Flower pots come in all shapes and sizes. If you are looking at standard ceramic, clay or plastic planters, the size and volume may be marked on the side or bottom. Any pot you use for plants should have holes in the sides or bottom for drainage.
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If a round flower pot has a measurement marked in centimeters or inches, it probably refers to the pot's diameter. To confirm, stretch a tape measure or ruler from one side of the rim to the other to measure the diameter. If you need to know the depth of the pot, place the tape measure at the side of the pot, and pull the it straight down to the ground. On a square or rectangular planter, measure the distance between each opposite rim for the length and width.