Potting Soil Calculator: How Much Do You Need?

Calculate potting soil by multiplying the planter’s inside length, width, and fill depth, then convert the result to the volume unit you need. Enter those three measurements below to get the soil volume in liters and cubic feet; the estimate assumes a rectangular planter filled to the measured depth.

Formula

For a rectangular planter, volume equals inside length × inside width × soil depth. With all three measurements entered in centimeters, dividing cubic centimeters by 1,000 gives liters. Dividing by 28,316.8466 gives cubic feet, because one cubic foot is 28,316.8466 cubic centimeters.

• l = inside length in centimeters.
• w = inside width in centimeters.
• d = the depth of soil required in centimeters.
• Potting soil volume = the resulting volume in liters or cubic feet.

Measure the planter’s interior, not its outside dimensions. If it narrows toward the bottom or has a curved shape, the rectangular formula is an estimate rather than an exact volume.

Worked Example

Using the calculator’s default measurements—30 cm long, 20 cm wide, and 15 cm deep:

1. Multiply the dimensions: 30 × 20 × 15 = 9,000 cubic centimeters.
2. Convert to liters: 9,000 ÷ 1,000 = 9.00 L.
3. Convert to cubic feet: 9,000 ÷ 28,316.8466 = 0.318 ft³, rounded to three decimal places.

The calculator therefore estimates 9.00 liters of potting soil, or 0.318 cubic feet, for this planter.

How to Measure a Planter

Measure the space that will actually hold soil. A pot’s rim can be wider than its base, and thick walls reduce the inside capacity. For a rectangular window box, measure the interior length and width at the soil surface, then choose the depth you intend to fill. For a round pot, measure the inside diameter and depth instead; the cylinder formula is π × radius² × depth. Radius is half the inside diameter.

Do not automatically fill to the rim. Leave enough headspace for watering, and account for any root ball, drainage layer, or other object that occupies space. If an item displaces soil, subtract its volume from the planter’s total capacity. For tapered containers, measuring the top and bottom dimensions and averaging their areas can give a more useful estimate than treating the whole container as a box.

Use the Result to Buy Soil

The result is a volume, not a weight. Potting mixes vary in density and can settle during transport or after watering, so a volume estimate cannot reliably be converted to pounds or kilograms without product-specific information. Compare the calculated volume with the package’s stated capacity, keeping units consistent.

• For liters, compare directly with a bag labeled in liters.
• For cubic feet, compare with a bag labeled in cubic feet. One cubic foot is about 28.32 liters.
• If a package lists quarts, check whether it means US liquid quarts: one US liquid quart is about 0.946 liters. Dry quarts and other regional units may differ.

When the calculated amount falls between package sizes, use the next suitable size if you need enough to fill the planter. A modest extra amount can be useful because soil may settle, but avoid buying a large surplus without considering storage: opened mix can dry out or become contaminated if stored carelessly. Follow the package’s storage directions.

When This Estimate Needs Adjustment

The calculator assumes a straight-sided rectangular volume and a uniform soil depth. Its estimate can differ from the amount used when the planter is tapered, rounded, divided into compartments, or partly filled with a large root ball. Measure unusual planters individually where practical, or use the manufacturer’s capacity if it is available and describes the usable interior volume.

Soil volume also changes somewhat as a mix settles. Do not compress potting mix heavily just to make the measured amount fit: excessive compaction reduces air spaces that help roots receive oxygen and water drain. Fill the container loosely, water as appropriate for the plant, and top up if settling leaves too little mix. Drainage holes should remain unobstructed; a layer of gravel at the bottom does not replace drainage holes and reduces the volume available for roots.

Potting Soil Versus Garden Soil

Potting soil is a general name for container-growing mixes, which may contain materials such as compost, bark, coir, peat, or mineral components. Their composition varies, so read the label to check whether a mix suits the plant and container. Garden soil taken directly from the ground is often denser and may drain poorly in pots; it is not automatically an equivalent substitute for a container mix.

Use the volume calculation for the medium you plan to put in the container. If you blend ingredients yourself, calculate the total finished volume rather than assuming each ingredient’s loose volume adds perfectly: mixing and settling can alter the final amount.

Frequently Asked Questions

How much potting soil do I need for a 10-inch pot?
A pot’s diameter alone is not enough to calculate its capacity; you also need its inside depth and shape. For a cylindrical pot, use π × radius² × depth, with radius equal to half the inside diameter. A tapered pot needs an estimate based on its top and bottom widths.

How many liters are in a cubic foot of potting soil?
One cubic foot equals about 28.32 liters. This is a volume conversion, not a conversion to weight.

Should I fill a planter completely with potting soil?
Usually, leave some space below the rim so watering does not immediately spill over. The needed headspace depends on the container and plant, so measure the intended soil depth rather than the planter’s full outside height.

Can I use garden soil in a pot?
Garden soil is often denser than a container mix and may drain poorly in a pot. Choose a growing medium labeled or formulated for containers unless you have a reason and suitable preparation for using another mix.

Why did I use less soil than the calculator estimated?
The planter may taper, contain a root ball or other object, or have a smaller usable interior than its outside measurements suggest. Soil can also settle, and the assumed fill depth may differ from the actual one.

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in l | Inside length | cm | 30 | 0
in w | Inside width | cm | 20 | 0
in d | Soil depth | cm | 15 | 0
out liters | Potting soil needed | L | l * w * d / 1000 | 2
out cubicfeet | Potting soil needed | ft³ | l * w * d / 28316.8466 | 3