About the Potting Soil Calculator
This potting soil calculator tells you how much potting mix it takes to fill your containers. Choose a straight-sided round pot, a tapered round pot or a rectangular planter or window box, enter its inside dimensions in inches and how many you are filling, and the tool returns the volume in US dry quarts (the unit printed on most potting-mix bags), liters and cubic feet.
It is handy before a trip to the garden center, when repotting houseplants, setting up a balcony container garden or planting a run of window boxes. Because bags are sold in 8 qt, 16 qt and 1, 1.5 or 2 cubic-foot sizes, the calculator also rounds up to whole bags for the size you pick and shows a quick cost estimate.
Measure the inside of the pot, not the outside. The calculator leaves an inch of headspace below the rim by default so water does not spill over; adjust it to zero if you want the pot filled to the brim. Fresh mix settles a little after the first few waterings, so it is worth buying a few extra quarts.
With the default inputs, the potting mix needed is 60.6 dry qt. Change any value above to recalculate instantly.
How to use the potting soil calculator
- 1Pick the container shape that best matches your pot or planter.
- 2Measure the inside diameter (or length and width) and inside depth in inches.
- 3Set the headspace you want below the rim — 1 inch is typical.
- 4Enter how many identical containers you are filling.
- 5Choose the bag size sold at your store to see how many bags to buy.
Formula and method
The soil depth h is the inside depth of the pot minus the headspace you leave below the rim. A straight-sided round pot is a cylinder, a rectangular planter is a box, and a tapered pot is treated as a truncated cone (frustum) whose top diameter is the width at the soil surface and whose bottom diameter is the base of the pot. All dimensions are in inches, so the volume comes out in cubic inches.
Potting-mix bags in the US are labeled in dry quarts; one US dry quart is 67.2 cubic inches, one liter is 61.02 cubic inches and one cubic foot is 1,728 cubic inches (about 25.7 dry quarts). The volume per pot is multiplied by the number of pots and divided by the bag size, rounding up to whole bags.
- D
- Top inside diameter (at the soil surface for tapered pots), inches
- d
- Bottom inside diameter of a tapered pot, inches
- h
- Soil depth = pot depth − headspace, inches
- 67.2
- Cubic inches in one US dry quart
Worked examples
Four 12-inch round pots, 10 inches deep
With 1 inch of headspace each pot holds a 9-inch-deep cylinder of soil: π × 6² × 9 ≈ 1,018 cubic inches, or 15.1 dry quarts. Four pots need about 60.6 quarts (66.7 L), so four 16-quart bags cover it for $32.
Two tapered pots, 14 in top, 10 in bottom, 12 in deep
Soil sits 11 inches deep, where the pot is about 13.67 inches wide. The frustum volume is π × 11 × (13.67² + 13.67 × 10 + 10²) ÷ 12 ≈ 1,219 cubic inches, about 18.1 quarts per pot. Two pots need 36.3 quarts, which is two 1 cu ft bags.
36 × 8 inch window box, 8 inches deep
The window box holds 36 × 8 × 7 = 2,016 cubic inches of soil after 1 inch of headspace. That is 30 dry quarts or about 33 liters, so you need four 8-quart bags (32 quarts).
Frequently asked questions
How many quarts of soil does a 12-inch pot need?+
A straight-sided 12-inch pot filled 9 inches deep holds about 15 dry quarts. A typical tapered 12-inch nursery pot holds closer to 10–12 quarts because it narrows toward the base, so measure your own pot for an accurate figure.
How many quarts are in a cubic foot of potting soil?+
One cubic foot is 1,728 cubic inches, which equals about 25.7 US dry quarts or 28.3 liters. A 2 cu ft bag therefore holds roughly 51 quarts and a 1.5 cu ft bag about 39 quarts.
Should I fill the pot to the top with soil?+
No. Leave roughly 1 inch of headspace in small and medium pots and 1–2 inches in large ones so that water can pool briefly and soak in instead of running over the rim and washing soil away.
Do I need gravel in the bottom of the pot?+
Horticultural research has shown that a gravel layer actually raises the water table inside the pot rather than improving drainage. Use a container with drainage holes and fill it with potting mix all the way down instead.
Can I use garden soil instead of potting mix?+
Garden soil compacts in containers, drains poorly and can carry weeds and diseases. Potting mixes are built from peat or coir, bark, perlite or vermiculite to stay light and airy, which roots in pots need.