Gallon to Cubic Feet of Soil Calculator
Convert gallons into cubic feet for soil, compost, potting mix, or raised bed planning. Choose the gallon type, add an optional settling allowance, and instantly see cubic feet, cubic yards, liters, and estimated bag counts.
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Tip: 1 US liquid gallon equals about 0.13368 cubic feet. If you are buying bagged soil, adding 5% to 15% extra is common to account for settling, uneven filling, and compaction.
How to Calculate Gallon to Cubic Feet of Soil Accurately
Knowing how to convert gallons to cubic feet of soil is one of the most useful skills in container gardening, landscaping, raised bed design, and bulk soil purchasing. Soil is commonly sold in cubic feet or cubic yards, while many planters, nursery pots, and household containers are described in gallons. If you do not convert properly, you can underbuy and end up short on material, or overbuy and waste money on extra bags of soil you did not need.
The core concept is simple: gallons measure volume, and cubic feet also measure volume. Once you know the conversion factor for the gallon system you are using, you can switch from one unit to another with precision. For most gardeners in the United States, the relevant unit is the US liquid gallon. That is the gallon used for common nursery pot descriptions, liquid measurements, and most retail references. In practical terms, 1 US liquid gallon equals 0.1336806 cubic feet. To convert gallons to cubic feet, multiply gallons by 0.1336806.
Primary formula: Cubic feet = gallons × 0.1336806 for US liquid gallons. If you are working with US dry gallons or Imperial gallons, the number changes slightly, so always confirm your gallon type before ordering soil.
Why the conversion matters for soil
Soil planning often crosses several measurement systems. A raised bed may be measured by length, width, and depth in feet. A decorative planter may be marketed as a 20 gallon container. Potting mix might be sold in 1.5 cubic foot bags, while bulk topsoil is often priced by the cubic yard. If you understand gallon to cubic feet conversion, you can compare these products directly and make better purchasing decisions.
This is especially important because soil is not always packed consistently. Potting soil can be fluffed and airy in one bag, while compost or topsoil can be denser and more compact. The conversion itself is mathematically exact, but the way soil settles after watering can affect how much material you should buy. That is why this calculator includes a settling or waste percentage. It lets you estimate a more realistic amount for actual projects.
Exact gallon to cubic feet conversion factors
Different gallon standards exist. In most home gardening situations, the US liquid gallon is correct. Still, for complete accuracy, here are the standard conversion factors used by professionals and reference sources.
| Gallon type | Exact or standard conversion | Cubic feet per gallon | Notes |
|---|---|---|---|
| US liquid gallon | 231 cubic inches | 0.1336806 cu ft | Most common for gardening containers in the US |
| US dry gallon | 1/8 US bushel | 0.1556229 cu ft | Less common in everyday gardening retail labeling |
| Imperial gallon | 4.54609 liters | 0.1605437 cu ft | Used in some UK references and legacy measurements |
As the table shows, the difference between gallon systems is meaningful. If you use the wrong conversion for a larger project, the error compounds quickly. For example, 100 US liquid gallons equals about 13.37 cubic feet, while 100 Imperial gallons equals about 16.05 cubic feet. That is a large enough difference to affect your bag count and budget.
Step by step method for converting gallons of soil to cubic feet
- Identify the gallon standard. Use US liquid gallons unless the product specifications clearly state otherwise.
- Write down the container or project volume in gallons. Example: 25 gallons.
- Multiply by the correct conversion factor. For US liquid gallons: 25 × 0.1336806 = 3.342015 cubic feet.
- Round to a practical buying amount. In retail terms, 3.34 cubic feet may mean buying two 1.5 cubic foot bags plus one 0.5 to 1.0 cubic foot bag, or simply three 1.5 cubic foot bags if you want extra fill margin.
- Add settling allowance if needed. For potting soil and compost blends, adding 5% to 15% is common depending on how loose the material is.
Common gallon sizes converted to cubic feet
The following table uses the US liquid gallon standard, which is the most relevant for soil calculators and planters in the United States. These values are widely useful for selecting bagged soil quantities for pots, grow bags, and bed amendments.
| Container size | Cubic feet | Approx. liters | Typical bag planning note |
|---|---|---|---|
| 1 gallon | 0.1337 cu ft | 3.79 L | Small nursery pot, often far less than one standard bag |
| 5 gallons | 0.6684 cu ft | 18.93 L | Fits within one 0.75 cu ft bag |
| 7 gallons | 0.9358 cu ft | 26.50 L | About one 1.0 cu ft bag |
| 10 gallons | 1.3368 cu ft | 37.85 L | Close to one 1.5 cu ft bag |
| 15 gallons | 2.0052 cu ft | 56.78 L | Often two 1.0 cu ft bags or one 2.0 cu ft bag |
| 20 gallons | 2.6736 cu ft | 75.71 L | Often two 1.5 cu ft bags with slight extra |
| 25 gallons | 3.3420 cu ft | 94.64 L | Common large planter size |
| 30 gallons | 4.0104 cu ft | 113.56 L | Common raised fabric grow bag size |
| 50 gallons | 6.6840 cu ft | 189.27 L | May justify bulk purchase over bagged soil |
| 100 gallons | 13.3681 cu ft | 378.54 L | Roughly 0.495 cubic yards |
How cubic feet relates to cubic yards and bagged soil
Once you have cubic feet, it becomes much easier to compare all soil products. Bulk soil vendors usually quote by cubic yard, and there are 27 cubic feet in one cubic yard. That means a project requiring 13.5 cubic feet is almost exactly half a cubic yard. If the project is larger, buying bulk can be more cost effective than purchasing many small bags.
Bagged soil is typically sold in standardized sizes such as 0.75, 1.0, 1.5, or 2.0 cubic feet. If your converted result is 4.01 cubic feet, for example, you might buy:
- Six bags of 0.75 cubic feet = 4.5 cubic feet
- Four bags of 1.0 cubic foot = 4.0 cubic feet
- Three bags of 1.5 cubic feet = 4.5 cubic feet
- Two bags of 2.0 cubic feet = 4.0 cubic feet
The most economical option depends on local pricing, transport limitations, and whether you want extra material for settling. For many gardeners, buying a little extra is worth the convenience.
Soil settling, compaction, and why exact math is not always enough
The gallon to cubic feet conversion gives a true volumetric answer, but gardening materials behave differently after filling and watering. Potting mix usually contains peat, coir, bark, or perlite, all of which can settle after irrigation. Compost also compresses over time. Mineral topsoil may compact less at first but can still slump once spread and tamped. That is why many experienced landscapers add a buffer amount to the exact volume.
A 5% adjustment may be enough for relatively stable material in shallow containers. A 10% or 15% adjustment is more practical when using fluffy potting mix, filling tall planters, or topping off beds that are likely to settle after several waterings. This is not changing the mathematical conversion. It is simply making the purchasing estimate more realistic.
Examples you can use right away
Example 1: You have a 10 gallon pot. Using US liquid gallons, 10 × 0.1336806 = 1.336806 cubic feet. If you add 10% for settling, you need about 1.4705 cubic feet. Buying one 1.5 cubic foot bag is usually appropriate.
Example 2: You are filling four 20 gallon grow bags. Total gallons = 80. Then 80 × 0.1336806 = 10.6944 cubic feet. Add 10% for settling and you get about 11.7638 cubic feet. You could buy twelve 1.0 cubic foot bags, eight 1.5 cubic foot bags, or compare that total with bulk soil pricing.
Example 3: You have a decorative planter listed as 25 gallons. The required soil volume is 25 × 0.1336806 = 3.3420 cubic feet. If the planter has a false bottom, drainage insert, or filler material at the base, the real soil requirement may be lower than the rated volume. Always consider the actual fill depth.
Mistakes people make when converting gallon to cubic feet of soil
- Using the wrong gallon type. US liquid and Imperial gallons are not interchangeable.
- Ignoring settling. The exact volume may not be enough after watering.
- Confusing pot size labels with actual internal volume. Nursery pots are sometimes nominally labeled.
- Forgetting drainage layers or filler inserts. These reduce the amount of soil needed.
- Mixing cubic feet and cubic yards. There are 27 cubic feet in one cubic yard, not 9.
How this applies to raised beds and landscape design
Although this calculator is focused on gallon to cubic feet conversion, the logic fits into broader landscape planning. A raised bed is often measured directly in cubic feet by multiplying length × width × depth. Containers, however, are frequently labeled in gallons. If you are combining planter boxes, pots, and garden beds in one project, converting everything to cubic feet gives you one common language for estimating materials.
For example, if you need 18 cubic feet for a bed and another 6.68 cubic feet for a 50 gallon stock tank planter, your total project volume is 24.68 cubic feet. That is about 0.914 cubic yards. This kind of integrated planning helps when ordering delivery from a soil yard.
Authoritative references for measurement and soil guidance
For readers who want to verify unit standards or learn more about soil properties, these reputable sources are excellent starting points:
- National Institute of Standards and Technology (NIST): unit conversion resources
- USDA Natural Resources Conservation Service (NRCS): soil and land management information
- University of Minnesota Extension: container gardening guidance
Final takeaway
To calculate gallon to cubic feet of soil, multiply the gallon value by the correct conversion factor. For nearly all US gardening situations, use 0.1336806 cubic feet per US liquid gallon. Then decide whether you should add a small extra percentage for settling, compare the result with common bag sizes, and if necessary convert again into cubic yards for bulk purchase planning.
If you are filling planters, grow bags, raised beds, or decorative containers, this calculator gives you a fast and practical answer. Enter the gallon amount, choose the gallon type, and review the estimated cubic feet, cubic yards, liters, and bag counts. That simple process removes guesswork, reduces wasted purchases, and helps you buy the right amount of soil the first time.