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About this tool
Turn pot size into litres of soil, compost, cocopeat and perlite using proven mix ratios for pots, grow bags and troughs.
This calculator converts a container's real dimensions into litres of each potting mix ingredient, using the parts-by-volume ratios gardeners actually work in. Pot volume is computed properly: a tapered nursery pot is a truncated cone, so its volume follows V = (π h / 3)(R² + Rr + r²) with the radius taken at the mix line rather than the rim — using rim diameter alone overstates a 12-inch pot by close to a third. Each ingredient then comes out in litres, in scoops of your chosen size, and in the units shops sell it by, including 5 kg cocopeat blocks.
Open Potting Mix Ratio 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.
Tapered pots, grow bags and rectangular troughs each use their own volume formula instead of a rough guess.
Recipes stay in the two-parts-to-one form gardeners use, and are converted to litres and scoops for you.
Soil and compost in kilograms, perlite in litres, cocopeat in compressed blocks — matching how each is actually sold.
About 13 litres for a typical tapered 12-inch pot roughly 28 cm deep, once you leave 3 cm of headspace below the rim. A straight-sided 12-inch grow bag of the same depth takes closer to 17 litres, because none of the volume is lost to the taper.
For general container plants, two parts garden soil to one part compost, one part cocopeat and half a part perlite works well: the soil gives body, compost feeds, coir holds moisture and perlite keeps the mix from compacting. Seedlings want no soil at all — three parts coir, one compost, one perlite — and succulents want two parts coarse sand with only half a part compost.
It is the most common reason potted plants fail. Garden soil compacts into a dense block in a container, holds water at the base and starves roots of air, and it can carry soil-borne fungi. Keep it to about half the mix at most, and always add something coarse such as perlite or sand for drainage.
About 3 litres for a 13-litre pot on a mix where coir is one part in four and a half. One 5 kg compressed block expands to roughly 75 litres, so a single block will fill the coir share of more than twenty such pots — buy blocks, not loose coir, and soak before measuring.
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