A container that truly holds 5 US liquid gallons has a full capacity of about 18.93 liters, 0.67 cubic feet, or 17.2 US dry quarts. Your fresh-potting-mix requirement is usually lower because you leave watering space and an existing root ball occupies volume.
Check the Actual Container
Nursery trade sizes are often nominal. Do not assume a pot sold as “5-gallon” has exactly that liquid capacity. Use the manufacturer’s volume or measure the inside dimensions. The intended fill height is below the rim, not necessarily the full container height.
Calculate a Round Pot
For a straight-sided cylinder, volume equals π × radius² × fill height. Measure all dimensions in inches; divide cubic inches by 1,728 for cubic feet. A tapered pot needs a tapered-volume calculation or the manufacturer’s specification, not the cylinder formula with only the top diameter.
For a circular tapered pot with straight sloping sides, volume is π × h × (R² + Rr + r²) ÷ 3, where R and r are the inside top and bottom radii and h is the height being filled. For a rectangular straight-sided planter, multiply internal length, width, and fill height.
Buy Mix in Matching Units
Check whether the bag label uses liters, cubic feet, or US dry quarts. A dry quart differs from a liquid quart; four dry quarts are not one US liquid gallon. Subtract the root-ball volume from the planned fill volume and allow a small practical margin for settling rather than assuming a universal percentage.
Fill for the Plant
Use a container mix suitable for the plant and drainage holes appropriate to the pot. Do not add a gravel layer as a drainage fix. Set the plant at its appropriate planting depth, fill around the roots, and water to settle the mix without packing it hard. Leave room to water without spilling over the rim.
Watering frequency depends on the plant, weather, and mix rather than the gallon label. For the different-capacity estimate, see our 10-gallon pot guide.
Unit conversions follow NIST; container-care guidance is available from University of Maryland Extension.
