Potting Soil Calculator for a Pot
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
The amount of potting soil needed is calculated with the frustum formula: (π × height ÷ 3) × (R² + R×r + r²), R and r being the top and bottom radii of the pot. A flared pot 30 cm wide at top, 20 cm at bottom, and 25 cm tall needs about 12.4 liters of potting soil.
Explanation
Most garden pots aren't simple cylinders: they're flared, wider at the top than at the bottom, to make unmolding easier and help root anchoring. The exact volume of such a shape, called a frustum (truncated cone) in geometry, is calculated with the formula (π × h ÷ 3) × (R² + R×r + r²), where R is the top radius, r the bottom radius, and h the height. This formula still holds for a perfectly cylindrical pot: just enter the same diameter at top and bottom, and it automatically simplifies to the classic cylinder formula (π × radius² × height). Knowing this volume ahead of time avoids the two common pitfalls of container gardening: buying too much potting soil (waste, opened bags that lose quality over the following months) or not enough (repotting interrupted halfway through). For a very long planter rather than a round pot, simply apply this same volume per section, or use a rectangular volume calculator directly for a rectangular shape. Once the pot is filled and planted, the amount of fertilizer to apply also depends on this substrate volume rather than the pot's surface area alone: see our garden fertilizer dosage calculator to adjust the dose accordingly — and for a raised vegetable bed rather than a single pot, our vegetable garden planting calculator helps plan how many plants that same volume of soil can support.
Example: a flared pot 30/20 cm, 25 cm tall
Inputs
Top diameter: 30 cm. Bottom diameter: 20 cm. Height: 25 cm.
Calculation
R = 15 cm, r = 10 cm. Volume = (π × 25 ÷ 3) × (15² + 15×10 + 10²) = 26.18 × (225 + 150 + 100) = 26.18 × 475 ≈ 12,435 cm³.
Result
This pot needs about 12.4 liters of potting soil to be filled completely.
Frequently asked questions
What if my pot is square or rectangular rather than round?
This calculator is designed for a pot with a circular cross-section. For a rectangular planter, the calculation is simpler: length × width × height, like any classic rectangular volume calculator.
Should the pot be filled to the brim?
No, it's recommended to leave 2 to 3 cm empty below the rim (otherwise watering would overflow), and to plan a drainage layer (clay pebbles or gravel) at the bottom for pots without a sufficient drainage hole. So use the height ACTUALLY filled with potting soil, not the pot's total height, in the "effective height" field.
Why use the frustum formula rather than a simple average of the diameters?
Because it's mathematically exact, unlike a simple average of the radii squared: on a strongly flared pot, the gap between the two methods can represent several hundred milliliters, enough to underestimate or overestimate an entire bag of potting soil on a large volume.