Enter the size of your pots or planters, how deep you will fill them and how many you have. The calculator gives the potting soil you need in dry quarts and cubic feet, and the number of bags to buy.
How to use the potting soil calculator
- Choose a round pot or a rectangular planter and enter the inside size at the top.
- Enter the soil depth: the depth of the container minus about an inch left at the top for watering.
- Enter how many containers of that size you are filling. For pots of different sizes, work out each size and add the bags.
- Pick the bag size on the shelf. Potting mix is sold in dry quarts or cubic feet.
Potting soil formulas
Round pot (ft³) = π × (diameter ÷ 2)² × depth Planter (ft³) = length × width × depth Dry quarts = ft³ × 25.71; liters = ft³ × 28.32 Bags = total ft³ ÷ bag size (ft³), rounded up
A US dry quart is 67.2 cubic inches (NIST), so a cubic foot (1,728 in³) holds 25.71 dry quarts. That means an 8 qt bag is 0.31 ft³ and a 25 qt bag is just under a cubic foot.
Worked example: four 12 in pots
- One pot, 12 in across and filled 11 in deep: 3.1416 × 6² × 11 = 1,244 in³ = 0.72 ft³.
- That is 0.72 × 25.71 = 18.5 dry quarts per pot.
- Four pots: 2.88 ft³, or 74 quarts.
- Bags: 3 bags of 1 cubic foot, or 10 bags of 8 qt.
Potting soil per round pot
For a pot filled to an inch below the rim, treating the pot as a straight cylinder:
| Pot diameter × depth | Soil | Dry quarts |
|---|---|---|
| 6 × 6 in | 0.08 ft³ | 2.1 qt |
| 8 × 8 in | 0.20 ft³ | 5.2 qt |
| 10 × 10 in | 0.41 ft³ | 10.5 qt |
| 12 × 12 in | 0.72 ft³ | 18.5 qt |
| 14 × 14 in | 1.16 ft³ | 29.8 qt |
| 16 × 16 in | 1.75 ft³ | 44.9 qt |
| 20 × 20 in | 3.45 ft³ | 88.8 qt |
Tapered pots hold less
Most nursery and decorative pots are narrower at the bottom than at the top. The calculator measures from the top diameter, so for a strongly tapered pot it gives a safe upper estimate: you may have a little mix left over, which is handy for topping up after the soil settles.
Tips for buying potting soil
- Use potting mix in containers. Garden soil and topsoil pack down in pots; potting mix is made to stay light and drain.
- Compare price per cubic foot. Divide the bag price by its volume; a 2 cu ft bag is often cheaper per quart than small bags.
- Moisten before filling. Dry mix settles once watered, so fill, water, and top up.
- Big beds need bulk. For a raised bed, use the raised bed soil calculator instead.
Questions
How much potting soil do I need for a 12 inch pot?
About 18.5 dry quarts, or 0.72 ft³, when filled 11 in deep. A 1 cu ft bag fills one pot with some left over.
How many quarts are in a cubic foot of potting soil?
About 25.7 dry quarts.
How many 8 quart bags of potting soil do I need?
Divide the total quarts by 8 and round up. Four 12 in pots need 74 quarts, so ten 8 qt bags.
Does a 25 quart bag equal a cubic foot?
Nearly: 25 dry quarts is 0.97 ft³, just under one cubic foot.
Sources
- NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically, National Institute of Standards and Technology.
Used for: US dry quart = 67.200625 in³; cubic foot = 1,728 in³ = 28.317 L.