Article — Retaining Wall Calculator
Retaining Wall Calculator: SRW Blocks, Gravel, and Geogrid
A 20 ft long × 3 ft tall segmental retaining wall using 12 in × 4 in face blocks (Allan Block, Pavestone, Versa-Lok) needs 180 blocks total — 20 blocks per course × 9 courses (with the bottom course buried). Base gravel: 0.65 cubic yards. Drain gravel behind the wall: 2.2 cubic yards. Maximum unreinforced height for typical SRW systems is 4 ft with good granular backfill (φ ≥ 32°), 3.5 ft with looser sand (φ ≈ 28°), 3 ft with silty soils. Above these heights, NCMA TEK 15-5B requires geogrid reinforcement and an engineer's stamp on the design.
This calculator outputs block count per course and total, courses needed, base trench and drain gravel volumes, Rankine Ka coefficient, lateral force per linear foot, and a flag for when geogrid reinforcement is required.
Segmental retaining wall basics
Segmental retaining walls (SRW) use concrete blocks dry-stacked with mechanical interlock (pin, lip, or tongue-and-groove). No mortar, no rebar in the blocks themselves. The wall resists overturning through its own mass and the mass of the retained earth bearing on the blocks' rear extension. The blocks' lip catches the course above and transfers shear load down to the base.
The standard Allan Block AB Classic, Versa-Lok Standard, and Pavestone Anchor Diamond run 12 inches wide × 4-8 inches tall × 9-12 inches deep, weighing 30-80 lb each. Smaller landscape blocks (12 × 4 × 9) weigh 25-30 lb and stack to 3 feet visible height. Larger structural blocks (18 × 8 × 12) weigh 75-80 lb and handle 4-6 feet without geogrid.
Retaining wall block count formula
Total blocks = blocks per course × number of courses. Blocks per course = wall length ÷ block face width, rounded up. Number of courses = wall height ÷ block height, rounded up. For a 20 ft long, 3 ft tall wall with 12 in × 4 in blocks: 20 × 12 / 12 = 20 blocks per course; 3 × 12 / 4 = 9 courses; total = 180 blocks.
Most SRW installations bury the bottom course or two below grade for stability. A "3-foot wall" might actually be 3 feet visible plus 1 bottom course buried = 4 feet of total block height. The calculator counts only the visible height; add one course for the buried base if your project requires it.
The standard 12 in × 4 in × 9 in landscape block weighs about 30 pounds. A 20-foot wall × 3 feet tall uses 180 blocks weighing roughly 5,400 lb (2.7 US tons). Two laborers can install that wall in a full day with a powered wheelbarrow; without it, expect 1.5-2 days. The labor-to-materials ratio for DIY SRW work is typically 60/40 — about $4-$5 of labor per $6-$7 of materials per square foot of wall face.
Max unreinforced retaining wall height
The NCMA TEK 15-5B technical note publishes maximum unreinforced SRW heights based on backfill soil friction angle. For granular backfill (clean sand, gravel) at φ ≥ 32°: 4 feet. For loose sand at φ ≈ 28°: 3.5 feet. For silty soil at φ ≈ 25°: 3 feet. Above these limits, geogrid reinforcement is mandatory and an engineer must stamp the design.
These limits assume no surcharge load (no buildings, driveways, or vehicles within 1.5× wall height behind the wall) and proper drainage. Add a surcharge from a parking pad or building footing and drop the unreinforced limit by 30-40 percent. Skip the drainage and effective Ka can double, dropping the limit similarly.
Rankine active earth pressure (Ka)
The Rankine active earth pressure coefficient Ka represents how much of the soil's vertical weight pushes horizontally against the wall. Ka = tan²(45° - φ/2), where φ is the soil friction angle. For φ = 32° (dense sand): Ka = 0.307. For φ = 36° (crushed gravel): Ka = 0.260. For φ = 25° (silty soil): Ka = 0.406.
The total lateral force on a wall per linear foot is Pa = 0.5 × γ × H² × Ka, where γ is the soil unit weight (typically 18 kN/m³ for compacted granular fill) and H is wall height. For a 1-meter (3.28 ft) tall wall with φ = 32° backfill: Pa = 0.5 × 18 × 1² × 0.307 = 2.76 kN/m of wall length. The force acts at H/3 above the wall base, contributing to overturning moment.
Retaining wall base and drain gravel
Every SRW needs a base trench: 6 inches deep × (block depth + 12 inches) wide, running the full wall length. The 12-inch over-width allows 6 inches in front and 6 inches behind the blocks for compaction. For a 20-ft wall with 9-in deep blocks: 20 × 1.75 × 0.5 = 17.5 ft³ = 0.65 yd³ of base gravel.
Behind the wall, install a 12-inch wide column of clean crushed stone (3/4-inch, no fines) running the full wall height. For a 20 × 3 ft wall: 20 × 3 × 1 = 60 ft³ = 2.2 yd³. The drain gravel relieves hydrostatic pressure — without it, saturated soil behind the wall can more than double the lateral force. Wrap the drain column with non-woven geotextile fabric to prevent fines from clogging the gravel.
Standard block 12 × 4 × 9 in, 30 lbBase trench depth 6 inchesDrain column width 12 inchesMax unreinforced (φ32°) 4 ftGranular fill Ka 0.307 at φ = 32°Geotextile Non-woven 4-6 oz/yd²When you need geogrid reinforcement
Geogrid is plastic mesh laid horizontally between courses of blocks; the tail extends 5-8 feet into the retained soil to create a reinforced earth zone behind the wall. Geogrid is mandatory above 4 feet (or 3 feet in clay soils), or when surcharge from buildings/driveways is present, or when the wall sits below a slope steeper than 3H:1V.
Common geogrid types: Tensar UX1100 (light duty), UX1400, UX1500. Place between courses every 2 courses (every 16 inches with 8-inch blocks, every 24 inches with 12-inch blocks). Pin to the front block lip; extend tail at least 60 percent of wall height into the retained slope, then compact backfill in 6-inch lifts.
Common retaining wall mistakes
The first mistake is skipping the drain gravel. Half of all retaining wall failures trace to drainage problems. Saturated soil behind a wall can have effective Ka exceeding 0.7 — more than double the dry value. The wall designed for 4 kN/m of lateral force suddenly sees 10 kN/m and falls over. The 12-inch drain column with geotextile fabric is non-negotiable.
Clay retains water, swells when wet, and shrinks when dry — all of which destabilize a retaining wall. Always backfill with granular soil (sand and gravel) for the first 12 inches behind the wall. Native clay can fill the space beyond that, but the immediate backfill must drain freely. Some SRW manufacturers won't honor warranty claims on walls built with clay backfill.
The second mistake is undersizing the base trench. The 6-inch deep gravel base supports the entire wall's vertical load and any soil mass above it. Skipping the base or shorting it to 2-3 inches causes the bottom course to settle unevenly, which propagates through every course above. The wall looks fine for a year, then develops a wavy top line as differential settlement accumulates.
- Total blocks = blocks per course × courses
- Blocks per course = ceil(length / face width)
- Courses = ceil(height / block height)
- Max unreinforced (granular) = 4 ft at φ ≥ 32°
- Rankine Ka = tan²(45° − φ/2)
- Granular Ka at 32° = 0.307
- Base trench = 6 in deep × (block depth + 12 in) wide
- Drain column = 12 in wide × full height