Verify the gallon definition
A grow-bag size may be a nominal label rather than a measured US liquid capacity. Confirm its usable volume or dimensions with the supplier. Do not equate liquid quarts, dry quarts and nursery trade sizes: their conversions differ. Our examples below explicitly use US liquid gallons.
Compare theoretical capacities
A measured capacity of 5 US liquid gallons is about 0.6684 cubic feet; 10 gallons is about 1.3368; 15 gallons is about 2.0052. These are conversion examples, not a guarantee about bags carrying those size labels. Use the actual volume to your intended soil line when available.
Example with four containers
Four containers each holding a confirmed 10 US liquid gallons to the chosen fill level need about 5.347 cubic feet. Four 1.5 cubic foot bags supply 6 cubic feet. Three 2 cubic foot bags also supply 6 cubic feet. Compare their entered prices rather than assuming the larger format is cheaper.
Estimate irregular flexible shapes carefully
If a filled bag approximates a cylinder, measure its inside diameter and intended fill height and use equal diameters in the pot calculator. Flexible sides introduce uncertainty. Do not use the flat packed dimensions as cylinder measurements or apply the same headspace percentage to every shape.
Use the shopping calculator
For a known capacity, convert to cubic feet or liters and enter the combined target in the bag tool. The tool does not directly accept gallons or dry quarts. Check the potting mix’s volume label, retain any allowance as a separate choice and follow container-growing guidance for mix selection and drainage.
Frequently asked questions
Are all 10-gallon grow bags the same size?
No. Confirm the manufacturer’s actual capacity.
Does 40 dry quarts equal 10 US liquid gallons?
No. Dry and liquid quarts differ; do not interchange them.