Growing food

How to Measure Plant Pots (Size, Diameter, Volume)

It’s essential to select the right size pot for the plant you want to grow in it. If the pot is too small, the roots will be too confined, and the plant won’t grow as best as it could. Therefore, you need to know how to measure plant pot sizes.

Plant pots are usually referred to by their diameter measurement or by their volume. To measure the diameter, you just place a ruler across the top rim and take the measurement from one side to the opposite one. To measure the volume, you need the diameter and height of the pot and then use a simple equation to determine the volume.

How are Plant Pots commonly Sized?

There are two basic ways that all plant pots are sized, by diameter or by volume. The diameter of the pot is how wide it is at the top. This is a relatively simple measurement and is measured in the same way, no matter the shape or size of the pot.

Sometimes, pots are also measured by their volume. This involves doing some basic calculations to get the volume based on the measurements of the pot. To work out the volume, you’ll need to measure the diameter and depth of a round pot or the width, height, and length of a rectangular or square pot.

The reason that volume is as important as the diameter is to ensure that there is enough space for the roots of the plant to develop easily without being too confined by the pot space.

Many new pots that you can buy will most likely have the correct size on them, either on the bottom or on the side. However, even this can get confusing because, in general, smaller pots will be sized by their diameter while larger pots will be sized by their volume.

That’s why it’s important for you to know how to measure the pots yourself so that you get the size that you need for the plants you want to grow in them.

Here are some more detailed explanations on how to measure both the diameter and the volume of plant pots.

herbs growing in pots

How to Work Out the Diameter of any Plant Pot

No doubt you’ve seen instructions that say you should use a 10-inch pot or maybe a 12-inch pot. But, what does this refer to these measurements refer to is the diameter of the pot, and this is extremely easy to measure. Here’s how:

  • Place a ruler or tape measure across the top rim of the pot at the widest part.
  • The diameter is the measurement from one side of the pot to the other.

Measuring the pot diameter is easy. However, to work out the volume, you’re going to have to do a little more calculation.

How to Measure the Volume of a Round Pot

To measure the volume of a round pot, you’re going to need the diameter of the pot and the height. Here’s how to work out the volume:

  • Measure the diameter of the pot.
  • Measure the height of the pot.
  • Use the following formula to work out the volume:
    Height x Pi x Radius2 (where Pi = 3.14 and Radius = half of the diameter).

Let’s look at an example. Say we have a perfectly round pot that has a diameter of 10 inches (radius of 5) and a height of 15 inches. Here’s the formula we would use:

15 x 3.14 x 52 = 15 x 3.14 x 25 = 1177.5 cubic inches

Then, we know that 1 gallon equals 231 cubic inches. So we need to divide 1177.5 cubic inches by 231 to calculate the number of gallons:

1177.5 / 231 = 5 gallons (approximately)

If you want to know the volume in liters, then you just multiply the gallons by 3.8 (in our case, 5 x 3.8 = 19 liters).

How to Measure the Volume of a Rectangular Pot

If you happen to have a rectangular pot, for example, one that is square at the top but taller in height, you need to use a slightly different formula:

  • Length x Width x Height

So for this, you need to take your ruler or tape measure and measure the length of the pot as well as the width and the height or depth.

Let’s use an example of a square pot first. You have a 6-inch square pot with a height or depth of 10 inches, so our formula would be:

6 x 6 x 10 = 360 cubic inches (or 1.5 gallons or close to 6 liters)

When doing these calculations, we generally round up or down to make it easier.

Here’s another example of a rectangular pot that is not square. The pot is 15 inches long, 6 inches wide, and 15 inches high. Here’s our formula:

15 x 6 x 15 = 1350 cubic inches (or almost 6 gallons or just over 22 liters)

By looking at these results, you can see that the volume is vastly different depending on the actual measurements of the pot.

Alternatively, if the thought of doing calculations to work out the volume sends you running to the hills, you can simply fill your chosen pot with soil and then get a measuring jug and empty the contents of the pot into the jug until you have a rough idea of how many gallons of soil the pot will hold.

To make things even easier, you can use an online calculator to calculate the volume for you. All you have to do is enter the measurements. This is particularly useful if you have a tapered pot because that calculation can be quite tricky.

How Do You Select the Correct Size Pot For Your Plant?

Now that you know exactly how to measure a plant pot, whether you want the diameter of the pot or the volume, you need to know how to select the correct pot for your plant.

Bear in mind that it’s not easy to give an exact size for every plant, but we can give you some general guidelines so that you can determine what size pot you should use. Primarily, it depends on the type of plant that you’re growing and the general growth habit of the plant.

Here are some tips:

  • For vegetables that are fairly shallow rooted, such as lettuce and spinach, you don’t need a tall pot. Just select a pot diameter that will give you enough space to grow the quantity that you want.
  • For root vegetables such as carrots and beetroot, you need a pot with extra height. Consider how long carrots normally grow and choose a pot that is of similar height. For beetroot, remember that these also grow in width, so you want a pot that is both wide and deep.
  • Fruit trees grown in pots need enough depth to develop a good strong root system. In general, the depth of the pot will determine how tall your tree is able to grow. The roots have to supply the tree with the moisture and nutrients that it needs, so they need to have enough depth to grow freely.
  • If you’re growing flowering plants or shrubs in pots, the general guideline is to use a pot that is roughly 1 to 2 inches larger than the current root ball of the plant. That is both in width and depth. This will allow the plant to continue to grow while in its new pot.
  • Other than vegetables, plants grown in pots should be repotted at least every couple of years. Once again, you want to choose a pot that is around 1 to 2 inches larger than the current one. This will allow the plant to put on more lush growth and will help to keep the roots healthy.

On a side note, you can restrict the growth of a plant by keeping it in the same pot. However, you’ll also need to trim it regularly to keep it healthy. Plus, the soil in the pot will need replacing at regular intervals.

Quick Guide to Some Standard Pot Sizes

To give you a head start on which size pot to select, here’s a general guide to standard nursery pot sizes.

Pot DiameterPot VolumeSuitable Plants
4-inch pot0.125 gallonsSeedlings
5- to 6-inch pot0.25 gallonsSmall succulents
7- to 8-inch pot1 gallonLarger succulents or annuals
10-inch pot3 gallonHerbs or annuals
12-inch pot5 gallonLettuce, spinach, strawberries
14-inch pot7 gallonLarger growing herbs or annuals
16-inch pot10 gallonSmall shrubs
18-inch pot15 gallonLarger vegetables such as tomatoes
24-inch pot25 gallonDwarf fruit trees or medium shrubs
30-inch pot30 gallonFruit trees or taller growing shrubs

Keep in mind that these are fairly standard guidelines, as the shape and height of the pot will affect the volume and the amount of soil available for the roots of the plants. You’ll also find that these standard sizes will vary from one manufacturer to the next.

Frequently Asked Questions

Are plant pots measured by height or diameter?

Plant pots are measured by diameter or by volume. Plant pots are not measured by their height.

How many gallons is a 12-inch pot?

A typical 12-inch pot is a 5-gallon pot.

How many gallons is a 10-inch pot?

A typical 10-inch gallon pot is a 3-gallon.

What is a 1-gallon pot size in inches?

A typical 1-gallon pot size is 7 inches.

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