Retaining Wall Calculator (Metric)
Enter your wall length and height and your block face size to get the number of retaining-wall blocks you need.
What your result means
Total blocks is your dry-stack count with 5% waste, based on the block face area, since these stack without mortar. Capstones for the top course are sold separately, so add those. And the big one: a wall over 1.2 m usually needs an engineer and a permit, plus drainage rock and pipe behind it.
How to use this calculator
- Measure wall length along the slope face.
- Measure or plan height (consult code - walls over 1.2 m typically need engineering).
- Enter block face dimensions from your supplier.
- Read total blocks needed.
- Order capstones separately for the top row.
The formula
Wall face area in square inches is length times height times 144. Divide by one block's face area (no mortar joint, since these dry-stack), add your waste, and round up.
Worked example
Say you've got a 6.1 m long, 0.9 m tall wall in standard 12 by 10.2 cm blocks with 5% waste. The face is 5.6 m², or 8,640 sq inches, and each block covers 48. That's 180 base blocks and 189 with waste. Order capstones on top of that.
Retaining wall quick reference
| Item | Value |
|---|---|
| Common block face | 12 x 10.2 cm |
| Blocks per sq ft | ~3 |
| Setback per row | ~2.5 cm |
| Permit threshold | usually over 1.2 m tall |
Tips & gotchas
- A wall over 1.2 m usually needs a structural engineer and a permit.
- Build with about a 2.5 cm setback per row so the wall leans into the slope.
- Bury the first row about half its height below grade.
- Put drain rock and a perforated pipe behind the wall, or water pressure will push it over.
- Capstones cost more than regular blocks, so figure those separately.
Frequently asked questions
How tall can I build a retaining wall without a permit?
Usually 1.2 m is the limit before you need engineering. Check your local code.
Do I need drainage behind a retaining wall?
Yes. Drain rock plus a perforated pipe behind the wall is essential.
What is setback on a retaining wall?
Each row sits about an inch behind the one below, which leans the wall into the hill and adds stability.
How many blocks per square foot?
About 3 standard 12 by 10.2 cm blocks per square foot of wall face.
How long do retaining walls last?
50-plus years built right. Bad drainage cuts that down fast.
Related calculators
Estimates only — see our full disclaimer.