Potting Soil Calculator: How Much Soil Do Your Pots Need?

How much potting soil for common pot sizes

A 6-inch pot needs about 2 quarts of potting soil, an 8-inch pot about 5, a 10-inch pot about 10.5 and a 12-inch pot about 18.5, if the pot is straight-sided, as deep as it is wide, and filled to an inch below the rim. Tapered pots hold roughly a quarter less.

I worked these out with the same geometry the calculator uses. Each pot is as deep as it is wide and filled to 1 inch below the rim; “tapered” means the bottom is three-quarters of the top width, which is my assumption for a typical nursery-style pot, not a measured standard. Shallow bowls, tall cylinders and pots with thick walls will come out differently, so measure yours if the number matters.

The quarter-less rule is the one that catches people.

A 12-inch tapered pot holds about 14 quarts, so a single 16-quart bag is enough, where the straight-sided version needs a second bag.

How the potting soil calculator measures a pot

Measure inside the pot, not the outside: width across the top inside the rim, width across the bottom, and depth from the inside floor to the rim. A thick terracotta or ceramic wall makes the outside measurement noticeably bigger than the space inside.

For a straight-sided round pot the volume is a cylinder. A tapered round pot is a truncated cone, and a square or rectangular planter that narrows toward the base is worked out the same way with length and width. The calculator fills from the floor up to your chosen gap below the rim, so the narrow part at the bottom is counted correctly instead of averaged.

The gap matters for watering. Washington State University Extension says that once the mix has settled, it should sit about an inch or so below the rim to leave room for water, and the calculator starts with that inch (2.5 cm in metric).

Quarts, liters and cubic feet on the bag

Potting mix bags in the U.S. use dry quarts, which are bigger than the liquid quarts on a milk carton. NIST defines the dry quart as 67.2 cubic inches, against 57.75 for a liquid quart, which is why an 8-quart bag also reads about 8.8 liters. Larger bags switch to cubic feet. One cubic foot is about 25.7 dry quarts or 28.3 liters, so a 1-cubic-foot bag holds a little more than one and a half 16-quart bags, and a 2-cubic-foot bag fills about five straight-sided 10-inch pots. Metric bags in liters convert directly: 1 liter is 61 cubic inches.

Mix settles, so round the bag count up rather than down. One grower on r/containergardening filled six or seven 7-gallon grow bags with about four 2-cubic-foot bags and said they could have used more; seven bags that truly hold 7 gallons come to about 6.6 cubic feet, or 3.3 of those bags, before any settling. Another reply in the same thread summed it up:

“Buy more than you think you need cos it does settle and it’s good to topdresss as the plant grows.”
– r/containergardening, February 2025 (5 upvotes)

Going up a pot size

Colorado State University’s PlantTalk advice is to choose a pot only one to two inches larger in diameter than the current one. Even that small step adds a lot of soil: from the chart above, a 6-inch pot holds about 2.1 quarts and an 8-inch pot about 5.2, more than double, and going from 8 to 10 inches doubles it again.

The soil you buy for the move is the difference, because the root ball comes along. Repotting from a 6-inch pot into a straight-sided 8-inch pot takes about 2.7 quarts once the root ball is subtracted, and from an 8-inch into a 10-inch about 4.5.

CSU says to set the plant at the same level it sat in the old pot, slightly below the lip, and fill around the sides. A tip from r/houseplants makes that easier:

“When repotting, take the plant out of its current pot and put that pot on top of enough soil in the new pot to set the plant’s crown where you want it. Then add soil around the inner pot (a spoon and chopsticks come in handy). Press to the sides a bit to hold the soil in place, then gently pull out the inner pot.”
– r/houseplants, March 2024 (13 upvotes)

When you need less soil than the pot holds

Repotting needs less than the full pot volume because the old root ball takes up space. Open the repotting option and enter the old pot’s size; the calculator subtracts it as a cylinder, which overstates the root ball from a tapered old pot, so the bag count stays on the safe side.

A big decorative container can also take a filler. The Master Gardeners of Northern Virginia suggest sealed, empty plastic bottles at the bottom of a large or deep container to cut the amount of mix you need, and they advise against Styrofoam because roots can grow into it. Enter the filler depth and the calculator starts the mix above it.

What they don’t suggest is a gravel layer for drainage.

The same article quotes Linda Chalker-Scott, an extension horticulturist at Washington State University: “Gravitational water will not move from a finely textured soil into a coarser material until the finer soil is saturated.” A gravel layer leaves the mix above it wetter, not drier, so drainage holes do that job and the gravel can stay in the yard.

For which mix to put in, the right recipe depends on the plant. I’ve written up the mixes for snake plants, pothos and ZZ plants, and the step-by-step for repotting a snake plant shows how much the root ball fills.

Potting soil questions