Retaining Wall Calculator

A retaining wall does real structural work — it holds back soil and the water in it — so the material estimate has to cover not just the visible blocks but the base and drainage that keep the wall standing. This calculator sizes the block count from the wall face, adds caps and base gravel, and the guide explains the drainage that the blocks alone do not show.

How wall quantity is calculated

A retaining wall is counted as a face area broken into courses (rows) and the blocks within each course. Multiply the two and you have the block count; the cap course finishes the top.

Measuring the wall

• Length: the run of the wall along its face.
• Height: from the top of the buried base course to the top of the wall. Remember the first course sits below grade.
• Block size: segmental wall blocks vary; enter the exposed face dimensions of your chosen block.

The hidden parts of a retaining wall

A worked example

A 20 ft wall, 3 ft high, using 12×6 in face blocks:

• Courses = (3 × 12) ÷ 6 = 6 courses
• Blocks per course = (20 × 12) ÷ 12 = 20
• Blocks = 6 × 20 × 1.05 = 126 blocks, plus 20 caps

Drainage makes or breaks the wall

The most common cause of retaining-wall failure is water, not weight. Soil behind the wall holds water, and saturated soil pushes with surprising force. The fix is to give water an easy escape: free-draining gravel behind the blocks, a perforated pipe at the base daylighting to the side, and landscape fabric to keep soil out of the gravel. Build this drainage in as you go — it cannot be added later.

Building level and stepped back

A retaining wall starts with a dead-level base course; everything above depends on it. Most segmental systems step each course slightly back (a built-in batter) so the wall leans into the slope it retains. Backfill and compact the drainage gravel in lifts as the wall rises. The related block and gravel calculators size the structural blocks and the base and backfill material, so the full wall — visible and hidden — is on one order.

Drainage is what keeps it standing

The leading cause of retaining-wall failure is not weight but water. Soil behind a wall holds water, and saturated soil pushes against the wall with enormous, growing force — hydrostatic pressure that can topple even a well-built wall. The defence is to give water an easy escape: a column of free-draining gravel directly behind the blocks, a perforated drain pipe at the base daylighting to the side or to a drain, and landscape fabric between the gravel and the retained soil to stop fines clogging the drainage. Build this in as the wall rises; it cannot be retrofitted, and omitting it is the single most common fatal shortcut.

Base, batter and burying the first course

A retaining wall starts with a level, compacted gravel base trench, and the first course is buried — about one-tenth of the wall height below grade — so the soil in front helps resist sliding. Get this base course dead level and well compacted, because every course above inherits its accuracy and any error compounds. Most segmental block systems build in a slight backward lean, or batter, so the wall tilts into the slope it retains, using the hill's own weight to help hold it. Follow the manufacturer's setback for each course to achieve the designed batter.

Backfill and compaction in lifts

As the wall rises, backfill the drainage gravel behind it in lifts, compacting each before adding the next, just as with a paver base. Do not compact heavily right against the blocks with a large machine, which can shove them out of line; use a hand tamper near the wall and the plate compactor further back. Keep the drainage gravel zone wide enough to relieve pressure — typically at least a foot behind the blocks — with the retained soil beyond it. Cap the wall with cap blocks, usually adhered with masonry adhesive, to finish the top and shed water.

Height limits, permits and engineering

Retaining walls do real structural work, and the rules reflect that. Many jurisdictions let homeowners build segmental block walls up to three or four feet without engineering, but taller walls — or any wall retaining a slope above it, supporting a driveway or structure (a surcharge), or built in tiers — require an engineer's design and a permit. Geogrid, a reinforcing mesh laid in the soil and tied back into the wall at intervals, is what allows taller walls to resist the increased pressure. Do not guess on a tall or loaded wall: the consequences of failure are serious and expensive, so size the blocks and base here for planning, then get a structural design for anything beyond a low garden wall.

Estimating cost and when to hire out

Segmental block is the most DIY-friendly retaining-wall system, but the full material list — blocks, caps, base gravel, drainage gravel, drain pipe, fabric and adhesive — adds up, and the labour of excavating, basing, draining and backfilling is substantial. The calculator sizes the blocks, caps and base; the related block and gravel calculators refine the rest. A low garden wall under three feet, with proper base and drainage, is within reach of a determined DIYer. But the moment a wall must be taller, retains a slope above it, supports a driveway or structure, or is built in tiers, it needs an engineer's design and likely geogrid reinforcement and a permit — and the consequences of getting a tall wall wrong, from bulging to collapse, are serious and costly. Use the calculator to scope and price a modest wall, and treat any tall or loaded wall as a job to design properly and, often, to build with professional help.