Container and growing pots come in a variety of sizes. In the United States nursery and garden centers sell pots by the size in inches and gallons. In the United Kingdom, Europe, and most of the rest of the world containers are sold by the size in centimeters and liters.
There are few standards when it comes to container sizes and volumes. To determine the size of a pot measure across the top from one side to the other to determine how many inches or centimeters it is. However, because some pots are long and others are squat and because the sides of some pots are straight and others are tapered, the volume can vary.
When it comes to filling a pot with soil estimating how much soil you need is an approximation. Often the volume of a container is measured in liquid quarts or liters, but, of course, when you purchase soil you are purchasing dry (potting soil is not liquid). A dry quart is equal to about 1⅛ liquid quarts. When deciding how much soil to purchase it’s best to factor soil compression that commonly results from moistening and pressing the soil into the pot. Soil compression can add another 15 to 20 percent dry soil to the container. Also take into consideration that when you transplant a plant from one container to another, you will be moving some soil around the roots of the plant.
Take notes on the pots you have and the soil they require. In short order, you will have a realistic estimate of how much soil you will need to purchase when potting plants.
Standard Clay Pots and Plastic Nursery Pots
This chart will help you translate container sizes for standard clay pots and black nursery pots and give you an approximation of how much soil each will require (again these are dry soil measures):
4 inch pot (10 cm) = 1 pint (0.5L)
5-6 inch pot (13-15 cm) = 1 quart (1L) = 0.03 cu. ft.
7-8 inch pot (18-20 cm) = 1 gallon (4L) = 0.15 cu. ft.
8.5 inch pot (22 cm) = 2 gallon (7.5L) = 0.3 cu. ft.
10 inch pot (25 cm) = 3 gallon (11L) = 0.46 cu. ft.
12 inch pot (30 cm) = 5 gallon (19L) = 0.77 cu. ft.
14 inch pot (36 cm) = 7 gallon (26L) = 1 cu. ft.
16 inch pot (41 cm) = 10 gallon (38L) = 1.5 cu. ft.
18 inch pot (46 cm) = 15 gallon (57L) = 2.3 cu. ft.
24 inch pot (61 cm) = 25 gallon (95L) = 3.8 cu. ft.
30 inch pot (76 cm) = 30 gallon (114L) = 4.6 cu. ft.
10 inch (25 cm) = 5.5 dry quarts (6L) = 0.21 cu. ft.
12 inch (30 cm) = 7.9 dry quarts (8.4L) = 0.3 cu. ft.
14 inch (36 cm) = 13.9 dry quarts (15.3L) = 0.5 cu. ft.
8 inch (20 cm) = 1.9 dry quarts (2L) = 0.07 cu. ft.
10 inch (25 cm) = 3.7 dry quarts (4L) = 0.14 cu. ft.
12 inch (30 cm) = 5.5 dry quarts (6L) = 0.21 cu. ft.
14 inch (36 cm) = 8.4 dry quarts (9.2L) = 0.29 cu. ft.
16 inch (41 cm) = 12.0 dry quarts (13.2L) = 0.46 cu. ft.
18 inch (46 cm) = 18.8 dry quarts (20.7L) = 0.73 cu. ft.
21¾ inch (55 cm) = 31.2 dry quarts (34.3L) = 1.21 cu. ft.
12 inch (30 cm) = 3.8 dry quarts (4.1L) =0.14 cu. ft.
16 inch (41 cm) = 7.3 dry quarts (8L) = 0.28 cu. ft.
20 inch (51 cm) = 9.4 dry quarts (10.3L) = 0.36 cu. ft.
12 inch (30 cm) = 11.2 dry quarts (12.3L) = 0.48 cu. ft.
15 inch (38 cm) = 23.0 dry quarts (25.3L) = 0.89 cu. ft.
24 inch (61 cm) = 11.7 dry quarts (12.8L) = 0.45 cu. ft.
30 inch (76 cm) = 15.6 dry quarts (17.1L) = 0.6 cu. ft.
36 inch (91 cm) = 19.7 dry quarts (21.6L) = 0.76 cu. ft.
5 gallon = 14 dry quarts (15.4L) = 0.54 cu. ft.