The plan
Three courses of 100 × 200 mm sleepers laid flat gives a frame 300 mm high, which is the depth the RHS rates as deep enough for dwarf bulbs, salad leaves and strawberries. The same guidance says most plants, especially fruit bushes, shrubs and small trees, want 45 cm or more — so a deeper bed is one more course, not a different job.
The growing mix is the part a calculator can size. Loose soil is full of air and slumps by roughly a tenth once it is watered, which is why the topsoil calculator adds its own settlement allowance on top of the depth you ask for. The RHS suggests filling a bed mainly with soil rather than compost, because compost breaks down and slumps within a few seasons — which is what the calculator’s default figure assumes.
Doing it, in order
- Set the bed out and check the width before you cut anything. The RHS rule of thumb is to keep it under 1.5 m so you can reach the middle without standing on the soil; this bed is 1.2 m.
- Clear the ground under the bed and level it. On soil, fork over the base; on a hard surface, make sure there is somewhere for water to go — the RHS calls that out before you build, not after.
- Lay the first course on a firm, level bed of sand or against a paved edge, checking the diagonals are equal so the frame goes down square.
- Fix each sleeper to the one below with a timber screw or a galvanised bracket at every joint, then stack the second and third courses the same way.
- Line the inside if you want the extra precaution — the RHS notes that new pressure-treated timber is fine for edible crops, and that lining is a choice rather than a requirement.
- Fill the mix in layers, firming each one and watering as you go, and leave the surface a little proud so it settles to the depth you wanted.
Don’t forget
- Reaching the middle A bed over 1.5 m wide is the one people end up standing on, which is what the RHS width figure is for. If you do go wider, keep a board to kneel on.
- Paths between beds The RHS figures are at least 450 mm for a wheelbarrow and 1 m if a wheelchair has to pass. Paths eat growing space, so they get decided first, not last.
- Watering, and a water butt A bed above ground dries out faster than soil at ground level — the RHS says so plainly, and suggests catching rainwater at the same time you build. Treat that as part of the job, not an optional extra.
- What to fill it with The RHS suggests filling mainly with soil, and gives a general mixture of topsoil, organic matter and sharp sand for most beds. The proportions are the supplier’s own, so there is no ratio on this page — and soil testing is the only way to know what your own ground needs.
- Drainage under the frame On a hard surface the bed cannot drain into the ground, so water needs a way out. That is a judgement on your own ground, not a number this page can give you.
Material take-off for this plan
Worked out by the site’s own Topsoil for this plan’s dimensions, not typed in by hand.
The working
| Working | Value |
|---|---|
| Area to fill | 2.9 m² 2.40 m × 1.20 m |
| Depth of topsoil | 300 mm The depth you want once it has settled, not the depth you tip in |
| Volume before settlement | 0.86 m³ 2.9 m² × 300 mm |
| Settlement allowance | 10% +0.09 m³ to allow for slumping |
| Order | 0.95 m³ ≈ 950 litres at 1.5 t/m³ |
| Bulk bags (850 kg) | 2 bags Rounded up to whole bags |
| 25 kg bags | 58 bags Rounded up. Only sensible for small beds |
What to order
| Material | Quantity | Notes |
|---|---|---|
| Screened topsoil (loose, by weight) | 1.43 tonnes | Loose loads are cheapest per tonne but need a tip point a wheelbarrow can reach. |
| Bulk bags (850 kg) | 2 bags | Delivered on a pallet — you need a hard, flat surface and somewhere dry to keep them. |
| 25 kg bags | 58 bags | Roughly 1,426 kg in total. Fine for a couple of raised beds. |
| Soil conditioner or compost (optional) | 1 part to 3 parts soilestimate | For raised beds and borders, mixing in compost improves structure and drainage. |
Settlement is the thing that catches people out: loose topsoil is full of air, and once it is watered and walked on it drops by roughly a tenth. That is why the calculator adds a settlement allowance on top of the finished depth you asked for. Fill the bed a little proud and let the rain do the rest.
Timber figures — from a named rate card, not from an engine
No calculator on this site sizes a sleeper frame, so these figures are not engine output. They are arithmetic on the product dimensions the merchant publishes, and the source is named underneath.
| Item | Quantity | Where the figure comes from |
|---|---|---|
| Treated softwood sleepers, 2,400 × 200 × 100 mm | 9 with butt joints, 12 with lapped corners | Three courses around a 2.40 × 1.20 m frame, using the merchant’s 2,400 mm stock length. Butt joints: two full sleepers per course for the long sides, plus one cut into the two 1.20 m ends. Lapped corners: each course runs 8.0 m once four 200 mm laps are added, so four sleepers. |
| Sleeper price | about £22–£27 each | Listed prices at six UK timber merchants on 25 September 2026. It is a range, so treat it as a planning assumption and get your own quote — delivered price depends on quantity and distance. |
Source: Quay Timber — 2400 × 200 × 100 mm green softwood railway sleeper, pressure treated — Read 25 September 2026. The stock length and section the timber count is worked out from. The same size is listed by Daws Heath Timber and Nationwide Supplies, some as a finished 95 × 195 mm.
Source: RHS — How to make a raised bed — Read 25 September 2026. The 30 cm and 45 cm depths, the 1.5 m width, the 450 mm and 1 m path figures, the topsoil, organic matter and sharp sand mixture, and the drainage point.
Source: RHS — Growing veg in raised beds — Read 25 September 2026. Filling mainly with soil rather than compost, and beds drying out faster than soil at ground level.
Both timber counts are stated rather than one being chosen for you, because the corner detail is your decision: butt joints are cheaper, lapped corners are stronger. Where a source gives a range rather than a figure, the page says so and labels the number a planning assumption.
A raised bed, planned out FAQs
How much soil do I need to fill a 2.4 m by 1.2 m raised bed?
1.43 tonnes of growing mix at 300 mm deep, from the topsoil calculator for 2.88 m². That is about two 850 kg bulk bags, or 58 bags of 25 kg if you are carrying it through the house.
How deep should a raised bed be?
This plan builds 300 mm, because the RHS rates 30 cm as deep enough for dwarf bulbs, salad leaves and strawberries. The same guidance says most plants, especially fruit bushes, shrubs and small trees, need 45 cm or more — so add a course for those.
How many sleepers does a 2.4 m by 1.2 m raised bed need?
Nine sleepers of 2,400 × 200 × 100 mm for a three-course frame with butt joints: two full sleepers per course for the long sides, plus one cut into the two 1.2 m ends. Lapping the corners the way a log cabin does takes four per course, so twelve.
Does a raised bed need a liner?
Not for the frame to work. The RHS notes that new pressure-treated sleepers are fine for edible crops, and that lining the inside is an extra precaution rather than something the bed depends on.