When planning a container garden, one of the most common questions gardeners ask is: how many cubic feet of soil for a 20 gallon pot? Whether you’re growing vegetables, flowers, herbs, shrubs, or small fruit trees, knowing the correct amount of soil helps you avoid waste, save money, and provide the ideal growing environment for your plants.
A standard 20 gallon pot requires approximately 2.67 cubic feet of soil when filled completely. However, several factors can influence the actual amount needed, including the container shape, soil type, drainage setup, and whether you leave space below the rim for watering.
This guide explains everything you need to know about calculating soil volume for a 20 gallon container, choosing the right soil mix, and avoiding common mistakes when filling large pots.
Quick Answer: How Many Cubic Feet of Soil for a 20 Gallon Pot?
The simple answer is:
20 gallons ÷ 7.48 = 2.67 cubic feet
Therefore, a standard 20 gallon pot holds approximately 2.67 cubic feet of soil.
For practical gardening purposes, it’s wise to round up slightly and purchase about 2.7 cubic feet of potting soil to ensure you have enough to fill the container completely.
Understanding Gallons and Cubic Feet
Before calculating soil requirements, it helps to understand how gallons and cubic feet relate to each other.
What Is a Gallon?
A gallon is a unit of volume commonly used in the United States. Garden containers are often labeled by gallon size because it provides a simple way to estimate root space.
Common container sizes include:
- 5 gallon pots
- 10 gallon pots
- 15 gallon pots
- 20 gallon pots
- 25 gallon pots
- 50 gallon containers
What Is a Cubic Foot?
A cubic foot measures volume in three dimensions.
Imagine a box that is:
- 1 foot long
- 1 foot wide
- 1 foot deep
The volume inside that box equals one cubic foot.
Potting soil, compost, mulch, and landscaping materials are usually sold by cubic feet.
Why Soil Is Sold in Cubic Feet
Manufacturers package soil according to volume rather than container size. This means gardeners often need to convert gallons into cubic feet to determine how much soil to purchase.
Understanding this conversion makes buying soil much easier.
How to Calculate Soil Volume for a 20 Gallon Pot
Fortunately, the calculation is straightforward.
Soil Conversion Formula
The standard conversion formula is:
Cubic Feet = Gallons ÷ 7.48
Using a 20 gallon pot:
20 ÷ 7.48 = 2.67 cubic feet
Therefore:
A 20 gallon container requires approximately 2.67 cubic feet of soil.
This formula works for any container size.
Soil Conversion Chart for Common Pot Sizes
The following chart can help gardeners estimate soil requirements for different container sizes.
| Pot Size | Soil Needed |
|---|---|
| 1 Gallon | 0.13 Cubic Feet |
| 3 Gallons | 0.40 Cubic Feet |
| 5 Gallons | 0.67 Cubic Feet |
| 10 Gallons | 1.34 Cubic Feet |
| 15 Gallons | 2.00 Cubic Feet |
| 20 Gallons | 2.67 Cubic Feet |
| 25 Gallons | 3.34 Cubic Feet |
| 30 Gallons | 4.01 Cubic Feet |
| 50 Gallons | 6.68 Cubic Feet |
Many gardeners keep this chart handy when planning container gardens.
How Many Bags of Soil Do You Need?
Most garden centers sell potting soil in bags ranging from 1 cubic foot to 2 cubic feet.
Here’s how many bags you’ll need for a 20 gallon pot.
Using 1 Cubic Foot Bags
Required soil:
2.67 cubic feet
Calculation:
2.67 ÷ 1 = 2.67 bags
You’ll need:
3 bags of 1 cubic foot soil
Using 1.5 Cubic Foot Bags
Calculation:
2.67 ÷ 1.5 = 1.78 bags
You’ll need:
2 bags of 1.5 cubic foot soil
Using 2 Cubic Foot Bags
Calculation:
2.67 ÷ 2 = 1.34 bags
You’ll need:
2 bags of 2 cubic foot soil
Most gardeners prefer buying a little extra because soil settles over time.
Does Container Shape Affect Soil Requirements?
Although gallon ratings represent volume, container shape can influence how soil is distributed.
Traditional Nursery Pots
These containers are deep and narrow.
Advantages include:
- Better root development
- Efficient watering
- Strong plant support
Most require approximately:
2.67 cubic feet of soil
Fabric Grow Bags
Fabric grow bags have become increasingly popular.
Benefits include:
- Better airflow
- Reduced root circling
- Improved drainage
A 20 gallon grow bag typically requires the same amount of soil:
Approximately 2.67 cubic feet
Decorative Planters
Decorative pots often contain thicker walls or built-in reservoirs.
As a result, some may require slightly less soil than their rated volume suggests.
Always verify the manufacturer’s specifications when possible.
Best Soil Mix for a 20 Gallon Pot
Choosing the right soil is just as important as using the correct amount.
A quality container mix should provide:
- Good drainage
- Adequate moisture retention
- Proper aeration
- Balanced nutrients
Recommended Potting Mix Recipe
A high-quality blend may contain:
- 60% premium potting soil
- 20% compost
- 10% perlite
- 10% coco coir or peat moss
This combination promotes healthy root growth while preventing compaction.
Why Potting Mix Is Better Than Garden Soil
Many beginners make the mistake of using garden soil in containers.
Garden soil can:
- Become compacted
- Restrict root growth
- Hold excessive moisture
- Reduce oxygen availability
Potting mixes are specifically designed for container gardening and generally produce better results.
Should You Fill a 20 Gallon Pot Completely?
Not always.
Many experienced gardeners leave 1 to 2 inches of space between the soil surface and the rim of the container.
This space provides several benefits:
- Easier watering
- Reduced runoff
- Less soil erosion
- Better moisture control
When leaving a watering gap, the actual soil required may decrease slightly to around 2.5 cubic feet.
However, purchasing the full 2.67 cubic feet is still recommended because soil naturally settles over time.
Practical Growing Examples
Tomatoes
Tomatoes thrive in large containers.
A single indeterminate tomato plant can comfortably grow in a 20 gallon pot.
Benefits include:
- Extensive root development
- Improved moisture retention
- Higher yields
Use approximately 2.67 cubic feet of nutrient-rich potting mix.
Potatoes
Potatoes perform exceptionally well in grow bags.
Many gardeners begin with a partially filled container and gradually add soil throughout the growing season.
By harvest time, the total soil volume often reaches approximately 2.67 cubic feet.
Peppers
Pepper plants appreciate large containers because they provide stable moisture levels.
A 20 gallon pot can accommodate:
- One large pepper plant
- Two medium pepper plants
- Several compact varieties
Dwarf Fruit Trees
Small fruit trees can thrive in 20 gallon containers.
Popular choices include:
- Lemon trees
- Lime trees
- Olive trees
- Fig trees
Large containers provide the root space necessary for healthy long-term growth.
Common Mistakes When Filling a 20 Gallon Pot
Avoiding common mistakes can improve plant health and save money.
Buying Too Little Soil
Many gardeners purchase only the exact calculated amount.
Because soil settles after watering, it’s wise to have extra on hand.
Overfilling the Container
Filling soil to the very top can cause water and soil to spill over during irrigation.
Leave a small watering gap.
Ignoring Drainage
Drainage holes are essential.
Without proper drainage, roots may suffer from:
- Rot
- Disease
- Oxygen deprivation
Using Low-Quality Soil
Cheap soil often contains excessive fillers and poor-quality organic material.
Investing in quality potting mix usually leads to healthier plants and better harvests.
Compacting the Soil
Avoid pressing soil down too firmly.
Roots need air pockets to grow properly.
Allow the soil to settle naturally through watering.
Expert Tips for Better Container Gardening
Add Compost
Compost improves:
- Soil structure
- Nutrient availability
- Water retention
Use Slow-Release Fertilizer
Large containers benefit from a steady nutrient supply.
Slow-release fertilizers reduce maintenance while supporting healthy growth.
Mulch the Surface
A thin layer of mulch helps:
- Reduce evaporation
- Moderate soil temperature
- Suppress weeds
Water Deeply
Deep watering encourages roots to grow downward and develop stronger systems.
Shallow watering often results in weaker plants.
Monitor Soil Levels
Over time, soil naturally settles.
Top off containers as needed throughout the growing season.
Frequently Asked Questions
Can I use exactly 2.67 cubic feet of soil?
Yes, but purchasing slightly more soil is recommended because some settling will occur after watering.
How much soil does a 20 gallon grow bag need?
A 20 gallon grow bag generally requires approximately 2.67 cubic feet of soil, just like a traditional pot.
Should I put rocks in the bottom of a 20 gallon pot?
No. Modern gardening research shows that rocks can actually reduce drainage efficiency. Quality potting mix is usually sufficient.
What plants grow best in a 20 gallon container?
Popular options include tomatoes, peppers, potatoes, cucumbers, dwarf fruit trees, herbs, and flowering shrubs.
How often should I replace the soil?
Most container gardeners refresh or replace potting soil every one to two growing seasons depending on crop type and soil condition.
Conclusion
If you’ve been wondering how many cubic feet of soil for a 20 gallon pot, the answer is straightforward: a standard 20 gallon container requires approximately 2.67 cubic feet of soil. This volume provides adequate growing space for vegetables, flowers, herbs, shrubs, and even small fruit trees.
By understanding the gallon-to-cubic-foot conversion, selecting a quality potting mix, and accounting for soil settling, you can fill your containers correctly and create an ideal environment for healthy plant growth. Whether you’re starting a vegetable garden, growing ornamental plants, or cultivating fruit trees, knowing exactly how much soil you need makes planning your garden simpler and more successful.
