GARDEN FIELD BOOK / CALCULATOR 07
Pot & Grow Bag Volume Calculator
Measure the usable inside of a container, not the printed pot size. Get the estimated capacity in US gallons, liters and cubic feet, then estimate how many bags of potting mix you need.
Measured cylinder · 1 container
About 22.2 liters
INSIDE MEASUREMENTS
Describe your container
CALCULATED CAPACITY
Volume & mix estimate
Results will compare one container with the total for your group. These are geometric estimates, not commercial pot size labels.
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Nursery container trade sizes, sloped shoulders, folded grow bags and unfilled headspace can change the real amount of mix required.
MEASUREMENT GUIDE
Measure the space the mix will actually fill
Straight-sided cylinder
Measure the inside diameter across the usable opening and the vertical fill height. Do not include an empty rim above your intended soil level.
Wide top, narrow bottom
Measure inside diameters at the top and bottom of the filled section. The calculator assumes straight sloping sides; irregular curves give an approximation.
Boxy planter or grow bag
Measure usable inside length and width and the fill height. Soft bags can bulge or fold, so their measured capacity may change after filling.
THE METHOD
How pot volume is calculated
π × (diameter ÷ 2)² × heightπ × height × (top² + top × bottom + bottom²) ÷ 12length × width × fill heightThe formulas first calculate cubic volume using your measured dimension unit, then convert to liters, US liquid gallons and cubic feet. One US gallon equals 231 cubic inches. For example, a cylinder with a 12-inch inside diameter and 12-inch fill height holds about 5.88 US gallons, or about 22.2 liters.
Bag estimates use the measured capacity times the number of identical containers. If you request an extra allowance, it increases only the purchasing estimate, not the measured volume. The calculated number of bags is rounded up.
READ THE ESTIMATE
What the result does not tell you
A measured volume does not determine which crop or root system belongs in that container. See the plant-specific guidance first.
US liquid gallons of measured space are not always the same as the nominal trade-gallon size shown on a container.
Bulging walls, tapered shoulders, liners, drainage materials and settling can change the amount of mix needed.
KEEP EXPLORING
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