Potting Soil Calculator
Estimate potting mix for tapered pots, planters or known container volumes, then round up to whole bags.
How to calculate potting soil
Use inside dimensions and measure only to the planned soil line. Round pots and rectangular planters are calculated as frustums, so their narrower bottoms are not treated as if they were as wide as the top. If the label already states container capacity, use known-volume mode.
Formulas
Round tapered pot volume = π × depth × (top diameter² + top diameter × bottom diameter + bottom diameter²) ÷ 12. Rectangular tapered planter volume = depth ÷ 3 × (top area + √(top area × bottom area) + bottom area). Multiply by the number of containers, subtract the entered displacement, add the allowance and divide by actual bag volume; bags are rounded upward.
Checked example
One round pot with a 12 in top diameter, 8 in bottom diameter and 10 in fill depth holds 795.87 in³, or about 13.04 L. Three pots with 10% extra need about 43.03 L. An 8 US qt bag supplies about 7.57 L, so purchase 6 bags.
Important limits
- Use inside measurements. Thick walls, false bottoms and drainage reservoirs reduce usable volume.
- A root ball is irregular; the percentage input is an estimate. Measure displaced volume when precision matters.
- Bag volumes are nominal and mix can settle after watering. The chosen allowance is explicit and is not a prediction of settling.
- The ratio section divides total prepared volume; it does not prescribe soil chemistry, fertilizer, pH, drainage or a plant-specific recipe.