Container potting soil calculator

Containers are sold by the inch and potting mix is sold by the quart, so the sum between them is easy to get wrong. This calculator gives you the volume of a pot, planter, window box or half barrel in the unit the bag is actually labelled in.

One conversion is worth stating plainly because so much of the internet gets it wrong: bagged potting mix in the United States is sold in US dry quarts, and a cubic foot is 25.71 dry quarts. The figure 29.92 that appears on many calculator pages is liquid quarts, a different unit, and using it overstates what a container holds by about 16%. If you only need that sum in isolation — a bag size turned into cubic feet or liters, or litres as the bag will say outside the US — the soil volume converter does it both ways.

How to use it

1. Start from a common container if one fits — pot sizes, window boxes, nursery trade sizes and half barrels are all in the dropdown. The measurements fill in and stay editable.
2. Or measure your own. For a round pot, the diameter across the top of the soil. For a square or rectangular planter, the inside width and length.
3. Enter the soil depth, not the full height of the pot. Fill to about an inch below the rim so watering does not wash mix over the edge.
4. Set how many containers you are filling, and the size of the bag you are buying — dry quarts in imperial, liters in metric.

How it is calculated

A round pot or a half barrel is a cylinder:

cubic inches = pi x (diameter / 2)^2 x depth

A square or rectangular planter is a box:

cubic inches = length x width x depth

Then everything divides by 1,728, the number of cubic inches in a cubic foot, and converts from there:

cubic feet    = cubic inches / 1728
US dry quarts = cubic feet x 25.714
US gallons    = cubic feet x 7.4805
liters        = cubic feet x 28.317

The dry quart is exactly 67.200625 cubic inches by definition, which is where 25.714 comes from: 1,728 / 67.200625.

A worked example

A twelve inch round pot, filled ten inches deep:

• Radius: 12 / 2 = 6 inches
• Volume: pi x 6 x 6 x 10 = 1,131 cubic inches
• In cubic feet: 1,131 / 1,728 = 0.654 cubic feet
• In dry quarts: 0.654 x 25.714 = 16.8 US dry quarts
• In liters: 0.654 x 28.317 = 18.5 liters

A 25 dry quart bag fills one of those pots with some left over. Four of them need three bags.

Common containers

Tips and common mistakes

Pots taper, so buy for the calculated figure and expect to use a little less. The cylinder maths treats a pot as straight-sided. A typical tapered pot holds ten to twenty per cent less than its top diameter suggests.

Do not fill to the rim. An inch of freeboard is the difference between watering a pot and washing half of it onto the patio.

Big containers are easier than small ones. The larger the volume, the longer it takes to dry out, and drying out is what kills most container plants. If you are choosing between two sizes, take the bigger one.

Buy mix by volume, not by weight. Bag weights vary with how wet the mix is. The quart or liter figure is the one that tells you whether it will fill your pots.

Nursery "gallon" sizes are trade sizes, not liquid gallons. A #1 "one gallon" pot holds roughly 0.7 US liquid gallons. The presets above use measured dimensions rather than the label.

Sources

• Gardeners can reuse, recycle last year's potting soil — Oklahoma State University Extension (David Hillock), 2023. Nutrient depletion, compaction, disease precautions, and the "plants that don't require rich soil" guidance.
• Growing Media (Potting Soil) for Containers — University of Maryland Extension, updated March 2026. The 50:50 reuse recipe, garden soil density in containers, and soilless mix composition.
• Types of Containers for Growing Vegetables — University of Maryland Extension, updated February 2023. Container depth ranges by crop size and the drainage-layer warning.
• Gardening Myth Busters — Mississippi State University Extension. The gravel-drainage myth.