Plant Pot Soil Volume Calculator
Calculate potting soil volume for round, square, or rectangular pots.
Enter dimensions to get liters and quarts needed with a bags-to-buy estimate.
Potting soil volume calculation works out how many bags or litres of potting mix you need to fill one or more containers, accounting for pot shape, any drainage layer, and the fill level below the rim.
Core formulas:
Straight-sided round pots: Volume = π × r² × h
Rectangular and square planters: Volume = Length × Width × Height
Where r = radius (half the diameter), h = fill depth, all in the same unit.
Conversion to useful units:
- Cubic inches → Liters: Cubic inches ÷ 61.02
- Cubic inches → Quarts: Cubic inches ÷ 57.75
- Cubic inches → US gallons: Cubic inches ÷ 231
Most pots are tapered, and that changes the answer a lot
A pot that measures 12 inches across the rim is not a 12-inch cylinder. It narrows toward the base, typically to about three quarters of the top diameter, and that removes roughly 23% of the volume a cylinder calculation would give you. Buy from the cylinder figure and you will be carrying a spare bag back to the shop.
The frustum formula handles it:
Volume ≈ (π × h ÷ 3) × (R² + R×r + r²)
Where R is the top inner radius and r the bottom one. With r = 0.75R that reduces to 0.771 × the cylinder volume, which is the factor this calculator applies when you pick a tapered pot.
Pot size to soil volume guide
Standard round pots, tapered, height about 0.85 × the diameter, filled to an inch below the rim:
| Pot Diameter | Soil Needed | Typical Use |
|---|---|---|
| 4 inches (10 cm) | 0.5 qt (0.5 L) | Seedling, small succulent |
| 6 inches (15 cm) | 1.5 qt (1.5 L) | Single herb |
| 8 inches (20 cm) | 3.9 qt (3.7 L) | Small annual, pothos |
| 10 inches (25 cm) | 7.9 qt (7.4 L) | Mid-size houseplant |
| 12 inches (30 cm) | 13.9 qt (13.1 L) | Standard patio container |
| 14 inches (36 cm) | 22.4 qt (21.2 L) | Large perennial, citrus |
| 16 inches (41 cm) | 33.8 qt (32.0 L) | Small shrub |
| 18 inches (46 cm) | 48.6 qt (46.0 L) | Small tree, large shrub |
These are volumes of mix, before the settling allowance. The pot size calculator uses the same model, so the two pages give the same answer for the same pot.
Important adjustments:
- Drainage layer: gravel at the bottom of a pot is a myth worth abandoning. It raises the saturated zone rather than lowering it. If you use one anyway, subtract its volume in the field above.
- Fill to 1–2 inches below the rim so water does not sheet off the top. This calculator takes 1 inch off the depth automatically.
- Settling allowance: add 10–15%. Potting mix compacts after the first watering, and the level always drops more than people expect.
Worked example: A 12-inch diameter, 10-inch tall tapered pot. Fill depth = 10 − 1 = 9 inches Cylinder volume = π × 6² × 9 = 1,018 cubic inches Tapered = 1,018 × 0.771 = 785 cubic inches Quarts = 785 ÷ 57.75 = 13.6 quarts (12.9 litres) With the 12% settling buffer: 15.2 quarts, so two 8-quart bags.
Run the same pot as straight-sided and you get 17.6 quarts. That gap, nearly four quarts on one patio pot, is the whole reason the taper question is the first thing this page asks.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.