Potting soil calculator

Enter the inside diameter and depth of your pots and how many you have. You get quarts, cubic feet and the number of bags.

Estimates. Bag sizes, material density and settling vary; confirm quantities with your supplier.

Good to know

Containers should be filled with potting mix (often soilless: peat or coir, bark, vermiculite, perlite), not plain garden soil, which drains and aerates poorly in a pot (Illinois Extension, Colorado State, UC Marin).

Illinois Extension: if garden soil is used, amend it as one part soil, one part peat moss, one part perlite or coarse builder's sand.

Colorado State: soilless mixes weigh two to three times less than soil-based potting soil; up to 10 percent of the container volume can be clean coarse sand to add weight.

Bagged potting mix is sold in quarts (8, 16 and 25 quarts are common) and cubic feet (1, 1.5 and 2 cu ft). One cubic foot is about 25.7 dry quarts.

The calculator treats each pot as a straight-sided cylinder, so a tapered pot needs a little less. Leave an inch or two at the top for watering.

The arithmetic

A round pot holds 3.14 times the radius squared times the depth, in cubic inches. Divide by 1,728 for cubic feet and multiply cubic feet by 25.7 for dry quarts.

Questions