How the calculator works it out
A sleeper bed is a stack of rectangles. On every course the two long sides run the full outside length, and the two short ends sit between them. The calculator lists every piece for every course. Sides longer than one sleeper are split into lengths. It then packs the pieces into the fewest whole sleepers, longest first, allowing 3 mm for each saw cut.
per course: 2 × L + 2 × ( W − 2 × thickness )
soil (L) = ( L − 2t ) × ( W − 2t ) × height (m) × 1,000
The packing matters more than you might expect. A 1.3 m wide bed with 50 mm sleepers needs 1.2 m ends, so each 2.4 m sleeper gives two ends and a course costs three sleepers. At 1.5 m wide the ends are 1.4 m, only one fits in a 2.4 m sleeper, and a course costs four sleepers with a metre of offcut each time. If you are flexible on size, try a few widths and watch the sleeper count.
Worked example: 2.4 × 1.2 m, two sleepers high
- Height: 2 × 200 mm = 400 mm – the depth most vegetables want. See how deep should a raised bed be?
- Long sides: 2 per course × 2 courses = 4 pieces at 2.4 m, which are 4 sleepers with no cut.
- Short ends: 1.2 m − 2 × 50 mm = 1.1 m. 4 pieces fit two to a sleeper, so 2 sleepers, each leaving a 0.19 m offcut.
- Total: 6 sleepers. Posts: 4 corners plus 1 in the middle of each long side, 6 in all, cut to 400 mm.
- Soil: inside 2.3 × 1.1 × 0.4 m = 1.01 m³, which is about 34 × 30 L bags. At that volume a bulk delivery is usually cheaper, as the bagged vs bulk guide explains.
Which sleepers? Treated pine, hardwood or cypress
About CCA. "H4" is the hazard class for timber in contact with the ground under the Australian treatment standard, AS 1604.1. Since 2006, CCA (copper chrome arsenate) may no longer be used to treat timber for garden furniture, picnic tables, exterior seating, children's play equipment, domestic decking or handrails. The reason is frequent skin contact. Landscaping sleepers are not on that list, so CCA pine is legal for a garden bed. For a bed you will grow food in, an arsenic-free treatment, untreated hardwood or cypress, or a plastic liner on the inside walls is the cautious choice.
Posts, joins and screws
- Corner posts go inside each corner: 75 × 75 mm or 100 × 100 mm treated pine or hardwood. Fix every sleeper to its post with two screws.
- Middle posts. Once a bed is two or more sleepers high, the soil pushes the long sides outwards. The calculator adds a post every 1.8 m or so along any side longer than that. It is cheap insurance against a bowed bed.
- Joins. If a side is longer than one sleeper, stagger the joins between courses like brickwork and put a post behind every join.
- Screws. Use galvanised or stainless batten screws long enough to go through the sleeper and at least 50 mm into the post: about 100 mm for 50 mm sleepers and 125–150 mm for 75 mm sleepers.
- On soil or on concrete. On soil you can run the posts 200–300 mm into the ground for extra stiffness. On concrete or pavers, cut them to the bed height and leave a gap at the bottom so water can drain away.
Lining and filling the bed
Lining the inside walls with builder's plastic slows rot and keeps soil off treated timber. Staple it along the inside, a few centimetres below the top edge. Line the sides only, never the base, or the bed becomes a bathtub. On open ground, lay plain cardboard on the base to smother grass, and fork the soil underneath if it is compacted.
The calculator's soil figure is the whole inside volume. You rarely need to fill it all with premium mix. The guide to filling a raised bed cheaply shows how to use cheaper material underneath. The raised bed soil calculator prices each layer and compares bags with a bulk delivery.
Sleeper bed questions
How many sleepers do I need for a 2.4 × 1.2 m raised bed?
Three per course with 2.4 m sleepers: two full-length sides, plus one sleeper cut into two 1.1 m ends if the sleepers are 50 mm thick. That makes 3 for one high (200 mm), 6 for two high (400 mm) and 9 for three high (600 mm).
How much soil does a sleeper garden bed take?
Use the inside dimensions: outside length and width minus two sleeper thicknesses, times the height. A 2.4 × 1.2 m bed two sleepers high holds about 1.01 m³ (1,010 L). One high holds half that, and three high one and a half times as much.
Are treated pine sleepers safe for a vegetable garden?
Arsenic-free H4 pine (ACQ or copper azole) is widely used for food gardens. CCA-treated pine is legal for garden beds, and the 2006 restrictions cover furniture, decking, handrails and play equipment rather than landscaping. If you use CCA pine, many growers line the inside walls with plastic as a precaution.
How high should a sleeper bed be?
Two sleepers (400 mm) grows almost every vegetable on open ground. Three sleepers (600 mm) suits beds on concrete and gardeners who want less bending. One sleeper (200 mm) is enough for herbs and salad greens if the soil underneath is loosened.
Sources and method
- Australian Pesticides and Veterinary Medicines Authority, Arsenic timber treatments review – restrictions on CCA-treated timber from March 2006. apvma.gov.au; see also Queensland Government, Restrictions on CCA-treated timber products.
- Standards Australia, AS 1604.1 Preservative-treated sawn and round timber – hazard classes H1 to H6; H4 covers outdoor in-ground use.
- Sleeper sizes and lengths from Australian landscape-supply and hardware listings (200 × 50 and 200 × 75 mm treated pine; 1.8, 2.4 and 3.0 m lengths).
- Cut list: first-fit-decreasing packing with a 3 mm saw kerf; soil volume is geometry only.
Written by DueRiva · Last reviewed 23 September 2026 · Report an error