Raised Bed Soil Calculator

In short A raised bed is a box, so the soil it needs is length times width times depth and nothing more — but the number that comes out surprises almost everybody, because volume grows in three dimensions and bagged compost is sold in two-cubic-foot lumps. A standard 4 by 8 foot bed filled 12 inches deep is 32 cubic feet. That is 1.185 cubic yards, 906 litres, and sixteen 2 cubic foot bags. Two beds is thirty-two bags, which is most of a car. This is the single most common reason a raised-bed project stalls halfway through a weekend. The conversions here are exact rather than approximate, and it is worth knowing why. A foot has been defined as exactly 0.3048 metres since 1959, so a cubic foot is exactly 28.316846592 litres and a cubic yard is exactly 27 cubic feet. Nothing on this page rounds until it prints. What is NOT exact is the mix. The recipes offered — a third each of compost, peat or coir and vermiculite; sixty-thirty-ten; a straight half and half — are conventions that different gardeners swear by for different reasons, and no one of them is a standard. They are shown as shares of the volume you already worked out, so you can price them against each other. Depth is the expensive dimension, and it is the one worth thinking about hardest. Doubling it doubles the bill. Most vegetables root in the top 25 to 30 centimetres, so a bed sitting on open soil rather than on a patio can have its bottom third filled with cheaper bulk topsoil, or with the branches and turf that the hugelkultur people use, and the growing mix only in the top. A bed on a hard surface cannot do that and needs the full depth. One last thing that is not a mistake: a compost-heavy fill settles noticeably over the first season. Filling to the rim in spring and topping up the following year is normal, and adding about a tenth extra at the start is the usual allowance.