Potting soil calculator
Enter pot diameter and height — and how many pots you're filling — to get dry quarts, cubic feet and the right bag count. Potting mix is sold by dry quart, so we speak quart.
Result
Potting mix needed
How much potting soil per pot?
Pots are (roughly) tapered cylinders, so the calculator uses 80% of the full cylinder volume:
Formula
quarts ≈ π × (d ÷ 2)² × fill height × 0.8 × 0.5143 qt per in³
Example: a 12-inch pot filled to 10 inches: π × 36 × 10 × 0.8 = 905 in³ ≈ 10.6 dry quarts. Four of them = 42.5 qt, so one 32 qt bag + one 16 qt bag covers it.
Common pot sizes
| Pot | Approx. volume | Fills from a 32 qt bag |
|---|---|---|
| 6 in (annual, herb) | ~1.8 qt | 17 |
| 8 in | ~4 qt | 8 |
| 10 in | ~6.5 qt | 4–5 |
| 12 in | ~10.5 qt | 3 |
| 16 in | ~22 qt | 1–2 |
| 20 in (half barrel ~26 in) | ~40–70 qt | less than 1 |
Tips that save soil (and money)
- Don't fill giant pots with mix: for plants with shallow roots, put an upside-down nursery pot or a layer of empty bottles in the bottom third, then landscape fabric, then soil. (Skip the gravel layer — it's a myth.)
- Grow bags: listed sizes are gallons — a “10 gallon” bag takes about 13 quarts less than its nominal volume when filled to 2 in below the rim; the rectangle tab handles them (1 gal ≈ 0.1337 ft³).
- Refresh, don't replace: dump last year's mix, break it up, blend 30% fresh compost and reuse — potting mix is mostly structure, not nutrition.
Frequently asked questions
How many quarts of potting soil are in a cubic foot?
One cubic foot = 25.71 dry quarts (US dry quart = 1.101 L). That's why 1.5 ft³ bags are labeled “38.8 qt”.
How much soil for a 5 gallon bucket?
Five gallons ≈ 0.67 ft³ ≈ 17 dry quarts — a 16 qt bag plus a top-up, or exactly one 20 qt bag filled to the rim.
How many 12 inch pots does a 32 qt bag fill?
About three (each takes ~10.5 qt filled to 10 inches). Six-inch annual pots get 17 fills per bag.
Do I need to change potting soil every year?
No — refresh it. Remove root balls, fluff, mix in ~30% compost and a slow-release fertilizer. Replace entirely only after disease or every 3–4 years.
How do I calculate soil for window boxes?
Use the rectangle tab: a 24 × 8 in box filled to 8 in is 24 × 8 × 8 × 0.8 (taper) ≈ 1,229 in³ ≈ 14.4 qt.