A nominal capacity is only the starting point
Five US liquid gallons equal approximately 0.6684 cubic feet, or 18.93 liters. This describes a stated liquid capacity, not the usable soil space of every bucket. Imperial gallons and nursery trade sizes differ; a pot labelled #5 is not automatically a measured five-liquid-gallon bucket.
Example: three buckets
Three containers each holding a measured five US gallons to the intended fill level total about 2.005 cubic feet. A 2 cubic foot bag is fractionally below that theoretical target, so whole-bag arithmetic rounds up to two. Actual headspace can reduce the target; measure it before applying a brim-capacity calculation.
Why gardening guidelines may look different
Maryland Extension gives a practical guideline of a 2 cubic foot bag for three 5-gallon buckets or grow bags. Such guidance reflects practical filling and nominal sizes, while the exact liquid conversion above uses stated full capacity. Do not mistake that small difference for a universal extra-bag requirement.
Measure the intended soil space
Leave your chosen watering space out of the fill height. For a tapered bucket, enter inside top and bottom diameters and the filled height in the round-pot calculator. If you know the usable volume already, enter it directly in the bag calculator. It does not accept gallon input; use cubic feet or liters.
Choose a suitable container and mix
Container suitability, previous use and drainage are separate from volume. Consult container-gardening guidance before repurposing a bucket. Root balls also take up space, so a planted bucket may need less fresh mix than an empty one. Use the label on the mix for its bag volume rather than its weight.
Frequently asked questions
Does a #5 nursery pot hold five liquid gallons?
Not necessarily; check the manufacturer’s capacity.
Must I fill to the rim?
No. Estimate to the soil line you plan to use.