Retaining Wall Calculator
Count segmental retaining wall blocks, caps and gravel from wall dimensions and block size.
Loading calculator…
What this tool does
Count segmental retaining wall blocks, caps and gravel from wall dimensions and block size. Assumes a standard segmental block wall with one buried course portion, a 12 in clear-gravel drainage zone behind the blocks and a compacted gravel base. Walls over about 4 ft, surcharged or terraced walls need engineering and geogrid — not estimated here.
How to use the Retaining Wall Calculator
- Enter or select wall length in ft.
- Enter or select wall height above grade in ft.
- Enter or select block length in in.
- Enter or select block height in in.
- Enter or select buried first course depth in in.
- Enter or select waste allowance (%).
- Read the calculated result; change any measurement to compare alternatives.
Formula
courses = ceil((height + buried/12) × 12 / block height); blocks per course = ceil(length × 12 / block length); blocks = courses × per course × (1 + waste/100); backfill yd³ = length × height × 1 ft / 27
- length
- Wall length (ft, before metric conversion)
- wallHeight
- Wall height above grade (ft, before metric conversion)
- blockL
- Block length (in, before metric conversion)
- blockH
- Block height (in, before metric conversion)
- buried
- Buried first course depth (in, before metric conversion)
- waste
- Waste allowance (%)
Assumes a standard segmental block wall with one buried course portion, a 12 in clear-gravel drainage zone behind the blocks and a compacted gravel base. Walls over about 4 ft, surcharged or terraced walls need engineering and geogrid — not estimated here.
Worked example
For retaining wall calculator, the following measurements illustrate the exact method: Wall length: 20; Wall height above grade: 3; Block length: 18; Block height: 6; Buried first course depth: 6; Waste allowance (%): 10.
Inputs
- Wall length20 ft
- Wall height above grade3 ft
- Block length18 in
- Block height6 in
- Buried first course depth6 in
- Waste allowance (%)10
Result
- Wall blocks108
- Courses (incl. buried)7
- Blocks per course14
- Cap blocks14
- Gravel backfill (12 in zone)2.22 cu yd
- Base gravel (6 in × 24 in trench)0.74 cu yd
Results explained
- Wall blocks
- Wall blocks from the formula above. Assumes a standard segmental block wall with one buried course portion, a 12 in clear-gravel drainage zone behind the blocks and a compacted gravel base. Walls over about 4 ft, surcharged or terraced walls need engineering and geogrid — not estimated here.
- Courses (incl. buried)
- Courses (incl. buried) from the formula above. Assumes a standard segmental block wall with one buried course portion, a 12 in clear-gravel drainage zone behind the blocks and a compacted gravel base. Walls over about 4 ft, surcharged or terraced walls need engineering and geogrid — not estimated here.
- Blocks per course
- Blocks per course from the formula above. Assumes a standard segmental block wall with one buried course portion, a 12 in clear-gravel drainage zone behind the blocks and a compacted gravel base. Walls over about 4 ft, surcharged or terraced walls need engineering and geogrid — not estimated here.
- Cap blocks
- Cap blocks from the formula above. Assumes a standard segmental block wall with one buried course portion, a 12 in clear-gravel drainage zone behind the blocks and a compacted gravel base. Walls over about 4 ft, surcharged or terraced walls need engineering and geogrid — not estimated here.
- Gravel backfill (12 in zone)
- Gravel backfill (12 in zone) from the formula above, expressed in yd3 before metric conversion. Assumes a standard segmental block wall with one buried course portion, a 12 in clear-gravel drainage zone behind the blocks and a compacted gravel base. Walls over about 4 ft, surcharged or terraced walls need engineering and geogrid — not estimated here.
- Base gravel (6 in × 24 in trench)
- Base gravel (6 in × 24 in trench) from the formula above, expressed in yd3 before metric conversion. Assumes a standard segmental block wall with one buried course portion, a 12 in clear-gravel drainage zone behind the blocks and a compacted gravel base. Walls over about 4 ft, surcharged or terraced walls need engineering and geogrid — not estimated here.
Frequently asked questions
With 18 × 6 in blocks: 7 courses including the buried portion × 14 blocks per course = 98 blocks, plus caps — about 108 with 10% waste.
A common rule is 1 in of burial per 8 in of wall height, with a minimum of about 6 in, on a compacted gravel base at least 6 in deep and 24 in wide.
A 12 in zone of clean drainage gravel with a perforated drain pipe carries water away — water pressure is the top cause of retaining wall failure.
Typically above 4 ft of exposed height, when supporting a driveway or slope above, or in tiered walls. Many jurisdictions also require a permit at that height.
Caps are counted separately here (one per block along the top course) because they are a different SKU from the wall blocks.