and a drain pipe.
Skipping the buried first row
The first course should sit partly below grade on compacted gravel. Plan one extra row for it.
Using soil as backfill
Soil holds water and presses on the blocks. Use clean crushed stone close to the wall.
Tools You Must Know
One number is rarely enough for a yard project. These calculators pair well with this one.
Retaining Wall Calculator FAQs
Quick answers before you order materials.
How many blocks do I need for a retaining wall?
Divide the wall height by the block height for rows, then the wall length by block width for blocks per row. Multiply them and add about 5 percent.
How tall can a DIY retaining wall be?
Many areas allow up to about 4 feet without an engineer. Check your local building code first.
How deep should the base be?
Six inches of compacted gravel is a common base for walls under 4 feet.
What is the best backfill behind a wall?
Clean crushed stone drains water away. Avoid using the soil you dug out.
Do I need a drain pipe?
Most walls benefit from a perforated pipe at the base that carries water to the side.
Is the Retaining Wall Calculator free?
Yes. It is free to use, with no account and with feet or metres.
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