Potting Soil Calculator

How much soil a pot holds

Potting soil is sold by volume, not weight, and the volume a pot holds is simple geometry. The confusion comes from the units on the bag: dry quarts on one label, cubic feet on another, pounds on a third. Those are not interchangeable, and the arithmetic between them is where most people get caught short.

A round pot is a cylinder, so the volume is π × radius² × depth. A rectangular planter is length × width × depth. That is the entire formula. The real work is converting the result in cubic inches into the units printed on the bag.

Below, the calculator does both jobs at once: it works out the volume of your container and tells you how many bags of each common size you would need to buy. Everything runs in your browser, nothing you type is uploaded or stored.

Dry quarts, cubic feet and what is in the bag

A US dry quart is 67.2 cubic inches. A cubic foot is 1,728 cubic inches. Divide one into the other and you get 25.71 dry quarts per cubic foot. That is the single number worth memorising.

So a 1-cubic-foot bag holds about 25.7 dry quarts, a 2-cubic-foot bag about 51.4 quarts, and a 32-quart bag is roughly 1.25 cubic feet. A 16-quart bag is a bit over half a cubic foot, 0.62 cubic feet to be exact.

One warning: some bags are sold by weight, 20 lb, 40 lb, and weight tells you nothing about volume. A heavy, moisture-retaining mix and a light peat-based mix can weigh different amounts for the same number of quarts. If the front of the bag only lists pounds, look for the quart or cubic-foot figure on the back.

Worked example: three 12-inch pots

Say you have three round pots, 12 inches across the top and 10 inches deep.

Radius is half the diameter, so 6 inches. Volume = 3.1416 × 6² × 10 = 1,131 cubic inches. Divide by 67.2 and each pot takes 16.8 dry quarts, which is 0.65 cubic feet.

You should not fill to the brim, though. Leaving 1 inch of headspace so you can water without it running over cuts the depth to 9 inches: 3.1416 × 36 × 9 = 1,018 cubic inches, or 15.1 dry quarts per pot.

For three pots that is 45.4 quarts. Your options are three 16-quart bags (48 quarts, about 2.6 quarts spare) or two 1-cubic-foot bags (51.4 quarts, about 6 quarts spare). Two 16-quart bags would come up short, 32 quarts against 45.4 needed. That is the classic potting-soil mistake: a 16-quart bag looks enormous until you pour it into a 12-inch pot and it barely reaches halfway.

Worked example: a long planter or raised bed

Now a rectangular planter, 48 inches long, 18 inches wide and 12 inches deep. Volume = 48 × 18 × 12 = 10,368 cubic inches. Divide by 1,728 and you get exactly 6.0 cubic feet, or 154.3 dry quarts.

With 2 inches of headspace the depth drops to 10 inches and the volume falls to 8,640 cubic inches, 5.0 cubic feet, or 128.6 dry quarts. A full cubic foot less, which at typical bag prices is real money.

To buy it: three 2-cubic-foot bags (6.0 cu ft) or six 1-cubic-foot bags. If your store only stocks 32-quart bags you need five of them (160 quarts, about 5.7 quarts spare). Add 10% for settling and you are at roughly 170 quarts, so six 32-quart bags or three 2-cubic-foot bags with a little left over for top-dressing.

Headspace, settling and how much extra to buy

Two small adjustments stop you from running short.

• Headspace. Leave 1 to 2 inches between the soil surface and the rim. Water needs somewhere to pool, and the gap stops mix washing over the edge. The calculator has a headspace field that subtracts this for you.
• Settling. Fresh potting mix compacts 5 to 10% in the first few weeks as it drains and the peat or coir breaks down. Buying 10% extra means you can top off after a month without another store run.

Roots also need air, and a pot with no drainage holes is a swamp. If your container has no holes, drill some or add a 1-inch layer of coarse material in the base and water less often. For plants that hate wet feet, mix in 20 to 30% perlite by volume: a 16.8-quart pot wants about 4 quarts of perlite and 13 quarts of potting mix.

One more habit worth building: measure once and write the number on the pot. Next season you will know to buy one bag, not three.

Quick reference: how much soil common containers hold

These are full-depth volumes with no headspace subtracted, using the formulas above. Round pots are given as diameter × depth.

A 24-inch pot is where the numbers get startling: it takes more than four and a half cubic feet, which is nearly two 2-cubic-foot bags for a single container.

Potting mix, garden soil and topsoil are not the same

These three products sit on the same shelf and cost similar amounts, but swapping one for another changes how your plants grow.

• Potting mix is light, mostly sterile and built for containers. It drains fast and holds air around the roots.
• Garden soil is heavy field soil blended with compost. Put it in a pot and it compacts into a brick; it can also carry weed seeds and soil-borne disease.
• Topsoil is for filling low spots in the yard, not for containers.
• Raised bed mix sits between potting mix and garden soil and is usually the best value for a large bed.

Weight deserves a thought too. A cubic foot of potting mix weighs roughly 20 to 25 pounds dry and more once watered, so the 6-cubic-foot bed above is carrying around 150 pounds of wet soil. That matters on a balcony, a roof deck or any surface with a load limit.

For soil health, drainage and container basics, Penn State Extension and Illinois Extension both publish free guides, and the EPA's home composting page explains how to make part of your own mix at home.