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About this tool
Work out soil, compost, cocopeat and perlite volume to fill raised garden beds, with settling allowance and block counts.
This calculator turns raised bed dimensions into the litres of soil, compost, cocopeat and perlite you actually have to buy. It works from length × width × fill depth, then applies the three corrections that catch people out: headspace left at the rim, any coarse bottom filler such as logs and dry leaves, and the 10-20% a fresh mix loses as it settles in its first season. Quantities come out in litres, cubic feet and kilograms, with cocopeat converted into 5 kg compressed blocks, since one block expands to roughly 75 litres.
Open Raised Bed Soil Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Volume converted into the compressed blocks shops actually sell, so you do not under-buy by a factor of ten.
A bed filled exactly to the calculated volume sinks by a hand's depth within weeks; the allowance prevents that.
Suppliers quote soil and compost by weight, so the kilogram figure is what you order against.
About 865 litres — roughly 30 cubic feet — for a bed one foot high, once you leave two inches of headspace and add 15% for settling. Without the settling allowance the bare volume is about 752 litres, and a bed filled to that will visibly sink in the first month.
A common general-purpose ratio by volume is 40% garden soil, 30% compost, 20% cocopeat and 10% perlite: the soil gives body and minerals, the compost feeds, the coir holds moisture and the perlite keeps it from compacting. The square-foot gardening alternative uses no soil at all — a third each of compost, coir and vermiculite — which stays lighter but needs feeding sooner.
A 5 kg compressed block expands to roughly 75 litres of loose coir once fully soaked, and a 650 g brick makes about 8-9 litres. Soak it in a large tub with warm water for an hour and break it up by hand; buying by weight rather than expanded volume is the most common ordering mistake.
Yes, in a bed deeper than about 45 cm. Logs, prunings, dry leaves and cardboard in the lower third break down slowly, hold water and cut the volume of bought mix — the hügelkultur idea. Keep at least 30 cm of proper mix on top, because most vegetable roots live in that layer, and expect the bed to sink noticeably as the filler decomposes.
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