Advanced options
A 60:40 topsoil to compost mix suits most vegetables; bulk bags (about 0.7 m3) beat 50L bags on price past half a cubic metre. Soil settles about 10-15% in the first season, so fill proud. Line timber beds' sides (not the base) and leave drainage open to the ground below.
What The Raised Bed Soil Calculator Works Out
- Soil volume: length x width x depth. A 2 m x 1 m bed filled 30 cm deep takes 0.6 m3, or 600 litres, weighing about 1.4 tonnes per m3. Every fill settles 10% to 15% in its first season, so fill proud and top up next spring.
- As bags: the volume in 50 litre garden-centre bags or 0.7 m3 bulk bags. The crossover is around half a cubic metre: beyond one small bed, a bulk bag delivered to the kerb is usually half the price per litre.
- Suggested mix: the standard veg mix of 60:40 screened topsoil to compost, blending structure and drainage with fertility. Pure compost slumps, dries oddly and overfeeds; pure topsoil starts hungry and compacts. 30 cm satisfies salads, roots and most vegetables; parsnips with ambitions want 40 cm plus.
- Number of beds (advanced): multiplies everything for a set of identical beds so one delivery covers the lot. Keep widths near 1.2 m so the middle reaches from either side without stepping on the soil.
- Woody base layer (advanced): the hugelkultur trick: logs, branches and rough prunings in the bottom of a deep bed, with the growing mix on top. It saves soil and feeds the bed as it rots, though it settles more, so expect to top up.
- Topsoil share (advanced): change the mix for the crop: 70:30 for roots and Mediterranean herbs that want drainage, 50:50 for hungry brassicas and squash, or 100% compost for a no-dig bed on open ground.
- Bagged versus bulk cost (advanced): whole bags at your bag size and price against bulk bags delivered. Typical UK prices are GBP 5 to GBP 8 per 50 litre bag of topsoil or peat-free compost and GBP 80 to GBP 120 per bulk bag, so a 0.6 m3 bed costs around GBP 70 in bags or one GBP 95 bulk bag with soil to spare.
- Barrow loads (advanced): the volume in 80 litre wheelbarrow loads, which is how a kerbside delivery turns into exercise. The depth table compares volumes and the cheapest route at 20 to 60 cm.
Further reading: RHS: raised beds.