Fill Every Pot Perfectly.

How potting soil volume is calculated

The volume of any container is simply its shape's formula applied to your measurements. For rectangular and square pots, multiply length × width × depth. For round pots, use π × (radius)² × depth. Our calculator handles both, then converts the result into cubic inches, cubic feet, and liters so you can compare with bag sizes.

A fill percentage lets you leave headspace for drainage or mulch — most pots are filled to about 90–95% of their total depth to avoid overflow when watering.

Container dimensions: what to measure

• Length & width — measure the inside top of the pot (not the outside).
• Depth — measure from the bottom to the rim you plan to fill to.
• Diameter (round) — measure across the top opening, inside edge to inside edge.
• Fill % — if you leave 1 inch of space at the top, that's roughly 10% less soil for a 10‑inch deep pot.

Round vs rectangular containers

Round pots are more space‑efficient for roots because there are no corners, but the volume formula differs. For a round pot, the volume is about 78.5% of a square pot with the same width and depth. That means a 12‑inch round pot holds less soil than a 12×12‑inch square pot — important when estimating bag counts.

Why bag volume differs from container volume

Potting soil is sold by volume (quarts, cubic feet, liters), not weight, because moisture content changes weight. But a bag labeled "2 cubic feet" may not fill a 2‑cubic‑foot container perfectly: soil settles during shipping and you may need extra to account for compaction. Always buy 10–20% more than your calculated volume.

Fill level & soil settling

Fill level matters: leave at least 1–2 inches of space below the rim for watering. Soil also settles after the first few waterings, so you may need to top off. Our calculator lets you set a fill percentage to automatically reduce the volume, giving you a realistic estimate.

Estimating soil bags

Common bag sizes: 1 cubic foot (≈ 28.3 liters), 2 cubic feet, 8 quart (≈ 0.27 cubic feet), and 16 quart (≈ 0.53 cubic feet). Our calculator divides your total volume by 1 cubic foot (the most common bag size) to give you a starting point. Always round up to the nearest whole bag.

Why different plants may require different planting depths

Root depth varies by plant: succulents prefer shallow pots, tomatoes need deep containers (12+ inches), and herbs like 6–8 inches. Using a pot that's too deep wastes soil; too shallow stunts growth. Our calculator helps you plan the actual soil volume for the depth you choose.

Frequently asked questions