What this calculator works out
Railway sleepers are sold by the piece, not the metre, so ordering the right number for a raised bed or a border comes down to some straightforward but easy-to-get-wrong arithmetic: how many sleepers fit along each side, how many courses stacked on top of each other reach the height you want, and how many screws hold the corners together. For a bed, there is a third question too: once the walls are up, how much topsoil actually fills the space inside them. This tool takes a bed's outer length and width, or an edging run's total length, a sleeper size, and a course count, and turns all of that into a sleeper count, a screw count, and, for a bed, a topsoil order in cubic metres and UK bulk bags.
It sits alongside the topsoil calculator as the natural next step once the walls are built, and shares that page's fill-volume logic exactly, including its 0.75 cubic metre bulk bag convention, so the two figures line up rather than using two different assumptions about bag size.
How the maths works
The obvious shortcut, dividing a bed's total perimeter by one sleeper's length and rounding up once, does not actually describe how a sleeper bed gets built: it can land a corner mid-sleeper, which is not how a square-cornered bed goes together in practice. This calculator instead works out each side on its own. A bed's length and width sides are each divided by the chosen sleeper's length and rounded up separately, since a side longer than one sleeper needs more than one laid end to end, and a side shorter than one sleeper still needs a full one, cut down to fit.
A raised bed built this way is usually laid log-cabin style: one pair of sides sits on top of the other pair at the corners, and the next course up swaps which pair sits on top, so the joints interlock and the whole bed locks square as it's built. That alternation changes how the corners are built, not how many sleepers each course needs, since every course still has the same four sides at the same lengths; it's mentioned in the results as a build note rather than folded into the sleeper count.
Fixing screws follow the same per-course logic: each course has four corners where two sleepers meet, and each corner joint is usually pulled together with a small number of long screws driven through the top piece and into the one underneath. This calculator defaults to two screws per joint, an editable field, so the total is four corners times the number of courses times the screws per joint.
Fill volume, for a bed, comes from the space left once the walls themselves are accounted for. The sleeper's narrower face becomes the wall's thickness once it's stood on edge, so that thickness is subtracted from both the length and the width, twice each, to get the inner floor size; the sleeper's wider face becomes the height added by each course, so the inner height is that figure multiplied by the number of courses.
Choosing a sleeper size
Two dimensions turn up again and again across UK timber merchants for new, pressure-treated softwood sleepers: 2.4m × 200mm × 100mm is the standard size stocked by suppliers including AVS Fencing and Suregreen, and a heavier 2.4m × 250mm × 125mm size is stocked by merchants including Chelford Farm Supplies and MKM for jobs wanting a chunkier finish or fewer courses for the same height. A 1.2m half sleeper, sharing the same 200 × 100mm cross-section as the standard size, is a common stock item too, useful for a short side or a small bed without cutting a full-length sleeper in half yourself; reclamation yards including Gardiner's Reclaims list it as a standard size in its own right.
Reclaimed oak sleepers are a different case: being genuinely ex-service timber rather than a manufactured product, no two are quite identical. Suppliers typically quote an approximate size around 2.4m to 2.6m long by roughly 250mm × 130 to 150mm in section, and say plainly that real pieces vary; this calculator uses 2.4m × 250mm × 130mm as a working figure, but measure what actually arrives before finalising a bed size that depends on an exact fit.
Fixing the courses and lining the bed
Railwaysleepers.com's own guidance on joining sleepers together recommends long "Timberlok"-style screws sized by a simple rule: about 50mm of the screw needs to bite into the second sleeper, so the total screw length is the first sleeper's own thickness plus 50mm. For the standard 100mm-thick softwood sleeper that comes to 150mm, which is exactly why 150mm sleeper screws are the size sold everywhere as the default; a 125mm-thick sleeper wants roughly 175mm, and reclaimed oak's thicker, harder section wants closer to 200mm and is worth pre-drilling a pilot hole for so the screw does not snap or bind partway in. This calculator works out a recommended length from whichever sleeper is selected and shows it next to the screw-count field.
A membrane or heavy-duty landscape fabric lining the inside faces of a sleeper bed is common good practice, keeping the timber's inner face off permanently wet soil, one of the more common causes of early rot at the join between courses. It should stay breathable rather than fully waterproof, since a sealed liner can trap water against the wood; a straightforward woven weed-control fabric does the job without sealing the bed shut. The same principle applies underground: wrapping any buried section, such as posts or a wall's lower edge, in a damp-proof membrane before backfilling protects the part that never dries out.
Worked examples
A compact vegetable bed, 2.4m by 1.2m outside, built two courses high from the standard 2.4m × 200 × 100mm sleeper: the long sides each take one sleeper (2.4 ÷ 2.4 = 1, no offcut), the short sides each take one sleeper too, cut down from 2.4m to 1.2m with a 1.2m offcut apiece. That's 2 + 2 = 4 sleepers per course, 8 sleepers over two courses. Four corners times two courses is 8 joints; at two screws each, that's 16 screws, each about 150mm long. For fill, the 100mm sleeper thickness takes 200mm off both the length and the width, leaving a 2.2m by 1.0m inner floor; two courses of the 200mm-tall sleeper give a 0.4m inner height, so 2.2 × 1.0 × 0.4 = 0.88 cubic metres of topsoil, which rounds up to 2 bulk bags.
A straight 10m garden edge, one course of the same sleeper: 10 ÷ 2.4 = 4.17, rounded up to 5 sleepers, with a 2m offcut spread across the run (commonly taken off the last piece rather than split evenly). A single course of edging is usually held with timber stakes driven in behind it rather than corner screws, since there's no corner to bolt together; going to two courses for a taller border adds a second row of 5 sleepers pinned down into the first, which is where the screw count in edging mode comes from.
Questions people ask
Are treated sleepers safe to use near vegetables?
The older arsenic and chromium (CCA) preservatives were banned from sale in the UK from September 2006. New treated softwood sold today uses modern preservatives, commonly Tanalith E, and the general position among growers and suppliers is that it is fine for raised vegetable beds. Lining the inside faces with membrane, as this calculator's bed mode assumes optional, gives extra peace of mind by keeping soil off the treated timber directly.
Should I use oak or treated softwood sleepers?
They trade cost against lifespan. Suregreen states its treated softwood sleepers have an expected lifespan of around 8 to 10 years in typical use. Reclaimed oak costs more and its sizes vary piece to piece, but suppliers such as Timberstore commonly quote 25 years or more in the ground, since oak is naturally far more durable than softwood even before any treatment.
How do I fix sleeper courses together?
Long structural screws, often sold as Timberlok or Timberfix screws, driven through the top sleeper into the one underneath at each corner joint, are the standard method. Railwaysleepers.com's own sizing rule is the first sleeper's thickness plus 50mm of bite into the next, which comes to 150mm for a standard 100mm-thick sleeper; pre-drill a pilot hole in oak or other hardwood so the screw does not snap.
Do I need a base under a sleeper raised bed?
A firm, level base, commonly a few centimetres of compacted hardcore or gravel under the first course, stops the bed settling unevenly and keeps the bottom sleeper's underside off standing water. It is not strictly required for a low, one or two course bed sitting straight on reasonably level soil, but it's cheap insurance against a bed that leans over time.
Do I need a membrane or liner?
It's optional but worth doing. A breathable membrane or landscape fabric against the inside faces reduces how much time the timber spends in direct contact with wet soil, which slows the rot that typically starts at the join between courses first. Avoid a fully sealed, waterproof liner, since trapped water against the wood is worse than none at all.
How many sleepers do I need for a raised bed?
Work out how many sleepers fit along each of the four sides on its own, rounding up any side that isn't an exact multiple of the sleeper's length, add the four sides together for one course, then multiply by how many courses tall the bed will be. A 2.4m by 1.2m bed, two courses high, in standard 2.4m sleepers, comes to 8 sleepers in total.
This calculator uses sleeper sizes checked against published UK timber and reclamation merchants including AVS Fencing, Suregreen, Chelford Farm Supplies, MKM and Gardiner's Reclaims, the Timberlok fixing rule and lifespan figures published by railwaysleepers.com, Suregreen and Timberstore Ltd, and the 0.75 cubic metre UK bulk bag convention shared with this site's topsoil calculator. Reclaimed sleeper sizes vary piece to piece; measure what you actually receive before finalising a bed that depends on an exact fit, and treat the fixing and lifespan figures as sound planning estimates rather than a fitting specification.