Potting Soil Calculator

Pot Size to Volume Table

A round pot holds soil equal to pi times the radius squared times the height. The radius is half the diameter. A 12 inch wide pot that is 12 inches tall holds about 0.6 cubic feet, or roughly 15 quarts. The calculator runs this for any pot size and any number of pots. One cubic foot equals about 25.7 quarts, the conversion that links cubic-foot bags to quart-labeled bags. Leave a 1 inch gap below the rim for watering, so fill to just under the top. The table below lists common pot sizes and their soil needs.

Common Pot Sizes (gal/qt)

Potting soil is sold in both quarts and cubic feet, and container plants are often described in gallons. One gallon equals 4 quarts. An 8 inch pot holds about 1 gallon, or 4 quarts, which is 0.15 cubic feet. A 5 gallon bucket measuring about 12 inches across and 15 inches tall holds about 20 quarts, or 0.77 cubic feet. Bagged potting mix comes in 8, 16, and 20 quart bags for small jobs, and 2 cubic foot bags for larger plantings. A 2 cubic foot bag holds about 51 quarts and fills roughly twelve 8 inch pots.

How Much Potting Soil Do I Need

The potting soil you need is the total volume of all your pots. Calculate each pot with pi times radius squared times height, add them up, and convert to quarts or cubic feet. For a square or rectangular planter, use length times width times height instead. The calculator multiplies a single pot volume by the number of identical pots. A planting of ten 8 inch pots needs about 40 quarts, or 1.5 cubic feet. Buy a slightly larger amount than calculated, since potting mix settles and you may want to top off after the first watering.

How much soil for a 5 gallon bucket?

A 5 gallon bucket holds about 20 quarts, or 0.77 cubic feet, of potting soil. The bucket measures roughly 12 inches across and 15 inches tall. Leave a 1 inch gap at the top for watering, so fill to about 14 inches. One 2 cubic foot bag fills about two and a half 5 gallon buckets. Drill drainage holes in the bottom before filling.

Can I reuse potting soil?

You can reuse potting soil for one or two more seasons if it is healthy. Refresh it by mixing in one-third fresh compost or new potting mix to restore nutrients and structure. Discard soil from plants that had disease or pests. Old potting mix compacts and drains poorly, so amending or partially replacing it each year keeps container plants thriving.

Choosing the Right Potting Mix

Potting mix is not garden soil. A good potting mix is light, drains fast, and holds moisture without compacting in a container. It usually blends peat moss or coconut coir, perlite or vermiculite for aeration, and compost for nutrients. Garden soil is too heavy for pots and packs down, suffocating roots and blocking drainage. Always use a dedicated potting mix or container mix for pots and window boxes.

Match the mix to the plant. Succulents and cacti need a fast-draining mix with extra sand or perlite. Vegetables and annuals want a richer mix with more compost. Seed-starting needs a fine, sterile mix with no large chunks. Most pots also benefit from a slow-release fertilizer mixed in, since frequent watering flushes nutrients out of containers. Refresh the top third of the mix each season and fully replace it every two to three years as it breaks down and compacts. A quality mix pays off in healthier, more productive container plants.

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