Soil Calculator for Pots

How this calculator works

Round mode uses a conical frustum from top and bottom inside diameters. Rectangular mode uses the prismoidal frustum relationship from top and bottom inside areas.

Matching container volume is reduced by entered root-ball or insert displacement and the chosen usable-volume factor. Whole bags use the printed package volume.

Pot labels often list outside dimensions, which overstate capacity. Settling, drainage layers and existing roots also change how much mix is physically needed.

Control example: four tapered round pots

Each pot is 18 in across the top, 12 in at the bottom and measured to a 16 in internal fill height.

The calculator uses frustum volume, subtracts displacement, applies a 95% usable-volume factor for headspace and settling, and rounds to whole 1.5 ft³ bags.

Assumptions and limits

• Use internal dimensions at the soil line and base, then enter real root-ball displacement and the volume printed on the potting-mix bag.
• Measurements describe one simple project geometry.
• Waste and package values are editable planning assumptions.
• It estimates volume only and does not recommend soil chemistry, drainage design, plant suitability, fertilizer, or watering.

Common measuring mistakes

• Mixing nominal dimensions with actual installed coverage.
• Skipping whole-package or stock-length rounding.
• Failing to verify the page’s stated measurement and product assumptions.
• Using a planning estimate outside the limitations stated on this page.

Sources

Product-specific values remain editable. These sources support the method and stated examples; they are not a universal substitute for your supplier’s current label.

• GQ Calc methodology Documents cylinder, frustum, unit-conversion and package-rounding methods.
• USDA container volume technical note Discusses nursery container dimensions and volume as physical properties rather than universal pot-size labels.

Questions people ask