Free Potting Soil Calculator
Enter your container type and dimensions and this potting soil calculator returns the cubic feet of mix you need, the bag count for 1 cu ft and 2 cu ft bags, and the gallon equivalent — updated live, as you type.
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Estimates only. Soil settles 10–20% after watering — add a small margin for top-ups.
Results are estimates. Consult a professional.
How the potting soil calculator works
The calculator finds the interior volume of your container, then converts it to cubic feet — the unit bags are sold in — and rounds up so you never run short. The math changes slightly by shape: rectangular pots multiply length by width by depth; round or cylindrical pots use the circle area of the opening multiplied by the depth; raised beds use feet for length and width, with depth in inches. All three end up in cubic feet, then in bag counts and gallons.
How full should you fill a pot?
Do not fill containers all the way to the rim. Most guidance recommends leaving one to two inches of space at the top to allow for watering without overflow — water pooling over the rim carries soil and fertilizer out of the pot. For large planters over 24 inches, leave two inches; for small four-to-eight-inch pots, one inch is enough.
Soil settles after watering
Fresh potting mix is fluffy and full of air pockets. After the first few waterings, it compresses by 10 to 20 percent. Buy slightly more than the calculator says — especially for raised beds where you cannot easily top up — or fill the pot to the rim knowing the soil level will drop to the right place on its own.
Drainage allowance and mix composition
Very large containers — half-barrels, gallon pots over 15 gallons, or any planter over 18 inches deep — are sometimes partially filled at the bottom with coarse material to improve drainage and reduce weight. Lava rock, perlite, or chunky bark chips take up the bottom 20 to 30 percent of the container while potting mix fills the growing zone above. If you plan a drainage layer, reduce your soil volume accordingly.
Bagged mix composition
Standard potting mix is roughly 50 to 60 percent peat moss or coir, 20 to 30 percent perlite or vermiculite, and the rest bark fines and compost. The high perlite content is what makes a bag feel light and airy — it compresses more than mineral soil would. Higher-end mixes add worm castings, kelp, or mycorrhizae, which increases cost but not the volume math.
Potting soil vs potting mix vs garden soil
The three terms are often used interchangeably at the garden center but they are meaningfully different. Garden soil is meant for in-ground planting — it is heavy, may compact in a pot, and often drains poorly in containers. Potting soil is a soil-based blend formulated to drain well in pots but can still be heavy. Potting mix (or container mix) contains no true soil — it is entirely peat or coir, perlite, bark, and amendments — and it drains and aerates best in containers.
| Product | Best for | In a pot? |
|---|---|---|
| Garden soil | In-ground beds | No — compacts, drains poorly |
| Potting soil | Containers | Yes — OK, may be heavy |
| Potting mix | Containers | Yes — best choice |
| Raised-bed mix | Raised beds | Yes — formulated for depth |
For containers, always choose potting mix or a product labeled "for containers." For raised beds, a 60/30/10 blend of topsoil, compost, and perlite or pine bark is widely recommended (Mel Bartholomew's "Mel's Mix" variant).
How deep should a raised bed be?
The right raised-bed depth depends on what you are growing. Shallow-rooted crops like lettuce, herbs, and radishes do well in six inches. Most vegetables — tomatoes, peppers, beans, kale — need at least twelve inches. Root crops like carrots and parsnips want eighteen inches or more. If the bed sits over compacted soil or concrete, go deeper to give roots room; if it sits over good in-ground soil, roots can escape the bottom, so shallower beds work fine.
| Depth | Suitable crops |
|---|---|
| 6 in | Lettuce, herbs, radishes, strawberries |
| 12 in | Tomatoes, peppers, kale, beans, squash |
| 18 in | Carrots, parsnips, potatoes |
| 24 in | Any crop, maximum flexibility |
Standard raised beds are 6–12 inches deep. The most common retail kit height is 11 or 12 inches — this lets roots reach into the native soil underneath.
Worked example: filling a 4 × 8 raised bed
Jordan is setting up a standard cedar raised bed — 4 ft wide, 8 ft long, 12 in deep. She wants to know how many 2 cu ft bags of potting mix to buy, plus how much to add for settling.
Step 1 — Calculate volume
Volume = 4 × 8 × (12 ÷ 12) = 32 cu ft.
Step 2 — Add a 10% settling margin
32 × 1.10 = 35.2 cu ft. Round up to 36 cu ft.
Step 3 — Convert to 2 cu ft bags
36 ÷ 2 = 18 bags of 2 cu ft potting mix.
How accurate is this calculator?
The volume math is exact for the dimensions you enter. The bag count rounds up because you cannot buy a partial bag, so you will always have a small amount left over — a useful reserve for topping up after settling. The gallon figure is the exact volume converted at 7.48 gallons per cubic foot, which matches US customary exactly (1 cu ft = 1728 ÷ 231 ≈ 7.4805 US gallons).
What the calculator does not account for: a drainage layer at the bottom of the pot reduces the growing-medium volume needed; soil settling of 10–20% means the filled pot will look lower after watering; and some bags are labeled by weight (quarts) rather than volume (cubic feet) — a 16-quart bag is about 0.5 cubic feet. Always check the bag label before ordering.
Frequently asked questions about the free Potting Soil Calculator
About this potting soil calculator
This potting soil calculator runs entirely in your browser. Nothing you enter is sent anywhere or stored — the volume, bag counts, and gallon equivalent update instantly as you change the container dimensions or switch between rectangular, round, and raised-bed modes.
It is part of our free home & garden calculators — alongside soil, sod, plant spacing, and garden-plot tools — and the full library of free calculators covering home, finance, health, and more.