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Yard Drainage Fixes

Retaining Wall Calculator: Blocks, Gravel, Drainage & Cost

Block calculators count blocks. Cost guides give a price per square foot. Neither one quantifies the part of the wall that decides whether it is still standing in fifteen years — the stone zone, fabric and drain pipe behind it. This tool does the whole bill of materials, prices it two ways, and tells you plainly when a wall has left DIY territory. For what the drainage is for, see retaining wall drainage; for what walls cost and why quotes differ, see retaining wall cost.

Wall unit

6 × 16 × 12 in, pinned solid units (Versa-Lok style) — the residential default

A DIY-scale wall — the drainage is still not optional

At 3 ft exposed, wall mass and setback usually resist the soil without reinforcement. What makes or breaks it is below and behind: a compacted base with the first course buried, clear stone the full height, fabric, and a pipe that actually leaves.

Wall face

120 sq ft · 7 courses incl. 1 buried · 3.5 ft total height

Units

231 blocks

  • 30 per course × 7 courses = 210, plus 10% for cuts
  • 32 cap units
  • About 9.5 tons to deliver

Base & excavation

2.1 tons base stone

  • Trench 24 in wide × 6 in deep of compacted crushed stone (1.5 yd³)
  • First course buried 6 in below grade
  • Roughly 17.8 yd³ to dig for wall, stone zone and working room

The drainage package

6.2 tons of clear stone (4.4 yd³)

  • A 12-inch zone behind the wall, full height, capped with 6 in of topsoil
  • 230 sq ft of filter fabric between stone and soil
  • 42 ft of 4-in perforated pipe at the base, 11 ft of solid pipe to the outlet — 6 ten-foot sticks
  • Materials about $300–$490. Cheap — the cost is the wider dig and placing it in lifts.

DIY materials

$2,000–$3,050

Units, caps, base stone, drainage stone, fabric, pipe. Not included: delivery, tool rental, a compactor, and hauling 17.8 yd³ of spoil.

Installed, planning range

$2,400–$6,600

$20–$55 per sq ft of face for this wall type. Of that, the drainage package is usually $240–$1,300 — the line item to look for on a quote.

What a quote missing drainage looks like →Check a quote you already have →

What's behind the wall

Retaining wall cross-sectionCross-section of a segmental block retaining wall. The first course sits buried below grade on a compacted gravel base. Behind the blocks is a 12-inch zone of clear stone the full height of the wall, separated from the retained soil by filter fabric, with a perforated drain pipe at the base of the stone falling toward an outlet that daylights beyond the wall. Topsoil caps the stone so lawn can grow to the wall.to an outletclear stone, 12 inwrapped in fabriccompacted base, first course buriedfinished graderetained soil

What this doesn't tell you

These are planning quantities and ranges, not a design or a quote. The tool assumes a straight wall on level ground with ordinary soil behind it and nothing heavy above it. Slopes above the wall, a driveway or structure behind it, poor soil, water pressure and a curved or stepped layout all change the base, the reinforcement and the price. Above about 4 ft the design belongs to an engineer, and most places want a permit. Check both before you dig.

How to read your results

  • Units and courses. The tool adds a buried course under the height you typed, because the first course belongs below grade. That is why a 3-foot wall in 6-inch blocks comes out at seven courses, not six. The tonnage figure is for the delivery — most block is pallet-dropped at the curb, and a 40-foot wall is a couple of tons you then move by hand.
  • Base. Six inches of compacted crushed stone in a trench 6 inches wider than the block on each side. It is the least visible material on the list and the one that does the most.
  • The drainage package. Clear stone 12 inches wide the full height, filter fabric between stone and soil, a 4-inch perforated pipe at the bottom, and solid pipe from the low end of the wall to somewhere lower. The materials are cheap. The cost is the wider excavation and placing it in lifts — which is exactly why it is the first thing dropped from a low quote.
  • Two cost figures. DIY materials is what the list above costs to buy. Installed is a planning range from the site's cost page, with the drainage share separated so you know what to look for on a quote. Neither is a quote.

The formula — check it yourself

courses = (exposed height + buried course) ÷ block height
blocks = courses × (wall length ÷ block length) × 1.1

Base stone in cubic yards is length × (block depth + 12 in) × 6 in, all in feet, divided by 27. Drainage stone is length × 1 ft × (total height − 6 in) ÷ 27. Multiply cubic yards of stone by about 1.4 for tons. Fabric is the back, bottom and top of the stone zone plus 15 percent for overlap. Geogrid, when the wall is tall enough to need it, is roughly a layer every 2 feet of height, each extending at least 4 feet back — a planning quantity until the manufacturer's chart or an engineer sets it.

When it stops being a DIY wall

The verdict card uses two thresholds that show up in almost every block manufacturer's literature and most local codes. Up to about 3 feet of exposed height a properly based, properly drained wall holds itself up by mass and setback. Between 3 and 4 feet, geogrid normally enters the picture and the excavation behind the wall gets much wider to fit it. Above about 4 feet, most places require an engineered design and a permit, and the numbers on this page become a way to sanity-check a contractor's quote rather than a plan to build from. Slopes above the wall, a driveway or structure behind it, and tiered walls can pull those thresholds lower. A wall that is already leaning or bulging is almost always a drainage failure — and sometimes the better answer is no wall at all.

Quantities and planning costs by wall size

Standard 6 × 16 × 12-inch block, cap row, drainage package included with a 10-foot outlet run. Installed figures are per-square-foot planning ranges with a typical project minimum applied.

WallBlocks (+10%)Drainage stoneDIY materialsInstalled
20 ft × 2 ft832.1 tons$780–$1,200$1,500–$3,000
40 ft × 2 ft1654.1 tons$1,550–$2,350$1,600–$4,400
60 ft × 2 ft2486.2 tons$2,300–$3,500$2,400–$6,600
20 ft × 3 ft1163.1 tons$1,000–$1,550$1,500–$3,300
40 ft × 3 ft2316.2 tons$2,000–$3,050$2,400–$6,600
60 ft × 3 ft3479.3 tons$3,000–$4,550$3,600–$9,900
20 ft × 4 ft1494.1 tons$1,350–$2,050$1,600–$4,400
40 ft × 4 ft2978.3 tons$2,650–$4,100$3,200–$8,800
60 ft × 4 ft44612.4 tons$3,950–$6,100$4,800–$13,000

Planning estimates from the site's cost tables, not quotes. Access, soil and what sits above the wall move every figure; anything over 4 ft adds an engineered design.

Frequently asked questions

How many retaining wall blocks do I need?
Divide the total wall height — exposed height plus one buried course — by the block height for the number of courses, and the wall length by the block length for blocks per course. Multiply, then add 10 percent for cuts. A 40-foot wall standing 3 feet tall in 6 × 16-inch blocks needs 7 courses of 30 blocks: 210, or about 231 with waste, plus 30 cap units.
How much gravel goes behind a retaining wall?
Plan on a zone of clear, angular stone about 12 inches wide the full height of the wall, capped with 6 inches of topsoil. That is roughly one cubic foot of stone per foot of wall for every foot of height — a 40-foot wall 3.5 feet tall including the buried course needs around 5 cubic yards, about 7 tons. It is separated from the soil by filter fabric so it never silts up.
How deep should the base be?
A compacted trench about 6 inches deep in crushed stone, extending 6 inches in front of and behind the block, with the first course set below finished grade. Bury at least one full course, or about 10 percent of the wall's exposed height, whichever is more. The base and the buried course are where cheap walls save money and where they fail first.
How much does a retaining wall cost per foot?
Priced per square foot of wall face, segmental block walls run roughly $20 to $55 installed, timber $15 to $30, and natural stone $40 to $80. Per linear foot is only meaningful with a height attached: a 3-foot block wall lands around $60 to $165 per linear foot. Most contractors carry a project minimum of $1,500 to $3,000. All of these are planning ranges, not quotes.
Do I need geogrid?
Usually above about 3 feet of exposed height, and always when the manufacturer's chart says so for your block and soil. Geogrid is a polymer mesh laid between courses and extended back into the retained soil, roughly every 2 feet of height, at least 4 feet deep. The calculator gives a planning quantity; the real layout depends on the soil and whatever sits above the wall.
When do I need an engineer or a permit?
Most jurisdictions require a building permit and an engineered design above about 4 feet, and some measure from the bottom of the base rather than the finished grade. Slopes above the wall, a driveway or structure behind it, or tiered walls close together can trigger the same requirement at lower heights. Check with your local building department before you order materials.
Why does the calculator include a drain pipe?
Because trapped water is what pushes retaining walls over. Rain soaks into the backfill, and without a way out the saturated soil weighs far more and presses harder than the wall was built for. A perforated pipe at the base of the stone zone, falling toward an outlet that daylights somewhere lower, is the part of the wall you cannot see and cannot cheaply add later.

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