Potting soil calculator
Measure the top and bottom of your pot across the inside, and its height — or enter its volume — to get how much potting mix to buy.
Enter your measurements
More options: Settling · Bag price
- In dry quarts (as on the bag)
- 12 dry quarts
- Bags of that size
- 1 bag · 1 cu ft bag
- Each container holds
- 3.3 gal (12.5 L)
Fill height 9 in; width at the fill line 11.7 in, bottom 9 in
3.1416 × 9 in ÷ 12 × (11.7² + 11.7 × 9 + 9²) = 3.3 gal per pot
+ 10% for settling = 0.48 cu ft (12 dry quarts) → 1 bag
- Use potting mix, not garden soil or topsoil — soil packs down in a container and drains poorly.
Quick answers for common sizes
| Example | Answer |
|---|---|
| How much potting soil for a 12 inch pot | 0.48 cu ft |
| How much potting soil for a 5 gallon bucket or grow bag | 0.74 cu ft |
| How much soil for a half whiskey barrel planter | 4.57 cu ft |
Potting mix by number of containers (size as above)
| Containers | Potting mix to buy | In dry quarts (as on the bag) | Bags of that size |
|---|---|---|---|
| 1 | 0.48 cu ft | 12 dry quarts | 1 bag |
| 2 | 0.97 cu ft | 25 dry quarts | 1 bag |
| 4 | 1.94 cu ft | 50 dry quarts | 2 bags |
| 6 | 2.91 cu ft | 75 dry quarts | 3 bags |
| 10 | 4.85 cu ft | 125 dry quarts | 5 bags |
| 20 | 9.7 cu ft | 249 dry quarts | 10 bags |
Most pots are tapered, so the volume is less than top width × height suggests. Potting mix is sold in dry quarts: 25.7 of them make a cubic foot.
How the potting soil calculation works
A straight-sided pot is a cylinder: 3.1416 × radius² × height. Most pots taper, though, so the calculator treats them as a cone with the top cut off (a frustum), filled to about an inch below the rim. For a square pot, trough or window box it is simply length × width × fill height.
tapered pot = 3.1416 × h ÷ 12 × (D² + D × d + d²) D = diameter at the fill line, d = bottom diameter, h = fill height rectangular = length × width × fill height 1 cu ft = 25.7 dry quarts = 7.48 gallons = 28.3 L
Dry quarts vs gallons
Bags of potting mix are labelled in dry quarts, a different unit from the liquid quarts on a milk jug. NIST defines the dry quart as 67.2 cubic inches, about 1.10 litres, so an 8 qt bag holds about 8.8 L, and 25.7 dry quarts make one cubic foot. Pot sizes, on the other hand, are usually given in (liquid) gallons. The calculator shows both so you can match the pot to the bag.
| Container | Approx. mix to fill | Notes |
|---|---|---|
| 6 in pot | About 1–2 dry qt | Depends heavily on taper |
| 10 in pot | About 5–7 dry qt | |
| 12 in pot | About 9–13 dry qt | Tall 12 in pots hold more |
| 16 in pot | About 22–30 dry qt | Roughly a 1 cu ft bag |
| 5 gal bucket or grow bag | About 15–19 dry qt | A 5 US gal container is 0.67 cu ft if completely full |
| Half whiskey barrel (24–26 in across) | About 3–4.5 cu ft | Measure yours — sizes vary |
Three sizes are worked through in detail: How much potting soil for a 12 inch pot, How much potting soil for a 5 gallon bucket or grow bag, and How much soil for a half whiskey barrel planter, the biggest planter most people fill.
Tips
- Use a potting mix labelled for containers. University of Maryland Extension puts garden soil at roughly 75 lb per cubic foot against about 10 lb for a soilless mix; the dense soil packs down and holds too much water in a pot.
- Every container needs drainage holes. Decorative pots and buckets often have none, so drill a few before filling.
- Moisten dry mix before filling — peat- and coir-based mixes shed water when bone dry.
- Big planters get heavy. Put them in position, on a caddy or feet, before filling.
- Buying for many pots? The 2 cu ft bags are usually the best price per quart.
What you’ll need
- Potting soil scoopBudget price
Fills pots from a bag with less mess than a trowel.
See options on Amazon (opens in a new tab) - Rolling plant caddyBudget price
A wheeled base for big planters — set it down before you fill the pot.
See options on Amazon (opens in a new tab) - Garden dump cartMid-range price
Easier than a wheelbarrow for hauling bags and pots, and it tips to unload. Check the rated load against what you will carry.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How much potting soil for a 12 inch pot?
Usually 9–13 dry quarts, depending on taper and height; a typical tapered pot 10 in tall takes about 11. Worked out in How much potting soil for a 12 inch pot.
How much potting soil for a 5 gallon pot?
About 17 dry quarts if it truly holds 5 US gallons, or 19 with 10% for settling; many "5-gallon" pots hold less. Details in How much potting soil for a 5 gallon bucket or grow bag.
How many quarts are in a cubic foot of potting soil?
About 25.7 dry quarts. Potting mix is labelled in dry quarts; a cubic foot is roughly 29.9 liquid quarts, which is why some online conversions look different.
Can I reuse potting soil?
Often, yes, for ornamentals — break it up, remove old roots and mix in some fresh mix or compost. If the previous plants were diseased, start fresh.
Related guides
Sources
- University of Maryland Extension: Growing Vegetables in Containers and Salad Tables (opens in a new tab) — Garden soil (~75 lb/cu ft) is too dense for containers; soilless mixes are ~10 lb/cu ft; minimum container sizes (5 gal for standard tomatoes, peppers, cucumbers, eggplant; 2 gal for cherry tomatoes).
- University of Maryland Extension: Types of Containers for Growing Vegetables (opens in a new tab) — All containers need drainage holes; half whiskey barrels and 5-gallon buckets as containers; growing-media volume and depth by plant size.
- NIST Handbook 44, Appendix C: General Tables of Units of Measurement (opens in a new tab) — 1 dry quart = 67.2006 cubic inches; 1 cubic foot = 1,728 cubic inches.