Retaining Wall Calculator
Plan a landscape retaining wall: wall blocks, cap blocks for the top course, base gravel for the trench and backfill gravel behind the wall.
How many blocks for a retaining wall?
A segmental retaining wall is three orders in one: the blocks you see, the cap blocks that finish the top course, and two kinds of gravel you mostly don't see — the compacted base in the trench, and the drainage backfill behind the wall that keeps water pressure from pushing it over. This calculator counts all four.
Blocks = wall face area (with waste) ÷ block face area · Caps = wall length ÷ cap length (with waste)
Example: a 20 ft long, 3 ft high wall in standard 6 × 18 in blocks: the face is 60 sq ft, each block covers 0.75 sq ft, so 80 blocks exactly — 88 blocks with 10% in, 15 cap blocks along the top, 0.56 cubic yards of base gravel in the trench and 2.22 cubic yards of backfill gravel behind the wall.
The parts that decide whether the wall lasts
| Part | Rule used here |
|---|
| Base trench | As wide as the block is deep + 6 in, and 6 in deep, compacted gravel |
| Buried course | At least one course below grade — include it in your height |
| Backfill | 12 in of clean drainage gravel directly behind the blocks, full height |
| Cap course | One cap per block length along the top — usually glued with landscape adhesive |
Water is the enemy this design is aimed at: the gravel column behind the wall gives groundwater somewhere to go that is not “through the wall,” and it weighs more than it looks — over 3 tons on the example wall. Walls over about 4 ft of retained height, walls holding a slope or a driveway, and walls on soft ground leave DIY territory in most codes and want an engineer and often geogrid reinforcement; keep this calculator to garden-scale walls and treat its numbers as the materials list, not a structural design.
Frequently asked questions
How deep should the base trench be?
6 inches of compacted gravel is the garden-wall standard used here, on undisturbed or compacted soil. Soft or clay ground wants deeper — and the first course sits buried so the base course is largely below the lawn level in front.
Do I really need gravel behind the wall?
Yes — it is the drainage layer. A wall backed directly by soil holds back wet soil after every rain, and wet soil is roughly twice as pushy as dry. The 12-inch gravel column plus a perforated drain at the base on longer walls is what lets the wall stay a wall.
How many cap blocks do I need?
One cap spans roughly one block length, so caps ≈ wall length ÷ block length, with the same waste allowance for the cuts at the ends and corners. Corner caps are cut from standard caps with a saw — that cutting is exactly what the allowance feeds.
Can I build a 6 ft retaining wall with this calculator?
The block arithmetic works at any height, but the engineering does not scale the same way. Above about 4 ft of retained height most building codes require a permit and many require a designed, reinforced wall. Use the count for materials on small walls; get a design for big ones.
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