Container Garden Calculator

Small pots (under 5 gallons / 19 L): herbs, lettuce, radishes, strawberries. Medium pots (5 to 10 gallons / 19 to 38 L): peppers, dwarf tomatoes, cucumbers, eggplant. Large pots (10 to 25 gallons / 38 to 95 L): indeterminate tomatoes, squash, large brassicas. Very large (25+ gallons): fruiting trees, berry bushes.

Container volume (gallons) x 0.134 = cubic feet of soil needed. Alternatively, volume in litres / 28.3 = cubic feet. Most bags of potting mix are 1 to 2 cubic feet (16 to 32 dry quarts). Allow 1 to 2 inches from the rim unfilled to prevent water runoff when watering. Do not use garden soil in containers: it compacts and does not drain properly.

Container plants dry out faster than in-ground plants because the root volume is limited. In hot weather, containers may need daily watering. Stick your finger 1 to 2 inches into the soil: if it is dry, water thoroughly until water drains from the bottom. Small pots and terracotta containers dry out faster than large glazed pots.

Potting mix is a soilless blend of peat or coir, perlite, and sometimes compost, designed for containers. It drains quickly and does not compact. Garden soil is heavier, compacts in containers, restricts drainage, and may contain weed seeds and pathogens. USDA Extension recommends using only quality potting mix or container blend in raised beds and pots.

A 5-gallon pot holds approximately 10 to 12 inches of growing depth and 12 to 14 inches of diameter. It supports 1 pepper, tomato, or cucumber; 2 to 3 bush beans; 4 to 6 lettuce plants; or multiple herbs. Spacing is based on mature plant width: use USDA Extension plant spacing charts for your specific species.