Retaining wall block calculator

Blocks = courses × blocks in a course: a 20 ft wall 2 ft high in 12 × 4 in blocks, with one course buried, is 7 courses of 20 blocks, or 147 blocks with 5% extra, on 0.74 cubic yards of base rock.

Your measurements

Example

Use the unit shown for each box, or type another one: 12 ft 6 in, 150 in, 3.8 m and 600 mm all work, as do the marks on a tape measure.

Wall length (ft) Wall height above the ground (ft) The face you will see. The buried course is added below it. Block face length From the label: the length of the face you see. Many big-box wall blocks are 12 or 16 in. Block height From the label: the height of one course.

Details Base, drainage, caps and your price

Buried depth Leave blank for Allan Block’s rule, 1 in for every foot of wall height, rounded up to a whole course. Base trench width Allan Block’s base trench is 24 in wide; for a wall under 4 ft it allows 18 in wide and 4 in of base rock. Compacted base rock At least 6 in of crushed rock in the trench, compacted; never sand. Drainage rock behind the blocks 12 in of wall rock behind the blocks, the full height of the wall. Cap length Leave blank if the wall has no caps. Extra blocks (%) Your price per block ($)

Example: replace these measurements

Your wall list

147 retaining wall blocks

40 sq ft measured

Your measured layout

The wall in section, to scale: the buried courses below the ground line, the compacted base rock under them and the drainage rock behind. The blocks step back slightly, as most segmental blocks do.

Your shopping list

147 retaining wall blocks

Quantities and package details

Courses: 7, 1 of them below the ground
Blocks in each course: 20
Blocks before extra: 140
Buried depth: 4 in (1 in a foot of wall height, a whole course)
Caps: Enter the cap length if the wall has caps
Base rock, compacted: 0.74 cu yd (a 24 in wide trench, 6 in deep)
Drainage rock behind the wall: 1.73 cu yd (12 in deep, the full 28 in height), plus the cores of hollow blocks
Drain pipe: 20 ft along the base if the site drains poorly

The calculation

Courses: (24 in of wall + 4 in buried) ÷ 4 in, rounded up = 7
Each course: 240 in ÷ 12 in, rounded up = 20 blocks
7 × 20 = 140 blocks; with 5% extra, rounded up = 147
Base rock: 24 × 6 × 240 in ÷ 46,656 = 0.74 cu yd
Drainage rock: 12 × 28 × 240 in ÷ 46,656 = 1.73 cu yd

Common questions

How many retaining wall blocks do I need?

Divide the wall length by the block face length for the blocks in a course, and the wall height plus the buried depth by the block height for the courses, rounding both up. A 20 ft wall 2 ft high in 12 × 4 in blocks, with one course buried, is 20 blocks a course and 7 courses: 140 blocks, 147 with 5% extra.

How deep should the first course be buried?

Allan Block buries 1 in for every foot of wall height, on top of at least 6 in of compacted crushed rock in the trench. In whole courses that is usually one course for a garden wall and two for a wall over 4 ft.

How much gravel goes behind a retaining wall?

Allan Block fills 12 in behind the blocks with crushed rock the full height of the wall, and the cores of hollow blocks too. Behind a 20 ft wall 28 in tall that is about 1.7 cubic yards, plus 0.74 cubic yards for a 24 × 6 in base.

When does a retaining wall need an engineer?

Under the International Residential Code (R404.4), a wall that holds back more than 48 in of soil needs a design, and so does one over 24 in high with a driveway, a building or another load above it. A slope above the wall adds to the load too. Your building department decides, so ask before you buy.

How to plan the quantity

Courses = the wall height plus the buried depth, divided by the block height and rounded up. Blocks in each course = the wall length divided by the block face length, rounded up. The buried course sits below ground so the wall cannot slide out at its foot.

The base is a trench of compacted crushed rock under the first course. Behind the blocks, a band of the same rock the full height of the wall lets water drain instead of pushing on the wall; hollow blocks are filled with it too.

Allan Block puts a drain pipe at the back of the base for any reinforced wall, for walls over 4 ft and on sites with poor drainage; it must run out to daylight.

Before you order

A wall that holds back more than 48 in of soil (the difference between the ground in front of it and behind it) needs a design under the International Residential Code (R404.4), and so does one over 24 in high with a driveway, a building or another load above it. A slope above the wall adds to the load too. Check with your local building department.
This counts a straight wall on level ground. Steps in the base, curves, corners, stairs and geogrid reinforcement change the count: follow the block maker’s guide for them.