Work out the volume of potting mix your container can hold before choosing a bag size. A planter’s outside dimensions often overstate the space available inside it, and a nursery root ball takes up some of that space. This calculator estimates a simple empty container; you can adjust the fill percentage to reflect how much new mix you intend to add.
Measure the usable space
Measure inside the planter, below its rim. For a rectangular box, record the internal length, width, and intended soil depth in inches. For a round container, record the internal diameter and depth. Keep all measurements in the same unit. If you want a gap below the rim for watering, measure to the planned soil line instead of the very top.
The round option assumes a straight-sided cylinder. Many decorative pots taper toward the base, so using the top diameter for the whole depth overestimates their volume. For a noticeably tapered or irregular container, treat the result as a rough buying estimate rather than an exact capacity. Thick walls, reservoirs, false bottoms, and inserts also reduce usable space.
How the estimate works
For a box, cubic inches equal length × width × depth. For a cylinder, cubic inches equal π × (diameter ÷ 2)² × depth. Multiply this volume by the fill percentage divided by 100. The calculator then converts volume to practical shopping units: 67.200625 cubic inches equal one US dry quart, 1,728 cubic inches equal one cubic foot, and 61.024 cubic inches are approximately one liter. Dry quarts differ from the smaller liquid quarts used in many kitchen measurements.
These are volume conversions, not weight conversions. A dry quart of potting mix does not have a fixed weight, and moisture and ingredients can change bag weight considerably. Compare the result with the volume printed on the bag. The shopping examples use 8-dry-quart, 16-dry-quart, and 1-cubic-foot bags; if your product uses another size, divide the matching volume result by that package size and round upward.
Worked example: a window box
A rectangular planter measures 21 inches long, 8 inches wide, and 8 inches deep inside. At 100% fill, its modeled capacity is 1,344 cubic inches: approximately 20.00 US dry quarts or 22.02 liters. If you enter 85% fill, the estimate becomes approximately 17.00 US dry quarts or 18.72 liters.
At 85% fill, one 16-dry-quart bag falls just short of the mathematical requirement; two such bags would cover it with substantial spare mix. One 20-dry-quart bag, if available, would meet the estimate. If your measurements were outside dimensions, the actual container may be smaller; if mix settles or spills, you may need more. A root ball should be accounted for separately rather than buried deeper to use up spare mix.
Choose a suitable growing medium
Container volume tells you how much to buy, not which mix suits a plant. Choose a medium appropriate for the plant, container, and drainage conditions. Burpee’s container-growing guidance recommends lightweight potting mix for containers. Follow the plant’s care needs and the product instructions rather than treating a volume calculation as a watering or fertilizer recommendation.
Common questions
Does this subtract the existing root ball?
No. The shape calculation does not know the root ball’s dimensions. Reduce the fill percentage to approximate the new mix required, or measure the surrounding space separately.
Does fill percentage mean moisture content?
No. It is a simple multiplier applied to modeled container capacity. It does not describe soil moisture, compaction, or a recommended planting depth.