How this calculator works
A sleeper retaining wall is built in courses (layers) of sleepers laid flat, held at each joint and each end by a post. The courses are the height of the wall divided by the height of a sleeper laid flat, rounded up. Each course takes the length of the wall divided by the sleeper length, rounded up, and there is a post at each of those joints and one more at the end.
Worked example: a wall 4.8 m long and 1 m high with 2.4 m sleepers 200 mm high has 5 courses of 2 sleepers, so 11 sleepers with 5% extra and 3 posts. Each post is the wall height plus 600 mm below ground, 1.60 m. A 7.2 m wall needs 16 sleepers and 4 posts. A wall 1.4 m high (7 courses) is above the height this calculator is meant for: ask an engineer.
Assumptions and sources
- Assumptions used: 5% waste added; sleepers laid flat in courses on the wide face; a post at every sleeper joint and at both ends; fixings at each sleeper end.
- This counts the timber for a low garden wall. A retaining wall holds back a lot of soil, and one over about 1 m high, one that holds a building, drive or slope, or one near a boundary needs advice from a structural engineer and may need approval under the Building Regulations. Fix a drainage layer behind the wall (gravel and a membrane) and a way for the water to leave, because water behind the wall is what makes walls fail. Use sleepers rated for ground contact. Posts need the right depth, and concrete is optional: follow your supplier's guidance.
The formula and a worked example are shown above, so you can check any result by hand. Read how we calculate and check our numbers. Results are estimates for planning: supplier products vary, so check the size printed on the label.
Sleepers for common walls
2.4 m sleepers laid flat (200 mm high), 5% extra, posts 600 mm below ground.
| Wall | Courses | Sleepers | Posts | Post length |
|---|---|---|---|---|
| 2.4 m long, 0.4 m high | 2 | 3 | 2 | 1.00 m |
| 4.8 m long, 0.6 m high | 3 | 7 | 3 | 1.20 m |
| 4.8 m long, 1 m high | 5 | 11 | 3 | 1.60 m |
| 7.2 m long, 1 m high | 5 | 16 | 4 | 1.60 m |
| 9.6 m long, 1 m high | 5 | 21 | 5 | 1.60 m |
Tips before you buy
- Keep the wall low. Above about 1 m a retaining wall carries large loads, and this is a job for an engineer.
- Line the back with a membrane and put gravel and a drain pipe behind the bottom course, so the water does not push on the wall.
- Stagger the joints between courses so no two joints sit above each other, and fix every sleeper to the posts.
- A shallow wall may be tied back to the bank with timber "deadmen". Ask the sleeper supplier for the detail.
- Wear gloves and eye protection when you cut: treated timber dust is not good for you.
Tools & materials for this project
Affiliate links As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Frequently asked questions
How many sleepers do I need for a retaining wall?
Divide the wall height by the sleeper height to get the courses, then multiply by the sleepers per course. A 4.8 m by 1 m wall with 2.4 m sleepers takes 11, including 5% extra.
How many posts does a sleeper retaining wall need?
One at each joint and one at each end, so 3 posts for a 4.8 m wall with 2.4 m sleepers.
How high can I build a sleeper retaining wall?
A low garden wall of up to about 1 m is a typical DIY job. Higher walls, and walls that support a building, drive or slope, need advice from a structural engineer.
How deep should the posts go?
Bury about a third to a half of the wall height as a rule of thumb, but this depends on the soil and the load. Ask your supplier or an engineer.
Do I need drainage behind a sleeper wall?
Yes. Put a membrane and gravel behind the wall and let the water drain away. Water pressure is a common cause of failure.
Add this calculator to your website
Running a blog, a contractor site or a class page? You can put this calculator on it for free. Copy the code below, and please leave the link underneath the calculator in place. The small script just resizes the frame to fit the results; if your site strips scripts, the calculator still works inside a scrolling frame.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.