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How Much Potting Soil Do I Need for My Plant? Types, Facts & FAQ

woman pouring store bought potting soil to a pot

Determining how much potting soil you need depends largely on your container. Then, you’ll have to consider how much you’ll need to fill the container. You probably don’t want to fill the container up, as this will lead to spillage. Plus, the plant will take up some room in the pot. Some plants will take up more room than others, of course.

All of these factors make it hard to determine how much potting soil you’ll need exactly. The only way to determine the exact amount is to get accurate measurements and run somewhat complicated math formulas.

Of course, we expect that you don’t want to do this.

Therefore, you can use the volume of the pot as a guideline. However, understand that you will probably need less than this. Most pots have their volume listed since this metric determines how much soil is needed. However, others might not. In this case, you may have to run a formula or two to get an accurate measurement.

You’ll also need to consider the shape of your pot. Different pot shapes have different formulas to determine their volume.

garden flower divider

Formulas for Determining Volume

If you can’t find a volume metric on your pot, you’ll need to determine it yourself. This requires utilizing various formulas, which are based on the shape of the pot. Some of these formulas are more complicated than others. However, they are all pretty straightforward and can be typed into your average phone calculator.

Here are the different formulas for each common pot shape:
  • Rectangular: Volume = Depth x Length x Width
  • Round: Volume = π x R² x depth, R is the radius of the pot.
  • Flower Pot: Volume = (1/3) x π x depth x (r² + r x R + R ²), r is the radius of the bottom of the pot, and R is the radius of the top of the pot
a person scooping potting soil with trowel
Image Credit: Teona Swift, Pexels

As you can see, flower pots are the hardest to determine the volume for. However, with our modern phones, these formulas are pretty easy. You can typically type them in—ensure that you add all the parentheses to ensure that the calculator correctly calculates the solution.

Of course, you’ll need to do these formulas for all your flower pots and then add them. This should give you the number of square inches of soil you need. One cubic inch is about 0.0173 quarts. Therefore, you can multiply your final number by 0.0173 to figure out how many quarts you need.

How Can I Calculate the Potting Soil Needed for My Terrace Garden?

Terrace gardens are a bit more complicated to determine, as there are often multiple pots. However, modern calculators can easily calculate how much potting soil you’ll need for your whole garden. We’ll work through how to do this step-by-step, including exactly what you need to type into a calculator.

First, you need to determine the volume of each pot. If you have a bunch of the same pots, this is easier. Otherwise, you’ll have to calculate each pot separately. Grab the appropriate formula from above and use it to calculate the volume. Do this for every pot. If you have a bunch, you’ll probably want to write down the answers as you go.

Next, add all the volumes together. This will give you the cubic inches of soil you need. Sadly, we don’t buy soil in cubic inches, so you’ll need to convert it to quarts. To do this, multiply your answer by 0.0173. This will give you the number of quarts you need. There are 4 quarts in a gallon. You can divide your final answer by 4 to determine how many gallons you need if you’d rather have the answer in that measurement.

garden flower divider

Standard Pot Sizes and Volume

Many pots come in standard sizes. These sizes all have the same volume, meaning they require the same amount of soil. Luckily, if you have one of these standard pots, you don’t need to do any calculations. Here is a quick breakdown of standard pot shapes and sizes and the amount of soil you’ll need to fill them.

You can get the size of your pot by measuring the top. Pots should also have their size listed on them or on the wrapping.

Circular Pots

Size Amount of Soil
4 in 1 pint
5–6 in 1 quart
7–8 in 1 gallon
8.5 in 2 gallons
10 in 3 gallons
12 in 5 gallons
14 in 5 gallons
16 in 10 gallons
18 in 15 gallons
24 in 25 gallons
30 in 30 gallons
close up of woman in gloves pouring soil to flower pot at home
Image Credit: Syda Productions, Shutterstock

Square Planters

Size Amount of Soil
12 in 11.2 quarts
15 in 23 quarts

Window Boxes

Size Amount of Soil
24 in 11.7 quarts
30 in 15.6 quarts
36 in 19.7 quarts
window box
Image Credit: RobbyFo, Pixabay

Oval Planters

Size Amount of Soil
12 in 3.8 quarts
16 in 7.3 quarts
20 in 9.4 quarts

Sadly, we can’t account for all potential pot sizes out there. Therefore, if your pot isn’t included in the tables above, you’ll need to use the volume equations we listed above to help determine how much soil you need.

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Do You Fill the Whole Planter with Soil?

You don’t need to fill an entire planter with soil. You want there to be some extra room on the top to prevent the soil from overflowing when you water the plant. Leaving a lip can prevent messes in the future. You also have to account for the space the plant takes up. Many plants will have sizable root balls, which take up quite a bit of space. Therefore, you won’t have to purchase soil for the whole pot unless you’re starting from seeds.

garden flower divider

Conclusion

Determining how much soil you need for a pot requires some math. You’ll need to determine the volume of the pot or planter, which will give you the amount of soil it will hold. Of course, this is the amount completely filling the pot would require. If you’re planting grown plants, you’ll need less than this due to their roots.

Plus, even if you’re starting from seeds, you don’t want to fill the pot all the way up. Doing this would lead to unnecessary messes and overflow.

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Featured Image Credit: progressman, Shutterstock

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