Potting Soil Calculator: Container Soil Volume

What is the potting soil calculator?

The potting soil calculator works out how much potting soil a container needs. Enter the length, width, and depth of a pot, planter, or raised bed, and it returns the volume in gallons, quarts, liters, cubic feet, or other units. This removes the guesswork of buying too little soil, or too much.

Potting soil is different from garden soil. It is a light, fast-draining mix made for containers, usually built from peat, coconut coir, bark, or compost, blended with perlite or vermiculite for drainage. Garden soil packs down too hard inside a pot and suffocates roots. Because potting soil costs more than garden soil, an accurate volume estimate matters for the budget.

How to calculate potting soil volume

For a rectangular or square container, volume is length multiplied by width multiplied by depth:

Volume = Length × Width × Depth

This is the same formula used to find the volume of any box shape. The calculator converts everything to a common unit internally, then reports the result in whichever output unit is selected — cubic inches, cubic feet, cubic centimeters, cubic meters, quarts, gallons, or liters. All three dimensions must be entered in the same unit (inches, feet, centimeters, or meters).

Unit conversion factors

The table below shows the multipliers the calculator applies to a volume already expressed in cubic inches.

One detail matters here: potting soil is a dry good, like flour or grain, not a liquid. The calculator reports quarts using the US dry quart (67.200625 cubic inches), not the smaller liquid quart used for milk or gasoline (57.75 cubic inches). One cubic foot equals about 25.7 dry quarts. Bagged soil at garden centers is normally labeled with this same dry quart, so the numbers match what is printed on the bag.

Because the calculator uses the dry quart for soil and the liquid gallon for gallons, the two units do not convert at the familiar "4 quarts per gallon" ratio. One gallon of soil volume equals about 3.44 dry quarts, not 4.

Worked example: rectangular planter

A planter box measures 12 inches long, 12 inches wide, and 6 inches deep.

Volume = 12 × 12 × 6 = 864 cubic inches

Converted to other units:

• Cubic feet: 864 ÷ 1728 = 0.5 cu ft
• Gallons: 864 × 0.004329 ≈ 3.74 gal
• Quarts (dry): 864 × 0.014881 ≈ 12.86 qt
• Liters: 864 × 0.016387 ≈ 14.16 L

Round containers

Most pots are round, not square, so the formula changes. For a cylinder:

Volume = π × (diameter ÷ 2)² × depth

The calculator itself only computes rectangular volume, so for a round pot, entering the diameter for both length and width gives the volume of the square that the circle fits inside. That square is always larger than the true round volume by the same fixed amount: a circle covers π ÷ 4, or about 78.5%, of the area of the square around it. So a round pot needs roughly 21% less soil than the square estimate, at any size.

Worked example: round pot

A round pot has a 12-inch diameter and a 12-inch depth.

Square estimate (12 × 12 × 12): 1,728 cubic inches, or 1 cubic foot.

True cylindrical volume: π × 6² × 12 ≈ 1,357 cubic inches.

• Cubic feet: ≈0.79 cu ft
• Gallons: ≈5.88 gal
• Quarts (dry): ≈20.2 qt
• Liters: ≈22.2 L

How to use the calculator

1. Pick a dimension unit (inches, feet, centimeters, or meters).
2. Enter the container's length, width, and depth in that unit. For a round pot, enter the diameter as both length and width.
3. Pick a volume unit for the result (cubic feet, gallons, liters, and so on).
4. Read the required soil volume. It updates as the inputs change.
5. Copy the result with the copy button to reference it while shopping.

Measuring a container correctly

Measure the inside of the container, not the outside. A thick ceramic or ceramic-glazed pot can have walls over an inch thick, and using the outer dimensions overstates the true volume.

For depth, stop measuring one to two inches below the rim. That gap is needed so water does not spill over the edge when the pot is watered.

For an irregular container, measure the longest length, the widest width, and an average depth. This slightly overestimates the volume, which is safer than running short partway through filling a pot.

Buying extra soil

Fresh potting soil is loose and full of air pockets. After the first few waterings it settles, typically losing 10 to 20% of its volume as those air pockets collapse and organic material compresses. Buying 10 to 15% more than the calculated volume covers this settling, so there is enough soil left to top off the container afterward.

Frequently asked questions

How much soil does a 12-inch round pot need? A 12-inch diameter, 12-inch deep round pot holds about 5.9 gallons (about 20 dry quarts, or 0.79 cubic feet), using the true cylinder formula. Leave about an inch of headspace below the rim.

Should I calculate the exact volume or just buy extra? Calculate the volume first, then buy 10 to 15% more than that figure. Soil settles after watering, so the extra covers the drop in volume.

How do I handle an oddly shaped container? Measure the longest length, widest width, and average depth, then use the rectangular formula. For very irregular shapes, fill the container with water, pour the water into a measuring jug, and convert: one US gallon of water is about 0.13 cubic feet.

Why does the calculator use dry quarts instead of the quarts I know from the grocery store? Potting soil is a dry good, so the industry standard measure is the US dry quart (67.2 cubic inches), not the smaller liquid quart used for milk or fuel (57.75 cubic inches). Bagged soil is labeled in the same dry quart.

Does this work for raised beds? Yes. Enter the bed's length, width, and depth just as for any rectangular container. For beds deeper than about 4 cubic feet, many gardeners fill the bottom half with cheaper garden soil or topsoil and reserve potting soil for the top half, where roots need the best drainage.

How deep should soil sit below the rim? About 1 to 2 inches for a standard pot, and 2 to 3 inches for anything over 15 inches wide. Seed trays need almost no gap, around a quarter inch.

Sources

• NIST Handbook 130, Uniform Laws and Regulations, section 7.3 (dry quart and dry measure definitions).
• NIST Handbook 133, Checking the Net Contents of Packaged Goods, Table 3-4 (bagged soil labeled in liters, cubic feet, and dry quarts).
• Cornell University Department of Horticulture, "Soil and Growing Media Basics."
• University of California Agriculture and Natural Resources, "Container Gardening."