Garden & Water Utilities
Potting soil for pots.
Enter the size of a round or rectangular pot or planter and how many you are filling. Get litres, cubic feet and dry quarts of mix, and how many bags to buy. Runs in your browser.
Pot size
Potting mix needed
Fill the form and press Calculate.
Worked example
A rectangular planter 24 in long, 8 in wide and 11 in high, filled to 1 in below the rim, holds 24 × 8 × 10 = 1,920 cubic inches, about 31.5 litres or 1.11 cubic feet. Three of them need about 94 litres, 3.3 cubic feet. That is 4 bags of 1 cubic foot mix.
How it works
A rectangular planter is length × width × soil depth. A round pot that narrows toward the base is a conical frustum: π × depth / 3 × (r₁² + r₁r₂ + r₂²), where r₁ is the bottom radius and r₂ the radius at the soil surface, found by scaling between the base and the rim. Soil depth is the pot height minus the gap below the rim. One litre is 1,000 cubic centimetres, a cubic foot is 28.317 litres and a US dry quart is 1.101 litres.
What this is not. Mix compresses and settles after watering, so buy a little extra or plan to top up. Drainage material is not needed in most pots with holes and takes space, so it is not subtracted. Bag volume labels can be loose, so check whether yours is a loose or compressed volume.
Sources and basis (checked 2026-10-06): geometry (right circular frustum volume) and unit definitions: 1 cubic foot = 28.3168 litres (2.831 685 E-02 m³); 1 US dry quart = 1.10122 litres (NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically).
FAQ
How much soil do I need for a 12-inch pot?
A straight-sided 12 inch round pot about 10 inches deep holds roughly 17 litres (0.59 cubic foot, about 15 dry quarts) when filled an inch below the rim. Tapered pots hold less, so use the calculator with the top and bottom diameters.
How many quarts are in a cubic foot of potting soil?
About 25.7 US dry quarts, or 28.3 litres.
Should I fill a pot to the rim?
Leave about an inch (2.5 cm) so water can pool and soak in without spilling. The gap is an input here.
Do I need rocks at the bottom of the pot?
Generally no. Pots with drainage holes drain from the mix itself, and a rock layer reduces root space. Use the calculator with the full soil depth.