Loan Retaining Wall Estimator
A retaining wall is three purchases, not one: the blocks you see, the caps that finish it, and the gravel nobody photographs — the buried base and the drainage stone that decide whether the wall stands straight in five years. This estimator counts all three.
For standard segmental blocks (about 16 × 8 in face). Height is above ground — the estimator buries the first course in the base count the way builders do. Walls over 4 ft usually need geogrid, drainage pipe and, in many places, a permit or an engineer.
How it works
A standard segmental block face (16 × 8 in with its joint) covers about 1.11 sq ft, so the wall face takes 0.9 blocks per sq ft. Caps run along the top: one cap per 16 in of length. The invisible half of the wall is gravel: a base trench about 24 in wide and 6 in deep under the wall, plus a 12-in drainage zone of clean stone behind the full height — that zone, with a perforated drain at its foot, is what keeps water pressure from pushing the wall over. Gravel is converted at 1.40 tons per cubic yard.
Formula & worked example
Formula: Blocks = wall length × height × 0.9 blocks per sq ft (16 × 8 in segmental face) plus waste; caps = length ÷ 1.33 ft per cap plus waste. Gravel = base trench (length × 2 ft × 0.5 ft) + drainage zone (length × 1 ft × height), converted at 1.40 tons per cu yd.
Worked example: A 20 × 4 ft wall: 72 raw blocks, 80 to buy with 10% waste, 17 caps, about 6 courses — and 5.19 tons of gravel between the base and drainage zone.
When to use this
- You are pricing a segmental block wall and need the full shopping list, not just blocks.
- A landscaper's quote lists block counts you want to verify.
- You are planning delivery: blocks, caps and gravel arrive on different trucks.
How to use it, step by step
- Measure the wall's length and its height above ground.
- Leave the waste allowance at 10% unless the wall is dead straight.
- Note blocks, caps and total gravel tons separately.
- If the wall exceeds 4 ft, stop and involve an engineer before ordering.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Wall length | End to end along the wall line | 10–40 ft typical garden walls |
| Height above ground | Exposed face height; the first course is buried in the base math | 2–4 ft typical; over 4 ft needs engineering |
| Waste allowance | Cuts, corners and breakage | 10% standard; 15% for curves and steps |
Another worked example
A longer, lower wall: 30 ft long, 3 ft above ground. Face 90 sq ft → 81 raw blocks, 90 to buy with waste; 25 cap blocks; base gravel 1.11 cu yd and backfill 3.33 cu yd, totalling 6.22 tons of stone — the gravel order outweighs the block order's surprises again.
Frequently asked questions
How many blocks do I need for a 20 × 4 ft wall?
The face is 80 sq ft × 0.9 = 72 blocks, 80 with a 10% cutting allowance — plus 17 cap blocks for the top course. The same wall needs about 5.19 tons of gravel between its base and drainage zone: the gravel order surprises everyone the first time.
How deep should the base be?
A trench about 24 in wide and 6 in deep of compacted crushed stone, with the first block course buried below grade. The rule builders use: bury at least 1 in of wall for every foot of height above ground, and never less than the full first course.
Why so much gravel behind the wall?
Because water is the load that actually fails walls. Soil holds rain like a sponge and pushes; clean drainage stone lets it fall to the drain pipe instead. Skipping the backfill stone is the single most common DIY wall mistake.
How many courses is a 4-ft wall?
Six courses of 8-in block reach 48 in — five showing and one buried, roughly. The estimator reports courses so you can sanity-check the count against your block's actual face height.
What does a retaining wall cost installed?
Segmental block walls typically run $40–$70 per sq ft of face installed — a 20 × 4 ft wall lands around $3,200–$5,600. The Home Renovation Estimator can fold a wall into a bigger landscape budget.
🧱 Building the wall from block instead?
For a mortared CMU wall rather than dry-stacked segmental block, the Concrete Block Estimator counts blocks and mortar bags for any wall size.
Segmental block or poured concrete wall?
Segmental (dry-stacked, interlocking) is the DIY- and small-crew-friendly system and dominates residential work under 4 ft. Poured and CMU walls suit engineered, taller or load-bearing cases — the Concrete Block Estimator covers the CMU route.
Do I need a drain pipe behind the wall?
Yes on almost every wall: a perforated pipe at the base of the drainage zone, daylighted at the ends or tied to a drain, is what turns the gravel column into a system instead of a reservoir.