Retaining Wall Calculator
Calculate retaining-wall materials — enter your wall length, height and block size to get the number of wall blocks, cap blocks and base gravel, plus an estimated cost.
How many blocks do I need for a retaining wall?
A segmental retaining wall is a stack of interlocking blocks on a compacted gravel base, with a cap row on top and free-draining gravel behind it. Estimating one means counting the wall blocks for the face area, adding a cap row, and sizing two lots of gravel — the base trench and the drainage backfill. The calculator above does all of that from your wall length, height and block size.
The block formula
Blocks are counted by wall face area. Find the area, then divide by the face area of a single block:
wall face (sq ft) = length (ft) × height (ft)
block face (sq ft) = block length (in) × block height (in) ÷ 144
blocks = wall face ÷ block face × (1 + waste %)
You can also think of it as courses × blocks-per-course: courses = wall height ÷ block height, and blocks per course = wall length ÷ block length. Both give the same total. Add about 10% for cuts, breakage and future repairs.
Worked example: a 30 ft × 3 ft wall
Say you are building a 30 ft long, 3 ft high wall with standard 12 in × 6 in face blocks and 10% waste:
- Wall face = 30 ft × 3 ft = 90 sq ft
- Block face = 12 × 6 ÷ 144 = 0.5 sq ft each
- Blocks = 90 ÷ 0.5 × 1.10 = ≈ 198 blocks
- Courses = 36 in ÷ 6 in = 6 courses of 30 blocks
- Cap blocks = one row along 30 ft = 30 caps
- Base gravel (6 in trench, twice block depth wide, +20%) ≈ 0.9 cubic yards, plus drainage stone behind the wall
Base and drainage: what makes a wall last
Two things fail retaining walls: a poor base and trapped water. Both are gravel problems.
| Layer | Spec | Why |
|---|---|---|
| Compacted gravel base | ≈ 6 in deep, twice the block depth wide | Level, load-spreading footing |
| Buried first course | ≈ 1/10 of wall height below grade | Stops sliding & heaving |
| Drainage gravel (backfill) | ≈ 12 in of clean stone behind the wall | Sheds water off the blocks |
| Drain pipe | perforated pipe at base (walls > 2 ft) | Carries water away |
Compacting gravel loses up to about 20% of the loose volume, so order roughly 20% extra base stone — the calculator adds that for you. Size any extra with the gravel calculator.
Cap blocks
The cap is the finished top row — often a flatter, different unit set with masonry adhesive. You need one row along the full length of the wall, so the cap count is the wall length divided by the cap block length. Caps hide the hollow cores and give the wall a clean edge.
How much does a retaining wall cost?
Cost is usually quoted per square foot of wall face. The figures below are typical 2026 US ranges and are estimates only.
| Item | Typical cost (2026 est.) |
|---|---|
| Segmental block wall, installed | $20–$50 / sq ft of face |
| 30 ft × 3 ft wall (90 sq ft), installed | ≈ $1,800–$4,500 |
| Wall blocks (material) | $3–$8 each |
| Cap blocks | $4–$10 each |
| Base & drainage gravel | $30–$60 / cubic yard |
Price rises with wall height (taller walls need deeper bases, reinforcement grid, and sometimes an engineer), block style, and how much excavation and backfill the site needs.
Common mistakes
- An unlevel base. The first course sets every course above it — a base that is not dead level and well compacted telegraphs all the way up.
- No drainage. Backfilling with soil instead of gravel traps water; hydrostatic pressure is the number-one reason walls bulge and topple.
- Not burying the first course. The bottom row should sit below grade so the wall can't kick out at the base.
- Skipping the batter and grid. Walls lean back slightly into the hill; taller walls also need geogrid reinforcement into the soil behind them.
- Building tall without an engineer. Walls over about 3–4 ft, or any wall holding a driveway or slope, usually need engineering and often a permit.
When to hire a pro
A low garden wall — under about 3 feet, on stable ground, holding back a modest slope — is a realistic DIY project if you are willing to do the base excavation and compaction properly. Hire a pro (and expect an engineer and permit) for taller walls, walls supporting a driveway, structure or steep slope, walls in wet or unstable soil, or any wall where failure would damage property. Use this calculator to size the blocks and gravel and to sanity-check bids. For the base and backfill stone, see the gravel calculator; for related hardscaping, the paver and concrete calculators.
Figures here are estimates only and vary by region, block system, wall height and supplier as of 2026 — verify quantities and engineering requirements with a local supplier or contractor, and check whether your wall needs a permit before you build.
Prices vary by location. Verify with local suppliers.
How many blocks do I need for a retaining wall?
Multiply the wall length by the wall height for the face area, then divide by the face area of one block and add about 10% for cuts and breakage. The calculator above also counts the cap row, sizes the compacted gravel base (with a 20% compaction allowance) and estimates drainage gravel for behind the wall.
blocks = (wall length × height) ÷ block face area × (1 + waste)
Sizing the gravel yourself? Cross-check with the gravel calculator.
Frequently asked questions
How many blocks do I need for a retaining wall?
Multiply the wall length by the wall height to get the wall face area in square feet, then divide by the face area of one block. Add about 10% for breakage and cuts. For example, a 30 ft × 3 ft wall is 90 sq ft; with a 12×6 in block (0.5 sq ft face) that is about 180 blocks plus waste. The calculator does this for any block size.
How much base gravel does a retaining wall need?
Set the wall on a compacted gravel base about 6 inches deep and roughly twice as wide as the block. Because compacting can lose up to 20% of the loose volume, order about 20% extra. The calculator sizes the base trench and adds the compaction allowance, plus drainage gravel for behind the wall.
What are cap blocks and how many do I need?
Cap blocks are the finished top row of the wall, often a different, flatter unit glued down with construction adhesive. You need one row along the full length of the wall, so the count equals the wall length divided by the cap block length. The calculator counts a single cap row for you.
How deep should a retaining wall base be?
Bury the bottom course so the base sits below grade on 6 inches of compacted gravel — a common rule is to embed about one-tenth of the wall height. This keeps the wall from sliding or heaving. Level the base carefully; the first course sets the accuracy of every course above it.
Do I need drainage behind a retaining wall?
Yes. Water building up behind a wall is the main reason walls fail. Backfill the space right behind the blocks with free-draining gravel and add a perforated drain pipe at the base for walls over about 2 feet. The calculator includes a drainage-gravel estimate for the zone directly behind the wall.
How much does a retaining wall cost?
A segmental block retaining wall typically runs about $20–$50 per square foot of wall face installed in 2026, so a 30 ft × 3 ft wall (90 sq ft) often costs $1,800–$4,500. Block and gravel material alone is a fraction of that. Walls over 3–4 feet, or ones holding back significant loads, may need an engineer.