Primary material
84 blocks
Retaining wall blocks
Use the block count as a starting point, then confirm the exact block face size, weight, and pallet quantity with the store.
Retaining wall calculator
Retaining wall block calculator.
Enter wall length, height, and block size to estimate wall blocks, cap blocks, and base gravel. A 20 ft long, 2 ft high wall needs about 84 blocks and 20 caps.
20 x 2 ft wall
84 blocks
20 caps
Courses
4 high
20 per course
Base gravel
0.37 yd³
6 in leveling pad
Quick reference
Retaining wall blocks for common wall sizes
12 x 6 in face blocks with the default 5% waste, cap blocks, and a 6 in deep, 1 ft wide gravel base.
| Wall size | Courses | Blocks | Caps | Base gravel |
|---|---|---|---|---|
| 10 ft long x 2 ft high | 4 | 42 | 10 | 0.3 tons |
| 20 ft long x 2 ft high | 4 | 84 | 20 | 0.5 tons |
| 20 ft long x 3 ft high | 6 | 126 | 20 | 0.5 tons |
| 30 ft long x 3 ft high | 6 | 189 | 30 | 0.8 tons |
| 50 ft long x 4 ft high | 8 | 420 | 50 | 1.3 tons |
Count the buried first course in the height. Walls over about 4 ft usually need engineering.
Quick wall answers
Common retaining walls
20 x 2 ft wall
84 blocks
plus 20 cap blocks
30 x 3 ft wall
189 blocks
plus 30 cap blocks
40 x 2 ft wall
168 blocks
plus 40 cap blocks
Drainage matters
A retaining wall needs gravel backfill and often a perforated drain pipe behind it so water does not build up and push the wall over. Estimate the base and backfill with the gravel calculator.
Shopping list
Blocks, caps, base, and reinforcement
Block and cap counts update from the calculator. Confirm the block face size and pallet quantity before ordering.
Top course
20 caps
Cap blocks and adhesive
Cap blocks finish the top row and are usually glued with masonry/landscape block adhesive.
Base and drainage
Base gravel and drainage
A compacted gravel base levels the first course, and gravel backfill plus a drain pipe keep water from building behind the wall.
Reinforcement
Geogrid and fabric
Taller walls need geogrid soil reinforcement and separation fabric. Check the block maker spec and local code for your height.
Formula shown
How the wall estimate works
1. Courses and per course
courses = ceil(height x 12 / block height); per course = ceil(length x 12 / block length)
2. Blocks and caps
blocks = ceil(courses x per course x (1 + waste)); caps = per course
3. Base gravel
base yd³ = length x base width x (base depth / 12) / 27
Tall walls need more
This counts blocks, caps, and a leveling base for a simple straight wall. Taller walls, slopes, surcharges, and walls near structures need geogrid, drainage, deeper base, and often an engineer and permit — check local code before building.
Sources
- Retaining wall block layout — courses, blocks per course, and base estimating
- Allan Block / Versa-Lok install guides — block dimensions, base, and geogrid guidance
Sources checked
Checked against
Bag yields checked against Quikrete and Sakrete product specs; price ranges against published concrete cost guides.
How we check our numbersLast updated: June 26, 2026
Estimate only.
This calculator is not a quote. Verify product yield, price, delivery minimums, and local conditions with your supplier or contractor before buying materials.
FAQ
Retaining wall questions
Block counts, cap blocks, base gravel, drainage, and when a wall needs engineering.
How many blocks do I need for a retaining wall? + -
Divide the wall height by the block height for the number of courses, and the wall length by the block length for blocks per course. A 20 ft long, 2 ft high wall with 12 x 6 in blocks is about 4 courses of 20, or roughly 84 blocks with waste.
How many cap blocks do I need? + -
Cap blocks finish the top course, so you need about one per block in the top row — about 20 caps for the 20 ft example. They are usually set with masonry or landscape block adhesive.
How much base gravel for a retaining wall? + -
The first course sits on a compacted gravel leveling pad, commonly about 6 in deep and a bit wider than the block. For a 20 ft wall that is roughly 0.37 cubic yards of base, plus gravel backfill behind the wall for drainage.
Do I need engineering or geogrid? + -
Short garden walls (often under about 3 to 4 ft) are usually DIY-friendly. Taller walls, walls holding a slope or surcharge, and walls near structures commonly need geogrid reinforcement, drainage, and an engineer or permit. Check local code.