Potting Soil Calculator

A potting soil calculator converts the inside dimensions of a container into the volume of mix needed to fill it. Potting mix is sold by volume rather than by weight, because its weight swings widely with moisture content, so the answer is given in the units printed on bags and pots: gallons, dry quarts, liters, and cubic feet.

Interior dimensions are the ones that matter. The wall of a ceramic or concrete pot can take an inch or more off each side, which removes a large share of the volume of a small pot. A rectangular planter is length times width times fill height. A straight-sided round pot or bucket is a cylinder, pi times the radius squared times the height, where the radius is half the inside diameter. Most nursery and terracotta pots narrow toward the base and form a frustum of a cone, so both the top and bottom diameters are needed. Height means fill depth, usually an inch below the rim to leave room for watering. Dimensions are in inches or centimeters and accept decimals, fractions such as 3/4, and mixed numbers such as 5 1/2. The container count multiplies one pot's volume across a set of identical pots.

Centimeters are divided by 2.54 so that every shape is computed in cubic inches. The frustum volume is pi times height divided by 3, times (r1 squared + r1 x r2 + r2 squared), with r1 and r2 the top and bottom radii. Cubic inches divide by 231 for US gallons, by 61.0237 for liters, by 1,728 for cubic feet, and by 67.2006 for US dry quarts. The dry quart is the unit printed on bags of potting mix in the United States, 25.7 of them to a cubic foot; using the 57.75 cubic inch liquid quart instead would overstate the bag count by about 16 percent. Bags needed is the total in dry quarts divided by the bag size, rounded up to a whole bag. Settings holds the bag size, a price per bag for a cost estimate, and a step-by-step view of each formula.

The result is the loose volume that fills the container once. Fresh mix compresses after the first thorough watering, so containers are often topped up afterward, and a root ball or a drainage layer already in the pot reduces what is needed. Treating a tapered pot as a cylinder of its top diameter overstates the volume: for a pot 14 inches across the top, 10 inches at the base, and 12 inches tall, the cylinder estimate is 35 percent too high.

A tapered pot 14 inches across the top, 10 inches across the base, and 12 inches tall has radii of 7 and 5 inches. The bracket is 49 + 35 + 25 = 109, and pi x 12 / 3 x 109 = 1,369.7 cubic inches. That is 1,369.7 / 231 = 5.93 gallons, 1,369.7 / 67.2006 = 20.38 dry quarts, 22.45 liters, or 0.79 cubic feet, so three 8-quart bags fill it (20.38 / 8 = 2.55, rounded up).

Knowing the volume a container requires prevents overbuying bags at the garden center and avoids mid-project trips for more material. Volume estimation applies to indoor houseplants, outdoor patio containers, raised bed liners, and commercial greenhouse operations.

• Patio Container Gardens: Identical pots on a deck or patio are one calculation multiplied by the count. Five 14-inch round pots filled 12 inches deep hold about 40 gallons, or 137 dry quarts, which is nine 16-quart bags.
• Window Box Planters: A window box is a rectangle. A 36 x 8 x 8 inch interior holds 2,304 cubic inches, about 10 gallons or 34 dry quarts.
• Repotting Houseplants: The full volume of the new pot is the most mix a repotting can take, and the root ball fills part of it, so the bag count for a repotting is an upper bound. Tapered ceramic pots are common indoors, so the frustum shape gives a closer estimate than treating them as cylinders.
• Raised Bed Liners: Fabric grow bags and raised bed inserts are typically rectangular, so their interior dimensions multiplied by the number of beds give the season's potting mix order.
• Greenhouse and Nursery Operations: One pot's dimensions multiplied by a count in the hundreds gives the mix order for a run of identical pots. Two hundred straight-sided pots 6 inches across and 6 inches deep need about 19.6 cubic feet, or 505 dry quarts.
• Vertical Garden Systems: Stackable planter towers use small tapered cups at each level, and the frustum volume of one cup times the number of planting slots gives the total for the tower.
• Budget Planning: A price per bag turns the bag count into a cost before shopping, and running the same containers with different bag sizes shows the cost per dry quart of each option.