How many quarts of soil does a 12 inch pot need?

About 17 US dry quarts filled to ten inches deep, which is two thirds of a 25-quart bag. A 10 inch pot takes around 10 dry quarts and a 14 inch pot around 27. Those are cylinder figures; a tapered pot holds ten to twenty per cent less.

Why do bags say quarts instead of cubic feet?

Because potting mix in the United States is sold by dry quart, a unit of volume for dry goods, while bagged soil and compost for beds are sold by cubic foot. One cubic foot is 25.71 dry quarts. Outside the US both are sold in litres, which avoids the confusion entirely.

Is a 2 cubic foot bag of soil the same as 50 quarts?

Close. Two cubic feet is 51.4 US dry quarts, so a 50-quart bag is a little under two cubic feet. If a bag is labelled in liquid quarts — which some are, incorrectly for this purpose — 50 of those is only 1.67 cubic feet, which is a meaningful difference when you are buying five bags.

How much potting soil a container needs

By Muzamil Ali · Last updated September 17, 2026

The volume of common pots in dry quarts and litres, why dry and liquid quarts differ, and how to avoid under-buying.

This is a short, practical question with one genuine trap in it. Container volumes are easy arithmetic, and the unit that potting mix is sold in is not the unit most calculators use.

The trap: dry quarts, not liquid quarts

Bagged potting mix in the United States is sold in US dry quarts. That is a distinct unit from the liquid quart most people think of.

One US dry quart is exactly 67.200625 cubic inches.
One cubic foot is 1,728 cubic inches.
So one cubic foot is 25.71 dry quarts.

The figure 29.92 appears on a great many websites, and it is the liquid quart conversion. Use it and you will believe a container holds about 16% more than it does, then buy 16% too few bags and make a second trip.

Everywhere outside the US the unit is litres, and one cubic foot is 28.32 litres. No ambiguity.

The arithmetic

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 divide by 1,728 for cubic feet, and convert from there:

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

Measure the diameter across the top of the soil, and use the soil depth rather than the full height of the pot — leave an inch of freeboard so watering does not wash mix over the rim.

Worked example

A twelve-inch round pot, filled ten inches deep:

Radius 6 inches, so volume = pi x 36 x 10 = 1,131 cubic inches
1,131 / 1,728 = 0.654 cubic feet
0.654 x 25.714 = 16.8 US dry quarts
0.654 x 28.317 = 18.5 litres

So one 25-quart bag fills that pot with a little left over. Four of them need three bags.

Common containers

Container	Soil depth	Dry quarts	Litres	Cubic feet
6 in round pot	5 in	2.1	2.3	0.08
8 in round pot	7 in	5.2	5.8	0.20
10 in round pot	9 in	10.5	11.6	0.41
12 in round pot	10 in	16.8	18.5	0.65
14 in round pot	12 in	27.4	30.2	1.07
16 in square planter	14 in	53.3	58.7	2.07
24 in window box, 7 in wide	6 in	15.0	16.5	0.58
36 in grow bag, 14 in wide	9 in	67.5	74.4	2.63
Half barrel, 26 in across	16 in	126	139	4.91
Nursery #1 trade gallon	6.5 in	3.2	3.6	0.13
Nursery #5 trade gallon	11 in	15.5	17.1	0.60

Two things to notice. The half barrel needs five 25-quart bags, which surprises people at the checkout. And the nursery trade sizes are not liquid gallons at all — a "one gallon" pot holds about 0.8 dry quarts short of a real gallon.

The container calculator will do any size, including tapered estimates and multiple containers at once.

Practical notes

Pots taper, so buy for the calculated figure and expect to use 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. That margin is useful anyway, because mix settles.

Do not put gravel in the bottom. It does not improve drainage — water will not cross from fine material into coarse until the fine material is saturated, so a gravel layer raises the waterlogged zone rather than lowering it. A drainage hole and a free-draining mix is what a pot needs.

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

Reusing old mix. Old mix has lost structure and nutrients and may carry disease. Mixing the top half with an equal volume of fresh mix and compost works well for annuals; start fresh for seedlings.

One big container beats two small ones. The larger the volume, the slower it dries out, and drying out is what kills most container plants. If you are choosing between sizes, take the bigger one.

For raised beds, switch units

Beds are big enough that quarts stop being useful. Bagged soil and compost for beds are sold by the cubic foot, and bulk by the cubic yard — 27 cubic feet. A four by eight foot bed twelve inches deep takes 32 cubic feet, which is 22 bags at 1.5 cubic feet, or about 1.2 cubic yards loose. The raised bed calculator handles that end.

The short version

Volume is pi r squared times depth for a round pot and length times width times depth for a square one, divided by 1,728 for cubic feet. Multiply cubic feet by 25.71 for dry quarts, which is the unit bags are sold in — not 29.92, which is liquid quarts. Measure to the soil line, not the rim, and buy the bigger pot.