Retaining Wall Block Calculator
Blocks, caps, and gravel base for a segmental retaining wall.
What it does
Segmental walls stack dry concrete blocks into gravity structures that lean on weight, batter, and drainage rather than mortar. Done right they outlast timber and look better doing it. Done wrong they lean, bulge, and march forward one frost at a time.
The tool above estimates a block wall from length, height, and catalog sizes. Enter the dimensions plus block face and cap length. It returns block count with 5 percent waste, cap count, and leveling pad gravel. Defaults of 20 by 3 ft open solved at 63 blocks, 14 caps, and 20 ft3 of pad.
The first course decides everything. It sits buried on 6 inches of compacted crusher run, twice as wide as the block is deep, leveled to the bubble. Every later course inherits its sins or its virtues, so pros spend half the job on base and backfill rather than stacking.
Water is the enemy, not soil. Twelve inches of clean stone behind the face plus a perforated pipe carry seepage out before pressure builds. Most failed walls the author has seen skipped drainage, not blocks; the soil wedge simply pushed the dry stack over.
Use the table to compare garden walls with bigger ones, then run the design. Anything above 4 ft tall, terraced, surcharged by driveways, or holding poor soils belongs to an engineer with geogrid and a stamp.
How to use
- Pick imperial (ft) or metric (m).
- Enter wall length and total height with burial.
- Enter block face area from the catalog.
- Enter cap length from the catalog.
- Read blocks first, then caps and gravel.
- Price drainage stone and pipe on top.
Formula
- Length
- Wall length: ft imperial, m metric.
- Height
- Exposed height plus buried base course.
- Block face
- One block face area from the catalog.
- Cap length
- One cap unit length from the catalog.
Face is length times exposed height plus buried course. Block face defaults to 1.0 ft2 (0.093 m2) and caps to 1.5 ft (0.46 m); adjust to the catalog. The gravel line covers the leveling pad only; backfill drainage stone is extra. Gravity walls above about 4 ft (1.2 m) need engineered design with geogrid.
Worked example
Example: 20 by 3 ft garden wall
A 20 ft long, 3 ft high wall with 1 ft2 blocks and 1.5 ft caps. Find blocks, caps, and pad gravel.
- Face area: 20 x 3 = 60 ft2.
- Blocks: 60 / 1.0 x 1.05 = 63, rounded up.
- Caps: 20 / 1.5 = 13.3, rounded up to 14.
- Gravel pad: 20 x 2 x 0.5 = 20 ft3.
- Add drain pipe and 12 in of drainage stone behind the face.
- Walls above 4 ft need an engineer and likely geogrid.
Reference table
| Wall | Blocks | Caps | Pad gravel |
|---|---|---|---|
| 20 by 3 ft (worked) | 63 blocks | 14 caps | 20 ft3 |
| 6.1 by 0.91 m (metric) | 63 blocks | 14 caps | 0.6 m3 |
| 100 by 4 ft | 420 blocks | 67 caps | 100 ft3 |
| 10 by 1 m (metric) | 117 blocks | 23 caps | 0.9 m3 |
Limits
- Gravity walls only; geogrid walls need design.
- Pad gravel only; backfill stone is extra.
- Catalog sizes vary; adjust face and cap.
- Soil, surcharge, and frost need engineering.
Common mistakes
- Skipping the buried base course.
- Leveling with sand instead of crusher run.
- Omitting drain pipe and backfill stone.
- Stacking vertical instead of battering back.
- Building above 4 ft without an engineer.
Try it yourself
Quick checks. Answers sit tucked under each question.
100 ft long, 4 ft high, same blocks?
Face 100 x 4 = 400 ft2. Blocks ceil(400 x 1.05) = 420. Caps ceil(100 / 1.5) = 67. Pad 100 x 2 x 0.5 = 100 ft3.
10 m long, 1 m high wall in metric?
Face 10 x 1 = 10 m2. Blocks ceil(10 / 0.09 x 1.05) = 117. Caps ceil(10 / 0.45) = 23. Pad 10 x 0.61 x 0.15 = 0.9 m3.
Metric default in face area?
6.1 x 0.91 = 5.6 m2 of face. Same ratios give 63 blocks and 14 caps, plus 0.6 m3 of pad.
Questions
How much waste should I add?
About 5% for cuts plus a few spare. Curves, corners, and steps eat more; long straight runs eat less.
How deep is the gravel base?
Six inches of compacted crusher run, about twice as wide as the block is deep. It levels the first course, which decides the whole wall.
Do I need drain pipe?
Yes: 12 in of clean drainage stone behind the face plus a perforated pipe daylighted out. Water pressure topples more walls than soil does.
How much block sits underground?
Bury about one tenth of the height or 6 in minimum. Burial resists sliding and hides the leveling course.
When does a wall need an engineer?
At 4 ft in most codes, sooner in some cities. Taller walls need geogrid layers, permits, and stamped design.
How are caps installed?
Caps glue on top with masonry adhesive, overhanging slightly. They shed water and finish the look.
Sources
- FHWA: durability of segmental retaining wall blocks, design context
- CMHA: segmental retaining wall best practices guide