How to Measure Plant Pots Size (Step by Step)

Learning how to measure plant pot size will help you avoid any costly mistakes when buying planters for your garden or houseplants.

But with all the different sizes, shapes and materials available, how do you get the right size and shape of pot for your plants to thrive?

Well, to help you navigate this often overwhelming process, we will cover the three important measurements used to determine a suitable container – diameter, height and volume.

I’ll then share tips on how to ensure your new pot is just right for your plant.

Let’s dive right in;

How to Measure Plant Pots

To measure plant pots, hold a ruler on one end of the pot and move it through the other side to the widest point for round-shaped pots. If your planter pot is square or rectangular, determine the length and width. The depth is measured from the bottom to the top.

Plant Pot Sizes Guide: How to Measure Pot Size

It’s easy to think that a few centimeters won’t matter. But it can make a big difference when it comes to growing your plants.

Plants need the right amount of space and soil to grow properly. And if your pot is too small or too big, it can be detrimental to the health of the plant.

How are Plant Pots Measured?

Most people measure plant pot sizes in two primary ways;

• Volume
• Diameter.

You can get the pot’s diameter by measuring the widest point at the top in inches or centimeters.

The most common pot sizes are small (4-6 inches or 10-15 cm), medium (7-10 inches or 18-25 cm) and large (11-14 inches or 28-35 cm).

The volume of your planter pot is determined by calculating the size of its interior, which is usually given in liters or gallons.

For round pots, diameter and depth are two factors that determine the volume. Whereas length, width and height are the three primary factors for square or rectangular pots.

1. How to determine pot size for a plant using the diameter

You’ve seen instructions telling you to use a 12-inch or even 10-inch pot. Well, those measurements always refer to the pot’s diameter and NOT height.

Here’s how to manually measure a pot diameter

• Place a tape measure or ruler across the pot’s top rim at its widest part/point. 
• The measurements of the pot’s one side to the other is the diameter.

2. How to Measure Planter Size Using Volume

This approach is largely dependent on the plant pot shapes.

Luckily, I have included the formulas for the most commonly used planter shapes.

– How to Measure the Volume of a Round Cylindrical Pot

You’ll need the round pot’s height and diameter to calculate its volume. So, here’s how to work out the pot’s volume:

• Measure the round pot’s diameter (see the method above)
• Measure the round pot’s height.
• Use the formula below to work out the pot’s volume:

Pot’s volume = π × Pot radius² × height

π = 3.14

Radius = half the diameter.

Here’s an example:

Say we’ve got a round pot with the following measurements

• A 10-inch diameter.
• A 15-inch height. 

Use the formula:

Pot’s volume = π × Pot radius² × height

15 x 3.14 x 5²

15 x 3.14 x 25 = 1177.5 cubic inches.

Then, as we know, one gallon represents 231 cubic inches. Thus, dividing (1177.5 cubic inches) by 231 will help us get the gallon amount:

1177.5 / 231 = 5.1 gallons.

If you’d like to get the volume measurement in liters, multiply the gallon amount by 3.8. In our case, that’ll be 5.1 x 3.8 = 19.3 L.

– How do you find the volume of an oval Pot?

To calculate the volume of an oval planter, use the formula below;

Volume = π abh/4

• a= Largest radius
• b= smallest radius
• h= Height of the pot

Finally, divide the result by 4.

– How to Measure the Volume of a Rectangular Pot

When you’ve got a rectangular planter, use the formula below;

• Volume = Length x Width x Height

Rectangular planter dimensions explained

• Length = Length of the longest side
• Width = Length of the shortest side
• Height = Vertical height

For this, let’s use a square pot as an example. 

So, for this instance, take your tape measure or ruler and measure the pot’s length, width and height.

The square pot is 6 inches, while the height is 10 inches. Thus, our formula is:

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

When doing this calculation, generally, we round down or up to make it easier.

– How to Measure Flower Pots

Most flower pots are shaped like a truncated cone i.e, wider at the top than the bottom. They may even have a saucer-shaped dish underneath to protect the surface on which you place it.

So here are the basic flower pot dimensions you need to know;

R = Top radius of the flow pot

r = radius of the base of the flower pot

Depth – the height of the flower pot

And the formula for calculating the size of flower pots is;

Volume = (1/3) × π × depth × (r² + r × R + R²)

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

• A tall pot isn’t necessary for shallow-rooted vegetables like spinach and lettuce. Just pick a diameter that’ll give you growing space for your crops.
• You’ll need an extra tall planter for root vegetables like beetroot and carrots. Consider the carrots’ average growth and pick a planter of similar height. It would help to remember that beetroots grow in width. So, getting a beetroot pot that’s both deep and wide will be best. 
• Fruit trees growing in pots need adequate depth. Otherwise, they won’t develop a good and strong root system. The tree’s roots need to supply it with enough nutrients and moisture it needs. Thus, they’ll need a plant pot with adequate depth for their roots to grow freely.
• Using a pot roughly 1-2 inches deeper than the plant’s current rootball is a general rule. But that’s if you’re growing shrubs or flower plants in pots. Again, this rule applies to the pot’s width. That’ll let your plant continue growing in its new planter.
• Besides vegetables, you should repot plants in pots for at least a couple of years. Again, like before, pick a plant pot that’s around 1-2 inches larger than the old one. That’ll let your plant put on an extra-lush growth. Thus, keeping its roots healthy.

Quick Guide to Some Standard Pot Sizes (Table)

Here’s a quick guide to standard pot sizes for nurseries. It’ll help you pick a plant pot that’s just the right size for your new addition.

Plant pot measurements

Pot Diameter	Pot Volume	Suitable Plants
4-inch pot	0.125 gallons	Seedlings
5- to 6-inch pot	0.25 gallons	Small succulents
7- to 8-inch pot	1 gallon	Larger succulents/annuals
10-inch pot	3 gallon	Herbs or annuals
12-inch pot	5 gallon	Lettuce, strawberries, spinach
14-inch pot	7 gallon	Larger growing herbs/annuals
16-inch pot	10 gallon	Small shrubs
18-inch pot	15 gallon	Larger vegetables like tomatoes
24-inch pot	25 gallon	Dwarf fruit trees/medium shrubs
30-inch pot	30 gallon	Fruit trees/taller growing shrubs

Remember the above are standard guidelines since the pot’s height and shape may affect its volume. Also, the amount of soil used will depend on the type of plant, its size and growth rate.

For example, if you’re growing a large tree or shrub in a pot, it will need more soil than a small succulent.

It’s worth noting that these standard sizes may vary among manufacturers.

Rules of Thumb for Measuring Pot Sizes

Ensure the planter is big enough to drain properly

Drainage holes play a critical role in plants’ health. So, ensure your plant pots have them. Moreover, without proper drainage, you may overwater your favorite plants.

In matters of repotting, one should re-home their houseplants every two to four years. Remember, the planter should be 5-10 cm larger. To be sure, check with a local gardening or nursery center. 

When you do so, you’ll get some crucial repotting advice when buying your plants.

Measure your space

There’s nothing worse than buying a pot and realizing it won’t fit within your space. To avoid such, always measure your interior or garden dimensions. Doing that ensures you get a plant container that’ll fit your area. 

Apart from that, consider your plant’s full-grown height and width. So ensure you get the tight-fit measurements. 

Generally, pots have small lips that add a few centimeters to their length. In some cases, your indoor pots may need saucers to harvest excess water. But that’s if you go for drainage holes.

Different Sizes of Plant Pots

Small pots

These pots are usually less than 30 cm in diameter and 10 L in volume. So, you have to consider shallow pot plants. The good thing is these sizes make perfect planters for tables and desks. Besides, this pot size will suit most house plants well. And that’s whether:

• You plan on brightening up your reception desk using a tiny succulent garden.
• Or add some poinsettias to your favorite lobby during the festive season.

Medium pots

These pot types are better suited to be floor planters. They measure 30-40 cm in diameter and 10-30 liters in volume.

They’ll look pleasing within the corners of your office, an office that needs a green splash. Further, one can create a room/space divider using several medium-sized pots. By arranging them together, you can achieve that.

Large pots

At more than a 30-liter capacity and a 40-cm diameter, these pots are made for the following:

• Large plants whose roots need wide spaces.
• Trees whose roots need wide spaces.

The roots will only grow and spread out over time if there’s enough growing space. These large pots function best within the following:

• An outdoor setting.
• A large or open lobby.

Custom pots

If you’re struggling to find a pot that will fit your houseplants perfectly, why not try a custom planter? They can be found anywhere and they can help you create the landscape of your dreams.

Custom pots have so many great features that make them ideal for any situation.

Maybe you have a room with special measurements that you need to use, or your houseplant needs a certain depth of soil to grow well. No matter the reason, custom pots can help your room design look even better.

Remember that size matters. For instance, the ideal snake plant pot size should be around 18 inches in diameter.

Pots that are too big can cause root rot and other issues due to too much soil being present and retaining too much moisture.

On the other hand, pots that are too small won’t allow enough room for the snake plant’s roots to grow.

What are Trade Gallons? 

Trade gallons are volume units for standard plant pots in horticultural industries. A single trade gallon equals 2.8 L (0.75 US gallons) or three US liquid quarts. 

Some sources note that a single trade gallon equals 2.7 L or 0.71 US gallons.

I recommend a single-gallon pot if we were to go by the volume of the potting soil. So:

• Should you pick a one-labeled gallon pot in nurseries that hold only 0.664-gallon potting soil?
• Or, should you choose a plant container you can fit within a (whole gallon) of soil?

Work out the difference between a trade gallon and the regular gallon to solve that challenging factor. Use the following gallon formula:

• 1 trade gallon = 0.71 regular gallon

The best 6-inch pot size recommendation

This pot size allows for plenty of root space and drainage, which helps to ensure that your plants thrive. Additionally, this size pot is light enough to be easily moved around when necessary.

For those looking for an even larger size pot, we recommend our 8-inch diameter pots

Conclusion

When picking a pot size, always consider the mature size of your plant.

You don’t want your pot to be too small and cause your plant to become root bound. On the other hand, a large pot may cause the soil to become overly saturated, preventing oxygen from reaching your plants’ roots.

So, how are plant pots measured? The best way to determine an appropriate pot size is by using this rule; your pot diameter must be no less than one inch wider than the root mass of your plant.

With these tips in mind, go forth and pick out the perfect pots for all of your plants.