Retaining Wall Block Calculator
Count the blocks and caps a retaining wall needs, course by course.
Last reviewed September 19, 2026
Result
Buy 95 blocksEstimate
Five per cent is the masonry default. Retaining wall blocks are heavy and a proportion arrive chipped, and corners and curves need blocks cut. A straight run delivered on a good pallet may need none of it.
- Blocks per course
- 15
- Courses
- 6
- Cap blocks
- 0Estimate
- Wall face area
- 60 ft²
https://calcgallery.com/masonry/retaining-wall-block-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use it
- Measure the wall length and the height you want to see above ground.
- Leave the buried first course turned on unless you have a reason not to.
- Enter the face length and height of the block you are buying.
- Turn caps on if the wall is being capped, and set the cap length.
- Read the block count, the courses and the caps.
How it is calculated
A block wall is a grid, so the count is two divisions and a multiplication:
blocks per course = ceil(length ÷ block face length)
courses = ceil(height ÷ block face height) + buried course
blocks = ceil(blocks per course × courses × (1 + waste))
caps = ceil(length ÷ cap length)Both divisions round up, because a part block still means buying a whole one and cutting it.
The height you enter is the wall you want to see. The buried course is added on top of that, so turning it on adds a full course of blocks rather than changing the visible height.
Safety and permits
A retaining wall holds back a mass of soil that weighs far more than the wall does, and the force grows quickly with height. Walls above roughly 4 ft, or 1.2 m, measured from the bottom of the base course rather than from the ground you stand on, commonly need an engineered design, a permit and inspection. That threshold varies by jurisdiction and some places set it lower.
The height is not the only trigger. A slope rising above the wall, a driveway or structure near the top, or soft or clay soils all increase the load, and several jurisdictions require a design regardless of height in those conditions. Two shorter walls terraced up a slope can behave as one tall wall if they are close together, and are often treated that way.
Nothing on this page is a structural design. It counts blocks. Whether a wall of that height is safe where you are building, and what has to go behind and beneath it, is a question for your local building department or an engineer.
Practical tips
The base course decides everything. Getting it level, fully compacted and properly buried takes a disproportionate share of the time and is what the rest of the wall depends on. Every error in the first course is repeated in every course above it.
Backfill with gravel, not the soil you dug out. Clay holds water against the wall and swells when it freezes. A foot of clean gravel behind the blocks and a drain at the base is what keeps a wall standing.
Compact as you go, in layers. Backfilling the whole height at once and compacting from the top does not work. Build a course, backfill, compact, repeat.
Order caps separately. Caps are a different block, often a different pallet and sometimes a different lead time. They are also the ones most likely to arrive chipped, since they are handled most.
Check what is above before you dig. Fences, trees and services near the top of a wall all change what you are actually retaining.
With your numbers
- Blocks per course = 20 ft ÷ 16 in = 15
- Courses = 3 ft ÷ 6 in = 6
- Blocks = 15 × 6, plus 0% waste = 90
Quick reference
Five per cent is the masonry default. Retaining wall blocks are heavy and a proportion arrive chipped, and corners and curves need blocks cut. A straight run delivered on a good pallet may need none of it.
| Job | Blocks to buy |
|---|---|
| 20 ft long, 3 ft high | 111 |
| 30 ft long, 3 ft high | 170 |
| 20 ft long, 2 ft high | 79 |
| 40 ft long, 4 ft high | 284 |
| 10 ft raised bed, 18 in high | 53 |
| 6 m long, 0.9 m high | 111 |
| 10 m long, 1.2 m high | 237 |
Common questions
- How many blocks do I need for a 20 foot retaining wall?
- Ninety for a 3 ft high wall in 16 by 6 in blocks, which is 15 blocks in each of 6 courses. Burying the first course takes it to 105, and a 5 per cent waste allowance to 111.
- Should the first course be buried?
- Yes, in almost every case. Standard practice is to bury the bottom course, or about a tenth of the wall height, so the base cannot slide forward under the load behind it. It is the single most important course in the wall and the one people are tempted to skip.
- How high can I build without an engineer?
- Many jurisdictions set the threshold at about 4 ft, or 1.2 m, measured from the bottom of the base course, but the figure varies and some places are stricter. Above it you usually need a design, a permit and an inspection. Slopes above the wall, loads near it and poor drainage all lower the threshold.
- Do I need drainage behind the wall?
- Yes. Water building up behind a retaining wall is what pushes them over, and it exerts far more force than the soil does. A gravel backfill and a perforated drain at the base are normal on any wall worth building, and are not optional on a tall one.
- Why is the count the same whether the wall is straight or curved?
- It is not, quite. A curve consumes slightly more blocks on the outside radius and needs some cut on a tight one. The waste allowance covers a gentle curve. For a tight radius or several corners, add more.
- What about the base course and the gravel under it?
- Not counted here. A retaining wall sits on a compacted gravel base, typically 6 in or 150 mm deep and twice the block width. Use the gravel calculator for that, and do not build on soil.