Sleeper Raised Bed Calculator

How Many Sleepers Do I Need for a Raised Bed?

The quick answer is the outside perimeter times the number of courses, divided by the sleeper length. It goes wrong in three places. The corners overlap, so the runs are not the same length as the sides. A run that is not a whole number of sleepers has to be cut. And a cut piece is only worth keeping if it is long enough for another job.

So this calculator lays out each course the way you would build it. It works out the four runs, splits any run longer than a sleeper into pieces with the joints staggered against the course below, and then packs every piece from every course into sleepers, longest first, so short pieces come out of offcuts. A 2.4m × 1.2m bed two courses high on 2.4m sleepers needs 6 in the cutting plan, which is 7 once 5% spare is added.

What This Calculator Is For, and What It Is Not

It is for free-standing rectangular raised beds and straight runs of edging. It is not for retaining walls. Anything retaining soil over about 600mm needs proper design, because the soil pushes outwards and a few screws through sleepers will not hold it. If you go over 600mm the calculator shows a warning, and you should get advice before you build.

Most garden raised beds are 300 to 600mm high, and at least 150mm is enough for most planting, according to Howarth Timber.

Overlapping or Butted Corners?

Overlapping (brick bond): one course has the top and bottom runs going the full outside length over the corners, and the next has the left and right runs going the full outside width over them. No corner joint sits on the one below, which is what makes it strong. It needs the side runs in alternate courses cut shorter by two sleeper thicknesses.

Butted: every course is laid the same way. It is simpler to cut and to think about, but each corner is a straight stack of joints. The calculator shows both so you can compare the sleepers needed. For a 3.6m × 1.8m bed three courses high on 2.4m sleepers, both need 16, so the choice is about strength and looks rather than cost.

The inside size is the outside size minus two sleeper thicknesses, in both styles. Laid on edge a 100 × 200mm sleeper is 100mm thick and 200mm high. Laid flat it is 200mm thick and 100mm high, which takes more room from the inside and more sleepers for the same height.

The Rules the Calculator Uses for Cuts and Joints

• Shortest piece: 300mm, when a run is split. If the last piece of a split run would be shorter than this, the piece before it is shortened. A run that is short on its own is left as it is.
• Staggered joints: at least 300mm. On runs longer than one sleeper, a joint in one course is kept at least 300mm away along the wall from any joint in the course below. The usual way is to start alternate courses with a part-length piece, often a half.
• Pieces can come out up to one saw cut (3mm) short. That lets a 2.4m sleeper make two halves, each about 1198mm, instead of one half and a short piece. It is well within normal sleeper tolerance. Beyond that first cut, each extra cut from the same sleeper costs 3mm.
• Offcuts are reused across courses. All the pieces are packed into sleepers together, not course by course.

What it does not do: it does not track which end of an offcut is square, and it does not allow for splits or bowing. The waste allowance is there to cover both. See how we calculate for more on where our calculators simplify.

Fixings: What to Use and How Many

Use timber or sleeper screws, not nails. Merchant guides agree on three things, and the calculator uses them:

• Get at least 50mm of screw into the adjoining sleeper. So the screw length is the thickness it goes through plus 50mm, rounded up. For 100mm-thick sleepers laid on edge that is 150mm at the corners. Suregreen uses 150mm screws there, and Howarth Timber uses 200mm.
• 2 screws at each corner of each course. Both of those guides say two per corner, and the calculator counts them that way. AVS Fencing suggests three at a lapped corner, so buy a few spare.
• Fix each course down into the one below with longer screws driven down through the top sleeper. On 200mm-high courses that is at least 250mm. Suregreen uses 250mm screws for this.

We do not give a count for the screws that fix courses together, because guides disagree: AVS Fencing says two per sleeper, about 150mm from each end, and another merchant says two at each end. Use two or more per sleeper piece, and more on long pieces. Pre-drill, and seal every cut end with preservative. Beds over about half a metre high are often built with M12 coach bolts instead (AVS Fencing), which is one more reason to get advice above 600mm.

Liner, Drainage and Soil

A breathable liner up the inside faces keeps soil in while letting water and insects through. The calculator gives the side area from the inside perimeter and finished height, and the base area separately if you want weed control underneath. Add a little extra for overlaps and stapling.

For drainage, some people lay a layer of gravel on the base of a deep bed. Work it out with our gravel calculator using the inside length and width. For the soil, the results link opens our topsoil calculator with the inside size and fill depth already entered.

Sleeper Sizes and Lengths

Every size and length in the dropdowns has a product page at Wickes, all pressure treated to Use Class 4 for ground contact. Not every size comes in every length, and stock varies by merchant, so check before ordering. Jewson and Travis Perkins also list 100 × 200 × 2400mm sleepers. Use Custom for anything else.

Common Bed Sizes: Sleepers Needed

Generated from the calculator above. Each bed uses 100 × 200mm sleepers on edge, 2.4m long, overlapping corners, 5% waste, and soil 25mm below the top.

Use the calculator for your own bed. A different sleeper length, laying the sleepers flat, or an extra course changes the answer more than you would expect.

Frequently Asked Questions

How many sleepers do I need for a raised bed?

It depends on how the corners are laid, not just the perimeter. Each course is four runs of sleeper, and a run that does not match a sleeper length has to be cut. This calculator lays out every course, cuts the pieces from your sleeper length and reuses offcuts across courses. A 2.4m × 1.2m bed two courses high on 2.4m sleepers needs 6 in the cutting plan, which is 7 with 5% spare.

Should I overlap the corners or butt them?

Overlapping is the stronger way, like brickwork: one course has the long sides running over the corners, and the next has the short sides running over them, so no corner joint lines up with the one below. Butted corners are laid the same way every course, which is simpler but leaves a straight vertical joint at each corner. For a 3.6m × 1.8m bed three courses high on 2.4m sleepers, both need 16 sleepers, so the choice is about strength and looks rather than cost. Overlapping needs a few part-length cuts to make the courses alternate.

What screws do I use to join sleepers?

Timber or sleeper screws, driven far enough into the next sleeper to bite. Merchant guides say to get at least 50mm of screw into the adjoining sleeper, and they use 2 screws at each corner of each course. On 100mm-thick sleepers laid on edge that means a screw of at least 150mm at the corners, and at least 250mm to fix a 200mm-high course down into the one below. Some guides use three screws at a lapped corner, so buy a few spare.

How high can I build a sleeper raised bed?

Most garden raised beds are 300 to 600mm high, and a height of at least 150mm is enough for most planting. This calculator is for free-standing raised beds and straight edging, not retaining walls. Anything retaining soil over about 600mm needs proper design, so get advice before building. Four courses of 200mm sleepers on edge is 800mm, and the calculator warns you when you go past 600mm.

Do I need a liner or weed membrane inside the bed?

A breathable liner up the inside faces keeps soil in while letting water through. The calculator gives the area for the sides, worked out from the inside perimeter and the finished height, and separately the area for the base if you want weed control underneath. Add a little extra for overlaps and stapling.

How much soil do I need to fill a sleeper bed?

Inside length times inside width times fill depth, where the inside is the outside size minus two sleeper thicknesses. For the 2.4m × 1.2m bed above that is 2.2m × 1m inside, and 0.83 m³ when filled to 25mm below the top. The results include a link that opens our topsoil calculator with those sizes filled in.

What sizes do garden sleepers come in?

At Wickes, garden sleepers come in 100 x 150mm, 100 x 200mm and 125 x 250mm, in lengths from 1.2m to 2.4m. Not every size comes in every length, so the length list follows the size you pick. They are pressure treated to Use Class 4 for ground contact. Stock varies by merchant, so check before ordering.