How the retaining wall calculator works
A retaining wall holds back soil, so it needs more than an ordinary garden wall: a wider footing, heavier construction and drainage behind it. The calculator works out each part:
Typical footing sizes
| Retained height | Footing width | Footing depth |
|---|---|---|
| Up to 600mm | 450mm | 225mm |
| 600mm – 1m | 600mm | 300mm |
| Over 1m | Get a structural engineer's design | |
Building tips
- Dig the footing to firm ground below topsoil and keep the base level. Step the footing on slopes.
- For hollow blocks, set reinforcing bars in the footing every 450mm and fill the cores with concrete as you build.
- Leave weep holes (gaps or 50mm pipes) about every metre in the bottom course.
- Backfill with gravel in layers behind a membrane, never with clay soil straight against the wall.
- Lean sleeper walls slightly back into the slope, and use treated sleepers rated for ground contact.
Worked example: a 6m wall holding back 600mm of soil
In hollow blocks: the wall is 6m long and 0.75m high (0.6m retained plus 0.15m below ground), so 4.5m² of wall. That's 45 blocks laid flat, plus 5% = 48 blocks. The 450 × 225mm footing and the filled block cores take about 1.1m³ of concrete. With bars every 450mm you need 15 lengths of 12mm rebar, about 1.1m each. Drainage behind the wall is about 1.8 tonnes of 20mm clean stone (3 bulk bags), 7.2m² of membrane and 7m of perforated pipe.
In sleepers: 600mm is three courses of 200mm sleepers, each course three sleepers long, so 9 sleepers. Add six posts of 1.2m (cut from three more sleepers), for 12 sleepers in all, plus 12 bags of Postcrete and 36 long timber screws. Switch the wall type above to compare the costs.
Which type of wall?
- Sleepers are quick and need no mortar, but timber in soil eventually rots. Expect 10–20 years from good-quality treated softwood or hardwood. They're best for low terraces and raised beds up to about 600mm.
- Reinforced hollow blocks are the strongest DIY option and can be rendered or faced. With rebar and filled cores they behave like a solid concrete wall.
- Brick looks best next to a brick house, but must be at least one brick (215mm) thick and use frost-resistant bricks and a strong mortar below ground.
When to get help
Retaining walls fail when they're too thin, poorly drained or holding back more than people think. Wet soil weighs around two tonnes per cubic metre. Get a structural engineer for anything over about 1m, any wall supporting a driveway, building, path or a neighbour's garden, and any wall on clay or a steep slope. Work near a boundary or a neighbour's building may also come under the Party Wall Act, so check before digging.
Common mistakes
- Backfilling with clay. Clay holds water and swells. Use free-draining gravel behind a membrane.
- No weep holes. Water with nowhere to go pushes the wall over in the first wet winter.
- Footing on topsoil. Dig down to firm, undisturbed ground. The foundation calculator covers the dig and concrete in more detail.
For a free-standing wall that isn't holding back soil, the garden wall guide explains footings and bricks. For planters, use the sleeper raised bed calculator.