Retaining Wall Calculator
Calculate blocks, concrete footing and backfill for a retaining wall.
Please note: Material estimates for planning only. Always confirm quantities against your drawings, local building codes and supplier specifications before ordering. Structural work requires a qualified engineer.
What the Retaining Wall Calculator does
A retaining wall needs three things calculated: the blocks for the face, a concrete footing wide enough to spread the load, and free-draining backfill behind it so water pressure cannot build up.
Formula
Courses = ⌈Wall height ÷ Block height⌉Blocks per course = ⌈Wall length ÷ Block length⌉Footing volume = Length × Footing width × Footing depthDrainage volume = Length × Backfill width × Height
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Wall length | number | Yes | Accepts more than 0. |
| Wall length unit | one of 6 options | Yes | — |
| Wall height | number | Yes | Accepts more than 0. |
| Wall height unit | one of 6 options | Yes | — |
| Block length | mm | Yes | Accepts 10 or more. |
| Block height | mm | Yes | Accepts 10 or more. |
| Footing width | number | Yes | Accepts more than 0. |
| Footing width unit | one of 5 options | Yes | — |
| Footing depth | number | Yes | Accepts more than 0. |
| Footing depth unit | one of 5 options | Yes | — |
| Drainage backfill width | number | Yes | Accepts more than 0. |
| Drainage backfill width unit | one of 5 options | Yes | — |
| Waste allowance | % | Yes | Extra material for cuts, breakage and offcuts. 5–10% is typical, more for complex layouts. |
How to use it
- Choose Wall length unit and Wall height unit.
- Enter Wall length, Wall height, Block length and Block height and 4 more.
- Select Calculate.
Worked example
A 10 m × 1 m wall with 440 × 215 mm blocks, 450 × 200 mm footing, 300 mm drainage, 10% waste.
- Length
- 10 m
- Height
- 1 m
- Block
- 440 × 215 mm
- Footing
- 450 × 200 mm
5 courses × 23 blocks = 115 blocks, +10% = 127. Footing 0.9 m³, drainage 3 m³ = 5.04 tonnes.
Frequently asked questions
Why does a retaining wall need drainage?
Saturated soil exerts far more pressure than dry soil. A drainage layer and pipe relieve that hydrostatic pressure, which is what usually topples undrained walls.