Retaining Wall Calculator

How the retaining wall calculator works

A 20-ft long, 3-ft tall retaining wall takes 189 standard 12″×4″ landscape blocks plus 20 cap blocks, 0.74 cubic yards of base gravel, and 2.22 cubic yards of drainage gravel. Three feet of height is 9 courses of 4-inch block, and 20 feet of length is 20 blocks per course — 180 blocks, or 189 with a 5% waste factor. Add 60 sq ft of landscape fabric and a run of 4-inch perforated drain pipe at the base.

The math: block count = courses (rows) × blocks per course. Courses = wall height ÷ block height. Per-course count = wall length ÷ block length. The first (buried) course is one full course, even though only the top of it is visible.

Base gravel sits in a 4–8" deep × 12–24" wide trench under the first course. It's compacted and screeded level. Without a proper base, walls fail within a few seasons.

Drainage gravel goes behind the wall, in the 6–18" of space between the wall and the soil it's holding back. Combined with weep holes or a French drain at the base, it prevents water buildup that would push the wall over.

Block types

• 12"×4" landscape block — the box-store standard (Anchor Diamond Pro, Pavestone, etc.). Locking lip on top to engage the next course. Good for walls up to ~3 ft tall (1 m). 6 blocks per face per square foot.
• 16"×6" landscape block — bigger profile, more mass. Walls to ~4 ft (1.2 m). Pricier but fewer blocks needed.
• CMU 16"×8" — structural concrete masonry unit. For walls over 4 ft and engineered designs (with rebar + footing). Requires permits in most jurisdictions.

Wall height limits without engineering

Most US municipalities allow DIY-built retaining walls up to 3–4 ft (1–1.2 m) without a permit or engineering. Anything taller usually requires a structural engineer's stamp + permit + inspections. Always check your local code before starting.

Read that carefully, because two details in it catch people out. First, the threshold is written into most local codes as an exposed height of 4 feet measured from the bottom of the footing — not from the ground surface in front of the wall — which is the language the IBC uses and which most residential jurisdictions mirror. Your buried course counts. Second, and more important: any surcharge removes the exemption entirely. A surcharge is any additional load bearing on the soil the wall retains — a driveway, a parking pad, a slope rising behind the wall, a pool, a building foundation, or a second wall above it. A 3-foot wall holding back a level lawn is a weekend project. A 3-foot wall holding back the edge of a driveway is an engineered structure in most jurisdictions, and it is the one that fails.

The tiering rule is the one that surprises people most. Splitting a 6-foot grade change into two 3-foot walls does not turn one engineered wall into two DIY walls — unless the upper wall sits far enough back that its load falls behind the lower wall's influence zone, the two behave as a single tall structure. The commonly used spacing rule is that the setback between tiers should be at least twice the height of the lower wall; anything tighter and you are building a 6-foot wall in two pieces.

The base: 10% plus six inches

Retaining walls do not usually fail at the block. They fail at the bottom, because the base was thin, unlevel, or built on soil that moved. The base is the single highest-leverage part of the job and the part most likely to be rushed on a Saturday afternoon.

The working rule is bury the wall by about 10% of its total height — rounded up to a full course — and put 6 inches of compacted gravel underneath that. A 3-foot wall buries roughly 3.6 inches, which rounds up to one full 4-inch course, so the trench is 4 + 6 = 10 inches deep. Taller walls get more of both.

Trench width should be roughly twice the depth of the block and never less than 24 inches, so the base extends behind the wall as well as under it. Use a well-graded angular crushed base — the kind with fines in it, sold as roadbase, ¾-minus, or crusher run — not clean drainage stone, because the base course needs to lock up and hold a level surface under compaction. Our gravel calculator covers the distinction and lets you price both materials.

Compact in 2–3 inch lifts with a plate compactor, and screed the top dead level. Time spent here is repaid many times: every course above inherits the level of the first, and a first course that is out by a quarter inch is out by an inch and a half nine courses later, with no way to correct it.

The drainage stack behind the wall

Water is what actually pushes retaining walls over. Saturated soil weighs far more than dry soil and exerts hydrostatic pressure directly against the back of the wall; in freezing climates it also expands. A gravity wall is designed to resist the load of drained soil, and if it never drains, the wall is holding a load it was never sized for. Build the drainage as a stack, back to front:

• Perforated drain pipe — 4" corrugated or rigid PVC, holes down, laid at the base of the wall on top of a couple of inches of stone, sloped to daylight or to a drain. Not a decoration: it needs somewhere to actually go.
• Clean drainage stone — 12 inches of washed, angular ¾" stone with no fines, filling the space between the block and the retained soil. Fines clog the voids and turn your drainage layer into a dam.
• Filter fabric — non-woven geotextile between the stone and the native soil, wrapped over the top of the stone as well. This is what keeps the soil from migrating into the stone and silting the drain shut over ten years.
• Capping soil — 6 inches of low-permeability soil at the top, graded to fall away from the wall so surface water sheds rather than percolating straight down into the drainage zone.
• Core fill — the hollow cores of open-back blocks get filled with the same clean stone as you build, which adds mass and continues the drainage path through the wall face.

The calculator's drainage-gravel figure assumes you fill the full height of the wall to the width you specify. That is the correct assumption — a drainage layer that stops halfway up simply moves the saturation line to where it stops.

Geogrid: what triggers it

Geogrid is a polymer mesh laid horizontally between courses and extending back into the retained soil. It converts the wall from a lump of blocks resisting by weight alone into a reinforced soil mass — the grid ties the block face to a wedge of compacted fill behind it, and the whole assembly resists tipping together. This is what lets a block wall exceed the height its own mass could hold.

Grid length is not arbitrary. Manufacturers typically specify a grid that extends back 60–80% of the wall height into compacted fill — a 4-foot wall wants roughly 3 feet of grid reaching behind it, in compacted soil, not the loose backfill you shovelled in. That excavation requirement is the reason geogrid walls are so much more work than they look: you are not building a wall, you are building a reinforced block of earth with a decorative face. If the space behind the wall is too tight to excavate that far back — against a property line, say — you cannot use geogrid there, and the design has to change.

Worked example: a 24 × 2.5-ft garden wall

Enter wall length 24 ft, wall height 2.5 ft, 12"×4" landscape block, all three checkboxes on, and leave the defaults for base depth 6", base width 2 ft, drainage width 1 ft, waste 5%.

Courses = 2.5 ft ÷ 0.333 ft per block = 7.5, rounded up to 8 courses. Blocks per course = 24 ÷ 1 = 24. Total = 8 × 24 = 192 blocks, and 5% waste rounds up to 202 blocks, plus 24 cap blocks. Base gravel = 24 × 2 × 0.5 ft = 24 cubic feet ÷ 27 = 0.89 cu yd. Drainage gravel = 24 × 1 × 2.5 = 60 cubic feet ÷ 27 = 2.22 cu yd. Fabric = 24 × (1 + 2) = 72 sq ft.

Notice the rounding on courses. Eight courses of 4-inch block is 32 inches of block for a 30-inch design height — and that is exactly right, because the bottom two inches of that eighth course disappear into the trench along with your buried course. If you want 2.5 feet of wall showing, enter the height as showing plus buried, not just what you want to see.

Worked example: a 30 × 4-ft wall — and why this one is different

Wall length 30 ft, height 4 ft, 16"×6" landscape block, base depth 8", base width 2.5 ft, drainage width 1.5 ft. Courses = 4 ÷ 0.5 = 8 courses. Blocks per course = 30 ÷ 1.333 = 22.5 → 23 per course, which tells you every course ends with a cut half-block; plan to rent a masonry saw and to alternate which end the cut lands on so the vertical joints stagger. Total = 184 blocks, plus 5% = 194 blocks and 23 caps.

Base gravel = 30 × 2.5 × 0.667 ft = 50 cubic feet ÷ 27 = 1.85 cu yd. Drainage gravel = 30 × 1.5 × 4 = 180 cubic feet ÷ 27 = 6.67 cu yd. Fabric = 30 × (1.5 + 2.5) = 120 sq ft. Note that the drainage gravel now outweighs the base by nearly four to one, and at roughly 1.4 tons per cubic yard that is about nine tons of stone to move by wheelbarrow.

More importantly, this wall is at the threshold. Four feet measured from the bottom of the footing is where the permit and engineering requirement bites in most jurisdictions, and this design needs geogrid regardless. Treat this example as a wall you price out and then take to your building department, not one you start on Saturday.

Failure modes — read this before you order

Retaining walls are the most consequential structure on this site. A failed wall is not a cosmetic problem: it releases the mass of soil behind it, usually all at once, usually in a storm, and often onto something. Walls fail in a small number of well-understood ways.

• Hydrostatic pressure. No drainage stone, no pipe, or a pipe with nowhere to discharge. Saturated backfill pushes the wall out at the middle. This is the most common failure by a wide margin and it is entirely preventable at the cost of a few yards of stone.
• Inadequate or unlevel base. The wall settles unevenly, courses separate, and the interlocking lips shear. Visible as a wall that has gone wavy along its top line.
• Overturning. The wall rotates forward about its toe because it lacks the mass or the reinforcement for the height. Visible as a wall leaning outward — a wall that is leaning is a wall that is failing, not a wall that has settled.
• Sliding. The whole wall translates forward along the base. Usually a base that was too smooth, too narrow, or built on soft soil.
• Global slope failure. The soil mass rotates beneath the wall, taking the wall with it. The wall was built correctly; the hillside was not stable. Only a geotechnical assessment catches this in advance.
• Backfill compacted too aggressively. A counterintuitive one: running a heavy plate compactor directly behind the block face can push the wall out of alignment. Compact in thin lifts and stay a few feet back from the face with heavy equipment.
• Frost heave. In cold climates, water in poorly drained backfill freezes and expands against the wall. Another argument for clean stone and a working drain.

Warning signs on an existing wall — leaning, bulging in the middle, separated or stepped joints, blocks sliding forward at the top, soil or water seeping through the face, a crack running down through several courses — are all reasons to stop using the ground above the wall and get an engineer out. Retaining walls rarely fail gradually once they start moving.

When to call an engineer

Directly, and without hedging: hire a licensed structural or geotechnical engineer if the wall exceeds 4 feet from the bottom of the footing, if anything at all surcharges the soil behind it, if the ground rises behind the wall rather than sitting level, if the wall would support or undermine a driveway, foundation, pool, or septic system, if you are tiering walls up a slope, or if there is any existing sign of soil movement on the site. Also call one if the soil is clay-heavy and you do not know its bearing capacity, or if the excavation would go deep enough to need shoring — trench collapse kills people every year and it does not care that the trench was only chest-deep.

The engineering fee on a residential wall is typically a small fraction of the wall's material cost, and it is a vanishing fraction of the cost of rebuilding a failed wall plus whatever the failure took with it. If the answer is genuinely borderline, build the wall shorter and use two well-separated tiers with planting between them, or regrade the slope instead. Neither of those is a compromise you will regret.

Common questions

For the gravel that goes around the wall, see the gravel calculator — sand for joints, see sand.