Work out how many retaining wall blocks and caps you need, plus gravel for the base and drainage stone behind the wall. Enter the wall size and your block dimensions to get a full materials list.
Formula checked 23 Oct 2026Free · no sign-up
Was this calculator helpful?
Found a bug or have a suggestion?
Leave it here and we will fix it and reply to you.
Embed this calculator on your website (free)
Copy this code into your page. It stays free and updates automatically.
How to use the retaining wall calculator
Enter the wall length and the exposed height above the finished ground.
Enter your block face length and height from the product label (common sizes: 16 × 6 in, 12 × 4 in, 18 × 8 in; 400 × 150 mm).
Keep at least one buried course.
Add caps, and check the base trench and drainage stone sizes.
Retaining wall formulas
Blocks per course = Wall length ÷ Block length → round up
Courses = Exposed height ÷ Block height → round up, + buried courses
Blocks = Blocks per course × Courses × (1 + waste %)
Caps = Wall length ÷ Cap length
Base gravel = Length × Trench width × Base depth
Drainage stone = Length × Drainage width × Total height
Worked example
A garden wall 30 ft long and 3 ft high using 16 in × 6 in blocks, one buried course:
Blocks per course = 360 in ÷ 16 in = 22.5 → 23
Courses = 36 in ÷ 6 in = 6, + 1 buried = 7
Blocks = 23 × 7 = 161, + 5% = 170 blocks
Caps (12 in long) = 30
Base = 30 × 2 × 0.5 ft = 30 ft³ = 1.1 yd³ of crushed stone
Drainage stone = 30 × 1 × 3.5 ft = 105 ft³ = 3.9 yd³
Step-by-step: building a block retaining wall
Call before you dig (811 in the US) to mark buried utilities.
Dig the trench about twice the block depth wide and deep enough for the base plus the buried course.
Compact 6 in (15 cm) of crushed stone in layers and level it carefully — the first course decides how straight the whole wall is.
Lay the first course and check level along and across every block.
Lay the perforated drain pipe behind the bottom course, sloped to drain out at one end.
Stack each course with joints staggered, filling the cores and the zone behind with drainage stone as you go and compacting the backfill.
Glue the caps with masonry adhesive and finish with topsoil behind the wall.
Common block sizes
Block (face)
Blocks per ft² of face
Typical use
12 × 4 in
3.0
Small garden and flower-bed walls
16 × 6 in
1.5
General landscape walls
18 × 8 in
1.0
Larger walls, quicker to build
400 × 150 mm
16.7 per m²
Metric landscape walls
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length by the block length to get blocks per course, and the height by the block height to get the number of courses. Add one buried course and multiply. A 30 ft wall, 3 ft high, with 16 × 6 in blocks needs 23 blocks per course × 7 courses = 161 blocks, or about 170 with 5% extra.
How deep should the first course be buried?
Most block manufacturers recommend burying the first course, or at least 10% of the wall height (about 1 inch per foot), whichever is more. This stops the base from being washed out or pushed forward.
How much gravel do I need under a retaining wall?
A typical base is a trench about 24 in (60 cm) wide with 6 in (15 cm) of compacted crushed stone. For a 30 ft wall, that is 30 × 2 × 0.5 = 30 ft³ — about 1.1 cubic yards.
Why put gravel behind a retaining wall?
Water pressure is the main reason retaining walls fail. A 12 in (30 cm) zone of clean drainage stone behind the blocks, with a perforated pipe at the bottom, lets water escape instead of pushing the wall over.
How tall can a block retaining wall be without engineering?
Many local codes and manufacturers allow gravity walls up to about 3–4 ft (1–1.2 m) without engineering, as long as nothing heavy sits above. Taller walls, or walls under driveways, slopes or buildings, usually need geogrid reinforcement, an engineer and a permit.
Do I need cap blocks?
Caps finish the top, protect the cores and look better. They are usually glued down with masonry adhesive. Count them by dividing the wall length by the cap length.
How we checked this calculator
We test this tool against worked examples before every release (last check: 23 Oct 2026, version 1.0.0). Results are estimates for planning — confirm critical figures with your supplier or a qualified professional.
