Pot Soil Volume Calculator
Calculate how much potting mix you need to fill a round or square container from its diameter (or side) and height.
How to use this tool
- Select whether your pot is round (cylindrical) or square.
- Enter the inside diameter (or side length) and inside depth in inches.
- The calculator returns the volume in quarts and cubic feet.
Find out how much potting mix to buy for any size container or planter.
⚠ This tool provides general estimates for education only and is not financial, tax or legal advice. Figures may not reflect your situation — verify with a qualified professional.
Formula
Round pot: Volume (in³) = π × (diameter ÷ 2)2 × height
Square pot: Volume (in³) = side2 × height
Volume (qt) = in³ ÷ 57.75
Volume (ft³) = in³ ÷ 1728
How it works
This calculator finds the internal volume of a cylindrical or rectangular pot from its diameter (or side length) and height, both in inches, then converts to quarts and cubic feet — the units most commonly shown on potting-mix bags.
The formula assumes the pot is a perfect cylinder or rectangular prism with straight sides; tapered nursery pots hold somewhat less than calculated. Leave 1–2 inches of headspace below the rim for watering, so fill volume is slightly less than total pot volume in practice.
Worked example
- Inputs: round pot, 12-inch diameter, 10-inch height.
- Radius = 12 ÷ 2 = 6 in. Volume in in³ = π × 6² × 10 = π × 360 ≈ 1,130.97 in³.
- Volume in qt = 1,130.97 ÷ 57.75 ≈ 19.58 qt.
- Volume in ft³ = 1,130.97 ÷ 1,728 ≈ 0.6545 ft³.
Volume = 19.58 qt (0.6545 ft³).
Common mistakes to avoid
- Using the outer diameter of the pot instead of the inner diameter — the pot wall thickness can reduce the usable volume noticeably in small containers.
- Forgetting that potting mix settles after watering, so filling to within an inch of the rim is practical but the calculator assumes the full height is usable.
- Applying the round-pot formula (pi x r squared) to a square pot or vice versa, producing a meaningfully wrong volume estimate.
Key terms
- Quart (qt)
- A US volume unit equal to 0.946 litres; potting mix bags are commonly sold in quart sizes (e.g., 8 qt, 16 qt, 32 qt).
- Cubic foot (ft³)
- A volume unit equal to 1,728 cubic inches or about 25.7 litres; larger bags and bales of potting mix are often sold by the cubic foot.
- Headspace
- The gap left between the soil surface and the pot rim (typically 1–2 inches) to allow water to pool briefly and soak in rather than running off immediately.
- Tapered pot
- A pot whose diameter is wider at the top than at the base; its actual volume is less than a true cylinder of the same top diameter and height.
Frequently asked questions
- How much potting soil does a 12-inch pot hold?
- A 12-inch round pot that is 10 inches deep holds about 19.6 quarts (0.65 cu ft) of potting mix. Most bags are sold in 8-quart or 2 cu ft sizes.
- How many quarts in a cubic foot of potting soil?
- One cubic foot equals 25.75 dry quarts. A standard 2 cu ft bag of potting mix holds about 51.5 quarts.