Retaining Wall Block Calculator

US & Canada

Enter the wall length and height and pick your block to get blocks, caps and wall rock, plus the code height triggers.

Your wall

1. Wall size

ft
in
in

Allan Block's guide buries 1 in of block for each 1 ft of wall height; follow your block maker's guide. Height is rounded up to whole courses.

2. Block

in
in
in
%

Custom sizes are used only when Custom block is picked. Enter your cap's length to count caps; VERSA-LOK caps use the maker's 0.8 per foot rule. None of our sources prints a waste figure for blocks, so the allowance starts at 0.

3. Base and drainage rock

in
in
in

Defaults from Allan Block's installation guide: 24 in wide trench, at least 6 in of wall rock, and 12 in of wall rock behind the block. Allan Block allows an 18 in wide trench for walls under 4 ft.

Result

0

Shopping list

Rock volume is the base trench plus the drainage zone behind the wall, before compaction (calculated). Allan Block also fills the hollow cores of its blocks, which is not included.

How the wall maths works

  • Courses = (height above ground + buried depth) ÷ block height, rounded up
  • Blocks = wall length × courses × block height × blocks per sq ft
  • Base rock = wall length × trench width × base depth
  • Drainage rock = wall length × depth behind the block × built height

Blocks per square foot is the coverage each maker prints, or a figure we worked out from what they print (marked calculated). For any other block, pick Custom and enter its face height and length: the calculator uses 144 ÷ (height × length in inches). Rock volumes are loose estimates before compaction; turn them into tons with the gravel calculator and your supplier's weight.

Block coverage

Retaining wall block face sizes and coverage
Block As printed Per sq ft Per m²
Allan Block AB Classic (8 x 18 in face) 1 sq ft approx. per block (11 per m2) 1 10.8 (calc.)
Pavestone RockWall Small (4 x 6.75 in face) 144 units per 46.5 sq ft pallet 3.1 (calc.) 33.3 (calc.)
VERSA-LOK Standard (6 x 16 in face) 151 units per 100 sq ft 1.51 16.3 (calc.)
Unilock PISA Standard, Ontario (150 x 200 mm face) 0.33 face sq ft per unit 3.03 (calc.) 32.6 (calc.)

Allan Block prints both "1 sq ft approx." and "11 blk per m²"; the calculator uses 1 per sq ft (about 10.8 per m², calculated). VERSA-LOK caps follow the General Shale bulletin: number of caps = 0.8 × wall length. For other caps, enter the cap length.

Base and drainage, from the guides

  • Allan Block installation guide: base trench 24 in (600 mm) wide; for walls under 4 ft (1.2 m), 18 in (460 mm) wide. Trench depth is 6 in (150 mm) plus 1 in (25 mm) for each 1 ft (300 mm) of wall height for buried block. At least 6 in (150 mm) of wall rock in the trench, and wall rock 12 in (300 mm) behind the block.
  • Allan Block Specification Manual: base 4 in (100 mm) minimum for walls under 4 ft, 6 in (150 mm) for walls over 4 ft.
  • Lowe's DIY guide: trench twice as wide as the block is deep; compacted base for a 6 in block of 3 in (wall under 1 ft 6 in), 4 in (2 ft) or 6 in (under 3 ft); drainage aggregate "such as 3/4 -inch angular/crushed free draining aggregate" 12 in behind the wall.
  • Unilock PISA (Ontario): 6 to 8 in of 3/4 in crusher run, compacted to 98% Standard Proctor Density. Gravity walls up to 39 in (1 m); geogrid reinforced walls up to 20 ft (6.1 m).

Height limits and permits

  • US, IRC 2021 R404.4: walls not laterally supported at the top that retain more than 48 in (1219 mm) of unbalanced fill, or walls over 24 in (610 mm) that resist other lateral loads too, must be designed to accepted engineering practice, with a safety factor of 1.5 against sliding and overturning.
  • Ontario, OBC 2024 Div. A, 1.3.3.5: a retaining wall exceeding 1 000 mm in exposed height adjacent to public property, access to a building, or private property to which the public is admitted is a designated structure. A wall over 1 m in a private back yard away from those is not caught by this clause.
  • Rest of Canada: we did not find a height trigger for landscape retaining walls in Part 9 of the National Building Code. Ask your municipality.

Sources

Blocks per square metre, rock volumes, and the Pavestone and Unilock per-square-foot figures are calculated.

Common questions

How many retaining wall blocks do I need?
Wall face area × blocks per square foot. A 30 ft wall of Allan Block AB Classic, 32 in above ground plus 4 in buried, is 5 courses (40 in) and 100 sq ft of face, so 100 blocks at Allan Block's 1 block per sq ft.
How many VERSA-LOK blocks and caps for a 20 ft wall?
At 24 in exposed plus 6 in buried, the wall is 5 courses of 6 in, 50 sq ft. At 151 units per 100 sq ft that is 76 Standard units, and 16 caps by the maker's rule of 0.8 × wall length.
How many Pavestone RockWall blocks per square foot?
Pavestone prints 144 units and 46.5 sq ft per pallet for RockWall Small, which works out to 3.1 blocks per sq ft (calculated). A 10 ft wall 12 in high takes 31 blocks, 1 pallet.
How tall can a retaining wall be without an engineer?
Under IRC 2021 R404.4, a wall that retains more than 48 in (1219 mm) of unbalanced fill, or more than 24 in when it also holds back other loads, must be designed to accepted engineering practice. Your local code and the block maker can set lower limits; the General Shale VERSA-LOK bulletin says "generally, any wall greater than 4' high should be engineered."
Do I need a permit for a retaining wall in Ontario?
OBC 2024 Division A, Article 1.3.3.5 makes a retaining wall a designated structure when it exceeds 1 000 mm of exposed height and is adjacent to public property, access to a building, or private property to which the public is admitted. It is not a blanket 1 m limit for every wall. Ask your municipality about permits for your site.
How much gravel goes behind a retaining wall?
Allan Block's guide fills 12 in behind the block with wall rock to the height of the block, over a base of at least 6 in in a trench 24 in wide. For the 30 ft AB wall above, that is about 4.8 yd³ (3.68 m³) of rock in total before compaction (calculated).
Do I need a drain pipe behind a retaining wall?
Allan Block: "A drain pipe is required for any reinforced wall, gravity walls over 4 ft. (1.2 m) tall or sites with poor drainage."
How many Unilock PISA units for a 1 m wall?
Unilock's Ontario sheet gives 0.33 face sq ft per Standard unit (about 3.03 per sq ft, calculated). A 10 ft wall 1 000 mm high rounds up to 7 courses and needs 104 units, 2 bundles of 72. Unilock lists gravity PISA walls up to 39 in (1 m).