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How the calculator works
- Round pots: it works out the volume from the top diameter and the height, then takes off 15 percent because most pots are narrower at the bottom.
- Boxes and troughs: length times width times height, measured inside.
- Headspace: it leaves 1 inch empty at the top, so water does not run over the rim.
- Settling: it adds 10 percent. Fresh potting mix sinks after the first few waterings and needs topping up.
For pots of different sizes, run the calculator once for each size and add the results.
Bag sizes
Potting mix in the US is sold by the dry quart or the cubic foot:
- 8 quarts: one or two small pots
- 16 quarts: one large pot or three small ones
- 1 cubic foot, about 25 quarts: a railing box, or two or three medium pots
- 1.5 and 2 cubic feet: a row of boxes or one very large planter
One cubic foot is about 28 liters.
Quick guide by pot size
For pots about as tall as they are wide:
- 6-inch pot: about 2 quarts
- 8-inch pot: about 5 quarts
- 10-inch pot: about 10 quarts
- 12-inch pot: about 17 quarts
- 24 x 8 x 8 inch box: about 22 quarts
Use potting mix, not garden soil
Garden soil packs down in a container, drains badly and is heavy, which matters on a balcony. Potting mix stays loose and weighs far less.
Do not fill the bottom of the pot with gravel. It does not help drainage. A drainage hole and a saucer do.
What to grow in those pots, and how to keep it alive, is in the balcony herb garden guide for beginners.
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