Calculator
Potting Mix Calculator
Pick the gallon size on the pot or grow bag, how many pots you have, and the bag size at the store. The tool uses US liquid gallons (what most grow bags and buckets mean). Nursery trade gallons can be smaller; measure if the plant looks cramped. Fill with potting mix, not garden soil.
Your result
Potting mix to buy
1bag1.0 cu ft
- Volume
- 0.61 cu ft15.8 dry quarts
- Liters
- 17 L
- Bags needed
- 11.0 cu ft, rounded up
Fits: Determinate tomato, eggplant, trellised cucumber, a comfortable pepper.
Plastic: Check two inches down. Plastic stays wet longer than fabric after the same rain.
How this calculation works
For a labeled gallon pot: cubic feet ≈ gallons × 0.1337, then about 8% off if you leave a watering rim. Bags = cubic feet ÷ bag size, rounded up. Custom round pots use π × radius² × fill height. Tapered pots use a frustum.
Example: two 5-gallon grow bags, 1 cu ft bags
One 5-gallon bag is about 0.67 cubic feet. Leaving a rim, call it 0.61 cu ft each. Two bags: 1.23 cu ft, so 2 bags of 1.0 cu ft potting mix (rounded up). A 16-quart bag is about 0.62 cu ft, so you would still need 2 of those.
Frequently asked questions
Is a 5-gallon nursery pot really 5 gallons?
Not always. Hardware-store buckets and many fabric grow bags are close to true US gallons. Nursery “#5” plastic pots are often smaller. If the plant dries out every afternoon in a “5 gal” nursery pot, measure it or step up a size.
Can I use garden soil or raised-bed mix in a pot?
No. Garden soil and mineral-heavy raised-bed mix pack in a closed container, stay wet, and cut off air. Use potting mix (peat or coir, bark, perlite). The potting mix vs garden soil guide on this site explains the difference.
Does the pot material change how much mix I buy?
No. Material changes how fast the pot dries, not the interior volume. Fabric needs more frequent watering. Plastic holds moisture. Unglazed terracotta dries faster than glazed ceramic.