Calculate mix before buying bags
Start with a measurement
Measure the usable inside width, length and fill depth in millimeters. Multiply them and divide by 1,000,000 to get liters. For a round pot, multiply π × radius² × fill depth, then divide by 1,000,000.
Worked example
A 4×8 ft bed filled 12 in deep holds 32 ft³, or 906.139 L. With 120 L already owned, the net is 786.139 L. At 40 L per bag, buy 20 bags (800 L), leaving about 13.861 L. Extra allowance is an editable choice.
Check the real conditions
Use a raised-bed growing mix for beds and a suitable potting mix for containers. Confirm useful container volume after leaving headroom; a labeled external size can differ. Do not convert kilograms to liters without known density.