Retaining Wall Calculator
Plan a segmental block retaining wall: blocks with a buried course, caps, compacted base gravel, drainage stone, drain pipe and fabric.
Your estimate
170 blocks
Wall blocks (incl. 5% waste)
- Courses × blocks per course
- 7 × 23
- Cap units
- 25
- Buried course depth
- 6 in
- Base gravel
- 1.11 cu yd (1.97 tons)
- Drainage stone
- 3.89 cu yd (5.24 tons)
- Wall face area
- 90 sq ft
Show the math
- Buried depth = max(6 in, 1 in per ft of height) = 6 in
- Courses = (exposed + buried) ÷ block height, rounded up = 7; per course = length ÷ block length = 23
- Blocks = 7 × 23 = 161; × (1 + 5%) → 170
- Base = length × max(24 in, block depth + 12 in) × 6 in = 1.11 cu yd; drainage = length × 12 in × full height = 3.89 cu yd
Shopping list & cost
| Item | Spec | Qty | Cost |
|---|---|---|---|
| Retaining wall blocks | Medium 6 × 16 in face, 10 in deep | 170 | — |
| Cap units | glue down with concrete adhesive | 25 | — |
| Base gravel (crushed, compactable) | 24 in wide × 6 in deep | 1.11 cu yd ≈ 1.97 tons | — |
| Drainage stone (#57 / 20 mm clean) | 12 in behind the wall, full height | 3.89 cu yd ≈ 5.24 tons | — |
| Perforated drain pipe | 4 in (100 mm), at the base behind the wall | 30 ft | — |
| Filter fabric | between drainage stone and soil | 144 sq ft | — |
Prices are what you enter under Advanced options. Estimates only — check stock and pricing with your supplier.
Quick answer A 30 ft retaining wall 3 ft high in 6 × 16 in blocks needs 170 blocks including a 6 in buried course and 5% waste, 25 caps, 1.11 cu yd (1.97 tons) of compacted base gravel and 3.89 cu yd (5.24 tons) of drainage stone behind it. A 9 × 0.9 m wall in 400 × 150 mm blocks needs 194 blocks. Courses = (exposed height + buried depth) ÷ block height.
Method & assumptions
Buried depth = max(6 in, 1 in per ft of exposed height) Courses = (Exposed height + Buried depth) ÷ Block height, rounded up Blocks = Courses × ⌈Length ÷ Block length⌉ × (1 + waste %) Base gravel = Length × max(24 in, Block depth + 12 in) × 6 in Drainage stone = Length × 12 in × (Exposed height + Buried depth)
- Exposed height
- from finished grade in front to the top of the wall (without cap)
- Base
- compacted crushed stone leveling pad
- Drainage zone
- clean ¾ in (#57) stone behind the blocks
Assumptions & limits
- A gravity segmental wall under about 3–4 ft, on firm native soil, with no slope or driveway above it.
- Densities: compacted base ≈ 2.1 t/m³ (about 1.8 tons per cu yd), #57 stone ≈ 1.6 t/m³.
- Block sizes are typical; units from your supplier may differ — enter the nearest size or check their chart.
- Taller walls, terraced walls and walls with loads above need geogrid and an engineered design.
Rounding: purchase quantities are always rounded up to whole units as sold. See our methodology.
Diagram
How to use this calculator
- Measure the wall length along the base and the exposed height above finished grade.
- Choose the block size; small 4 in units suit garden edging, larger 6–8 in units suit taller walls.
- The buried course is added automatically: 1 in per foot of height, minimum 6 in.
- Decide on caps, keep 5% waste for straight walls and 10% for curves, corners and steps.
- Use the base and drainage quantities to order gravel by the ton.
Worked examples
Imperial: 40 ft × 2 ft garden wall, small 4 × 12 in blocks
- Buried = 6 in → (24 + 6) ÷ 4 = 7.5 → 8 courses
- × 40 per course = 320; +5% = 336 blocks; caps = 42
- Base = 40 ft × max(24, 7 + 12) in × 6 in = 40 × 2 × 0.5 = 40 cu ft ≈ 1.48 cu yd
Metric: 12 m × 0.6 m wall, 400 × 150 mm blocks
- Buried = 150 mm → (600 + 152) ÷ 150 = 5.01 → 6 courses
- × 30 per course = 180; +5% = 189 blocks
- Drainage = 12 × 0.3 × 0.75 ≈ 2.75 m³ ≈ 4.4 t
In depth: retaining wall planning notes
The base and the buried course
A segmental retaining wall is a gravity structure: its weight and the friction between courses hold back the soil. That only works if it sits on a firm, level foundation. Dig a trench wide enough for the block plus about 6 in in front and behind, compact the native soil, then place and compact at least 6 in of crushed, well-graded gravel as a leveling pad. Allan Block's guidance is to bury the first course 1 in for every foot of wall height, with a minimum of 6 in, so the toe of the wall is locked in by the soil in front.
Take the most time on the first course. Level each block front to back and side to side, and check the line with a string — every error is repeated in every course above. Most blocks have a lip or pins that set each course back slightly, so the wall leans into the hill. Keep the setback consistent by brushing each course clean before laying the next.
Drainage, height limits and geogrid
Water, not soil, is what pushes most failed walls over. Saturated soil behind a wall can more than double the pressure on it. Fill at least 12 in behind the blocks — and the cores of hollow units — with clean, angular ¾ in stone, wrap it in filter fabric to keep fines out, and lay a 4 in perforated drain pipe at the base that daylights or connects to a drain. Compact the native backfill behind the stone in thin lifts.
Height matters. Most block manufacturers allow walls of roughly 3 ft or less as simple gravity walls; taller walls, walls on slopes, terraced walls and walls with a driveway, building or patio above them need geogrid layers and often an engineered design. In the IRC, walls over 4 ft measured from the bottom of the footing, or that support a surcharge, are not exempt from permits. This calculator is for the common short garden wall; for anything larger, use it to budget and follow the manufacturer's design charts or an engineer.
Reference table
| Block | Face · depth · per sq ft |
|---|---|
| Small (edging, low walls) | 4 × 12 in · 7 in · 3.0 |
| Medium | 6 × 16 in · 10 in · 1.5 |
| Large (Allan Block size) | 8 × 18 in · 12 in · 1.0 |
| Cap | usually same length as the block |
Key facts
- Permit trigger (IRC R105.2)
- over 4 ft from bottom of footing, or with a surcharge
- Base
- 6 in compacted gravel, at least 24 in (610 mm) wide
- Drainage
- 12 in of clean stone + 4 in perforated pipe
Tips & common mistakes
- The base course decides how the wall looks — level it carefully in both directions and check every block.
- Clean the top of each course of grit so the next course sits flat and the setback stays consistent.
- Use clean, angular stone behind the wall, not soil; water pressure is what pushes most failed walls over.
- Run the drain pipe to daylight or a drain so water can actually leave.
Frequently asked questions
How many retaining wall blocks do I need for a 50 ft wall?
With 6 × 16 in blocks and 3 ft of exposed height: 7 courses (incl. a 6 in buried course) × 38 blocks = 266, about 280 with 5% waste, plus 40 caps.
How much gravel do I need behind a retaining wall?
About 12 in of clean drainage stone the full height of the wall: a 30 ft × 3.5 ft wall needs about 3.9 cu yd (5.2 tons). Add the base pad (24 in wide × 6 in deep): about 1.1 cu yd for the same wall.
How deep should the first course of a retaining wall be?
Bury 1 in for every foot of wall height, with a minimum of 6 in, on a 6 in compacted gravel base (Allan Block guidance; check your block manufacturer).
How tall can a retaining wall be without a permit?
Under the IRC, walls up to 4 ft measured from the bottom of the footing to the top that don't support a surcharge are usually exempt. Local rules vary.
Do I need geogrid?
Most manufacturers call for geogrid on walls over about 3–4 ft, on poor soil, or with slopes or loads above the wall.
Sources
- Allan Block – Commercial Installation Manual (PDF) — accessed : “Dig the trench … a minimum of 24 in (610 mm) wide and 6 in (150 mm) deep … Buried block should be a minimum of 6 in (150 mm) or 1 in (25 mm) for each 1 ft (300 mm) of wall height.”
- ICC – 2021 IRC R105.2 work exempt from permit — general reference — not cited for a specific figure
- Concrete Masonry & Hardscapes Association – segmental retaining walls — general reference — not cited for a specific figure
Default values (yields, coverage rates and densities) are typical figures from manufacturer literature. Always check the label on the product you buy.
Related calculators
- Gravel CalculatorEstimate gravel or crushed stone in cubic yards, tonnes and short tons, including a 12×20 ft driveway at 4 in.
- Paver CalculatorCount patio pavers plus base gravel and bedding sand, including a 12×12 ft patio with 4×8 in pavers.
- Concrete Block CalculatorCount 8×8×16 cinder blocks (CMU) for a wall, plus bags of mortar and the grout or concrete to fill the cores — with the math shown.
Learn more in our guides
Method, waste and worked examples: Loose materials: depth, volume, bags and tonnes.
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DIYCalcLab. “Retaining Wall Calculator.” Oct 19, 2026. https://diycalclab.com/landscaping/retaining-wall-calculator/
Estimates only. Verify quantities with your supplier and local building codes. For structural, electrical or safety-critical work, consult a licensed professional. Full disclaimer.