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Landscaping & Outdoor

Retaining Wall Calculator

Blocks, base gravel, drainage stone and caps for a segmental retaining wall.

Your measurements

Height is measured from the top of the buried base course.

Wall
One course below grade for every foot of wall height, minimum one.
Block and base
Clean #57 stone, full height of the wall, with a perforated pipe at the bottom.

Your result

Sample values

240

Blocks

6

Courses

3.0

Tons of base gravel

5.9

Tons of drainage stone

40 ft wall, 3 ft exposed plus one buried course, 12 in block.

What to order

Item Quantity
Wall block 240
Cap block 40
Crusher run, base 3.0 tons
#57 drainage stone 5.9 tons
Perforated drain pipe 40 ft
Geotextile fabric 160 ft²
Construction adhesive 6 tubes

How to use this calculator

Enter the exposed height you want and the number of courses that will sit below grade. The rule of thumb is one buried course per foot of wall, with a minimum of one — the buried courses are what stop the wall sliding forward at the base.

Do not skip the drainage stone. Water pressure behind a wall is what pushes retaining walls over, and a foot of clean stone with a perforated pipe at the bottom relieves it. Fabric between the stone and the backfill soil keeps the stone from silting up and losing its drainage.

Formula

Courses = (Exposed height + Buried) ÷ Block height

Blocks per course = Wall length ÷ Block length

Blocks = Courses × Blocks per course

Block height
8 in for a standard segmental block
Buried
one course per foot of exposed wall

Cap blocks are counted separately, one per block length along the top course.

Worked example

A 40 ft wall, 3 ft exposed, one buried course, 12 × 8 × 6 in block, 6 in base.

  1. 3 ft exposed = 4.5 courses at 8 in each
  2. Plus 1 buried course = 6 courses total, rounded up
  3. 40 ft ÷ 12 in per block = 40 blocks per course
  4. 6 courses × 40 = 240 blocks
  5. Base: 40 ft × 2 ft × 0.5 ft ÷ 27 × 1.35 = 3.0 tons
  6. Drainage: 40 ft × 1 ft × 4 ft ÷ 27 × 1.35 = 5.9 tons

240 blocks, 40 caps, and about 9 tons of stone

Understanding your results

Height is the number that decides whether this is a weekend job or an engineered structure. Most jurisdictions require a design by an engineer above three or four feet, and many require geogrid reinforcement layers extending back into the soil. Terracing two shorter walls is often simpler and cheaper than one tall one, provided they are far enough apart not to load each other.

The base course takes the longest and matters the most. It should sit on compacted crushed stone, be level in both directions, and be checked along the whole run before any second course goes on. Every error in the base course is multiplied by the number of courses above it.

Questions

About this calculator

Ask something else

Commonly three to four feet, but it is set by local code and by what the wall is retaining. A wall with a driveway or slope above it carries far more load than one holding a flat garden bed, and that surcharge often triggers an engineered design at a lower height.

It resists the wall sliding forward at the bottom, which is how low walls usually fail. Burying one course per foot of height also puts the base below the zone where soil freezes and heaves in most climates.

Above about three feet, or wherever there is a slope or load above the wall, usually yes. Geogrid layers extend back into the retained soil and turn the block face plus the soil behind into a single mass. Whether you need it, and at what spacing, comes from an engineered design.

Most segmental blocks have a lip or pin that sets each course back about an inch, leaning the wall into the slope. That batter uses the retained soil weight to help hold the wall and gives it a margin before it could ever go past vertical.

Keep going

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Estimates, not specifications. Results are based on the values you enter and on typical material properties. Densities, coverage rates and product sizes vary between suppliers, so confirm quantities against your supplier's data before ordering. Structural work must be signed off by a qualified professional and meet your local building code.