Potting Soil Calculator
Enter your container shape and dimensions to find out how many cubic feet of potting soil you need, how many bags to buy, and an estimated cost. The calculator handles rectangular raised beds, round planters, and tapered flower pots, supports both metric (centimetres) and imperial (inches or feet) units, and adds an optional settling allowance so you always have enough.
How to calculate potting soil volume
The amount of potting soil you need depends on the interior shape of your container and how deep you plan to fill it. For a rectangular bed, multiply length by width by depth. For a round planter, use the cylinder formula: pi times the radius squared times the depth. For a tapered flower pot, use the truncated cone formula: (pi/3) times the depth times (R squared plus R times r plus r squared), where R is the top radius and r is the bottom radius. All three formulas give volume in cubic inches, which you then divide by 1728 to convert to cubic feet. Multiply by the number of identical containers if you have more than one.
Why add a settling allowance?
Dry potting mix is fluffy and full of air pockets. After the first watering, the mix settles and its volume shrinks noticeably. A 10% allowance is the industry standard for typical peat-based or bark-based mixes. Use 15% for very light, peat-heavy mixes, and 20% if you plan to work compost or perlite into the mix after filling, which can add further compression. Skipping the allowance usually means a second trip to the garden center.
Bags vs. bulk: which is cheaper?
Bagged potting soil is convenient for small projects but costs more per cubic foot than bulk. Once you need more than about 1 cubic yard (27 cubic feet), it is worth calling a local nursery or landscape yard for a bulk quote. Bulk prices typically run 30 to 50 percent less per cubic foot, though delivery fees can offset the saving for small orders. This calculator shows your volume in cubic yards so you can compare quotes easily.
Tips for filling containers correctly
Fill containers to within 1 to 2 inches of the rim, not all the way to the top. The gap lets you water without overflow and gives roots room to push up as the plant grows. For deep raised beds (over 12 inches), some gardeners fill the bottom third with twigs, straw, or old leaves to save on soil cost, since most vegetable roots stay in the top 8 to 12 inches. Make sure any container has adequate drainage holes before filling.
Standard potting soil bag sizes and volumes
| Bag size | Volume (cu ft) | Volume (litres) | Approx. dry weight |
|---|---|---|---|
| 8 qt | 0.50 | 14.2 | ~37 lb |
| 12 qt | 0.75 | 21.2 | ~56 lb |
| 1 cu ft | 1.00 | 28.3 | ~75 lb |
| 1.5 cu ft | 1.50 | 42.5 | ~112 lb |
| 2 cu ft | 2.00 | 56.6 | ~150 lb |
| 3 cu ft | 3.00 | 84.9 | ~225 lb |
Common retail bag sizes sold in the US. Always check the label: quart sizes vary by brand.
Frequently asked questions
How many bags of potting soil do I need for a 4x8 raised bed?
A 4-foot by 8-foot raised bed filled 12 inches deep holds 32 cubic feet of soil. Adding a 10% settling allowance brings that to about 35 cubic feet. Using 1.5 cu ft bags (the most common size), you would need 24 bags. Exact counts vary with bag size and fill depth, so use the calculator above with your actual dimensions.
What is the difference between potting soil and garden soil?
Potting soil (or potting mix) is a soilless blend designed for containers. It drains freely, stays light enough for roots to penetrate easily, and does not compact as heavily as outdoor soil. Garden soil is denser and meant for in-ground beds. Using garden soil in pots can lead to poor drainage, root rot, and compaction. Always use a bagged potting mix for raised beds and containers.
How deep should I fill a raised bed?
Most vegetable roots grow in the top 12 inches of soil, so 12 inches of potting mix is the standard for productive raised beds. Leafy greens and herbs can thrive in as little as 6 inches. Root vegetables like carrots or parsnips need at least 12 to 18 inches. For ornamental planters, 6 to 8 inches is usually enough for annual flowers.
Why is my soil volume in litres different from what the bag label says?
Bag labels sometimes show compressed or settled volume, while this calculator gives raw, pre-settled volume. Manufacturers also measure volume before and after compression differently. A useful rule: 1 cubic foot equals 28.3 litres, so a "50-litre" bag contains about 1.77 cubic feet by volume.
Can I use the calculator for bulk soil orders?
Yes. The cubic yards output is what landscape suppliers use for bulk quotes. One cubic yard equals 27 cubic feet. If your project totals more than 0.5 to 1 cubic yard, it is worth comparing a bulk price to the cost of buying individual bags, which the calculator estimates for you.
How heavy is potting soil?
Dry, loose potting soil weighs roughly 70 to 80 pounds per cubic foot. This calculator uses 75 lb per cubic foot as a middle estimate. After watering, the weight can roughly double. For large containers on decks or balconies, check the load rating of the structure before filling, and consider lightweight mixes that substitute perlite or coir for some of the heavier components.