How it works
soil per pot = π × (diameter ÷ 2)² × depth
The calculator treats a round pot as a cylinder: it halves the diameter to get the radius, squares it, multiplies by π and by the fill depth, and multiplies the result by the number of pots. Everything is computed in metric internally and shown back in US dry quarts — the unit bagged potting mix is sold in — where one cubic foot equals about 25.7 dry quarts (28.3169 litres divided by 1.101 litres per quart). Because pots taper inward toward the base, a true cone-sided pot holds a little less than the cylinder figure, so treat the answer as a small over-estimate that conveniently allows for settling. The bag count is the total volume divided by your chosen bag size, always rounded up to whole bags.
Sources
- University of Maryland Extension — Growing Media for Containers Containers should be filled with a well-drained soilless potting mix (not garden soil or topsoil); the amount needed is the volume of the container.
- Cylinder volume & quart conversion (exact by definition) Volume of a cylinder = π r² h. 1 cubic foot = 28.316846592 L = 25.71 US dry quarts (1 dry quart = 1.101220942715 L).
FAQ
How much potting soil does a 12-inch pot need?
A round 12-inch pot filled about 10 inches deep holds roughly 17 dry quarts, near 0.65 cubic foot, of mix. Pots that taper sharply toward the base hold a little less. Enter your pot’s diameter and fill depth above for a figure matched to your container.
How many quarts of potting mix are in a bag or a cubic foot?
Bagged potting mix is sold in dry quarts, and there are about 25.7 dry quarts in a cubic foot. So a 1-cubic-foot bag is roughly 25 quarts, a common 16-quart bag is about 0.6 cubic foot, and a 50-quart bag is close to 2 cubic feet. The calculator converts for you and rounds up to whole bags.
Can I use garden soil or topsoil in containers?
It is not recommended. Garden soil and topsoil compact in pots, hold too much water and drain poorly, which suffocates roots. Extension services advise a light, well-drained soilless potting mix for containers instead. Save topsoil for the ground and use the topsoil calculator for that.
Why is my estimate different from the pot’s stated size?
Pot sizes are often quoted by nominal diameter or trade-gallon, which do not translate exactly to soil volume, and pots taper so the real cavity is smaller than a straight cylinder. This calculator gives a slightly generous cylinder estimate; you will typically use a touch less once the pot is packed and settled.
How do I estimate soil for a window box or many small pots?
For a rectangular window box, use the raised bed soil calculator with the box’s length, width and depth. For many identical round pots, enter one pot’s diameter and depth here and set the number of pots — the total and the bag count scale automatically.
Do I need fresh potting mix every year?
You do not have to replace it entirely. Most gardeners refresh containers by removing the top third of old mix and stirring in fresh potting soil and a little compost each spring. Fully replace mix if plants struggled with root disease the previous season.
Volumes are estimates. Real pots taper and vary, and mix settles after watering, so buy a little extra. Use a soilless potting mix for containers, not garden soil. General gardening guidance, not professional advice.