How Are Planting Pots Measured? | Size Standards Explained

Planting pots are measured by the diameter of the top opening (rim to rim) in inches or by soil volume in gallons, with depth as a secondary measurement for soil capacity.

Walking into a nursery and facing a wall of pots labeled “4-inch,” “#1,” or “1 gallon” can feel confusing when none of the numbers seem to match. One wrong size means a root-bound plant or a pot that drowns the soil. The standard measurement system is simpler than it looks — once you know where the ruler goes and what each number actually refers to.

Where To Place The Ruler

The primary measurement for any round pot is the diameter of the top opening — the straight-line distance across the center from one inside rim edge to the other. This is not the outside lip or the widest part of the pot body. For square or rectangular planters, you measure the length and width of the inside opening instead.

Depth is the second measurement: from the inside bottom to the top rim. Exclude any feet, external water reservoirs, or decorative ridges that don’t hold soil. A pot listed as “6 inches” almost always refers to the top diameter, not the height. Garden Spots’ measuring guide confirms using the inside edge for both diameter and depth.

Standard Inch Sizes And What They Mean

Typical indoor pot sizes start at 2 inches and increase in 2-inch increments: 2, 4, 6, 8, 10 inches, and so on up to 15-plus inches for floor plants. Outdoor nursery pots can reach 60 inches. A plant described as a “10-inch plant” means it is growing in a pot with a 10-inch top diameter, not that the plant itself is 10 inches tall.

Gallon Pots Are About Volume, Not Diameter

When a nursery sells a “1-gallon” pot, it refers to the soil volume the pot holds — not a liquid gallon. These numbers use a “trade gallon” standard that is smaller than a true liquid gallon. The same pot diameter can vary in volume depending on its shape and depth, which is why checking depth matters for deep-rooted plants.

Here is how common inch diameters and nursery pot numbers line up with gallon volumes:

Notice the #5 container has a slightly smaller diameter than the #3 despite holding more volume — the #5 pot is taller and deeper, not wider. This is why depth cannot be ignored.

How Nursery Pot Numbers Work

Nursery pots stamped with a # sign followed by a number use a sizing system that roughly corresponds to gallons. A #1 pot holds about 3 quarts, a #3 holds about 3 gallons, and a #7 holds 7 gallons. The system is not perfectly standardized across manufacturers, so check the actual diameter or volume on the pot tag if root space is critical.

Regional Differences You Will Encounter

American nurseries typically use even-numbered inch sizes (4, 6, 8, 10), while European and some UK sellers often list odd numbers (7, 9, 11) or centimeters. A 7-inch European pot and an 6-inch US pot are not the same size — always look at the actual measurement, not the number sequence. Metric labels like 18cm refer to the top diameter in centimeters, and you still need to check base diameter and depth since pot shapes vary.

Measuring Depth The Right Way

Depth tells you the actual soil capacity and is the number most often misread. Place the ruler inside the pot against the bottom. Measure straight up to the top rim. Skip any external feet, attached saucers, or raised decorative bases — those add height but not usable soil space. A pot that is 8 inches tall but has 2 inches of feet holds only 6 inches of soil depth. Shallow pots under 12 inches may struggle to support deep-rooted perennials or shrubs without frequent watering.

How To Size A Pot For A New Cutting

Lay the cutting flat and measure the root length. Add 2 inches to that length — that is the minimum pot diameter you need. For example, a cutting with 1-inch roots gets a 3-inch pot. If that exact size does not exist, round up to the next available diameter. For fast-growing plants or pots already 10 inches or larger, add 3 to 4 inches to allow room for root expansion. Jumping up one pot size is sufficient for most houseplants; oversizing can lead to soggy soil and root rot.

Once you know the measurement method — inside diameter for width, inside depth for height, and trade gallons for volume — the numbers on any pot label become readable. The same ruler that measures a 4-inch starter pot also works on a 14-inch floor planter. Just keep the tape measure inside the rim and ignore the feet.