Retaining Wall Calculator
A 20 ft long, 3 ft tall retaining wall needs about 130 standard blocks, including a buried base course, a cap row, and 10% waste. Enter your wall length and height, set your block and cap dimensions, and this retaining wall calculator returns the blocks per course, course count, and total blocks to order.
Quick answer
Blocks per course come from the wall length divided by the block face length, rounded up because you cannot buy a partial block.
What this tells you
- •Blocks per course come from the wall length divided by the block face length, rounded up because you cannot buy a partial block.
- •Course count comes from the wall height divided by the exposed block height, and multiplying the two gives the visible wall blocks.
- •A buried base course and a cap row add to the total, and a waste factor covers cuts and breakage.
How to Use
- 1Enter the wall length and the exposed wall height in feet.
- 2Enter the block length and exposed block height in inches, or use the 18 by 6 in defaults for a standard retaining wall block.
- 3Keep the base course option checked to add one buried course below grade for stability.
- 4Keep the cap course option checked and enter the cap unit length to add a finished top row.
- 5Set the waste percentage, usually around 10% for cuts and breakage, then click Calculate.
How It Works
Formula
Blocks per Course = ceil(Wall Length / Block Length)
Courses = ceil(Wall Height / Block Height)
Total = ceil((Blocks per Course x Courses + Base Course + Cap Blocks) x (1 + Waste%))Blocks per course is the wall length divided by the block face length, rounded up. Courses is the wall height divided by the exposed block height, rounded up. Multiplying the two gives the field of visible wall blocks, and the buried base course and cap row use the same length math with their own dimensions. The waste factor adds extra for cuts and breakage before the final round up.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Standard block wall, 20 by 3 ft with base and cap
Wall length is 240 in, so 240 / 18 rounds up to 14 blocks per course. Wall height is 36 in, so 36 / 6 gives 6 courses, for 84 wall blocks. The buried base course adds 14 more blocks and the cap row adds 20 blocks at 12 in each, for a subtotal of 118. A 10% waste factor rounds that up to 130 blocks.
Low garden wall, 10 by 2 ft, no base or cap
Wall length is 120 in, so 120 / 18 rounds up to 7 blocks per course. Wall height is 24 in, so 24 / 6 gives 4 courses, for 28 wall blocks. With no base course or cap row, a 10% waste factor rounds 28 up to 31 blocks.
Common Retaining Wall Block Sizes
Typical face length and exposed height for popular retaining wall block styles. Check your block's spec sheet for the exact figures, since sizes vary between manufacturers.
| Block Type | Face Length | Exposed Height |
|---|---|---|
| Small garden block | 12 in | 4 in |
| Standard landscape block | 18 in | 6 in |
| Large format block | 16 in | 8 in |
| Interlocking retaining block | 24 in | 8 in |
Cap units are often shorter than wall blocks, commonly 12 in, so enter your cap length separately.
How Tall Can a Block Retaining Wall Be
A stacked, unreinforced gravity wall built from standard retaining wall blocks generally tops out around 3 to 4 ft of exposed height. Beyond that, the soil pressure behind the wall usually calls for geogrid reinforcement, a wider base, or a full engineered design.
Every retaining wall also needs drainage behind it. A gravel backfill zone with a perforated drainage pipe at the base keeps water from building up pressure behind the blocks, which is one of the most common causes of wall failure.
The buried base course in this calculator accounts for burying the first row below grade for stability, which most block manufacturers recommend. It does not size the gravel backfill or drainage pipe. The gravel calculator can help estimate that backfill volume once you know your trench dimensions.
Common mistakes
- Skipping the buried base course, which lets the wall shift and lean over time
- Forgetting the cap row in the block count and running short on the final row
- Using a block's overall depth instead of its exposed face length for the course math
- Building a gravity wall taller than about 3 to 4 ft without an engineer or geogrid reinforcement