Guide
How much soil for a grow bag
Every common size, with the settling allowance included.
Short answer
A 5 gallon grow bag takes about 21 litres, or 0.7 cu ft, of potting mix once you allow for settling, so one 2 cu ft bag of mix fills 2 of them. A 10 gallon bag takes about 1.5 cu ft. Use potting mix, not garden soil.
Soil for each grow bag size
| Grow bag | Litres | Cubic feet | 2 cu ft bags |
|---|---|---|---|
| 1 gallon | 4 | 0.1 | 1 |
| 2 gallon | 8 | 0.3 | 1 |
| 3 gallon | 13 | 0.4 | 1 |
| 5 gallon | 21 | 0.7 | 1 |
| 7 gallon | 30 | 1.0 | 1 |
| 10 gallon | 42 | 1.5 | 1 |
| 15 gallon | 64 | 2.2 | 2 |
| 20 gallon | 85 | 3.0 | 2 |
| 25 gallon | 106 | 3.7 | 2 |
| 30 gallon | 127 | 4.5 | 3 |
| 50 gallon | 212 | 7.5 | 4 |
| 100 gallon | 424 | 15.0 | 8 |
The figures include the roughly 12 percent a fresh mix settles by after the first good watering. Fill to the rim, water, and top up once.
A grow bag’s gallon size is a name, not a measurement
Grow bag and nursery pot sizes are trade sizes. A 5 gallon fabric pot is typically about 12 in across and 10 in tall, which holds about 4.9 US gallons when it is full to the brim, and a bag that slumps when filled holds less than one that stands straight. If you are buying mix for a lot of bags, measure one and put its size into the container soil calculator.
What to fill a grow bag with
Potting mix, or a mix of two parts potting mix to one part compost for heavy feeders such as tomatoes and squash. Garden soil packs down in a container and stays wet, because a grow bag has no ground under it to drain into. Raised bed soil is fine in the 25 gallon and larger bags, where there is enough depth for it to behave like a bed. Fabric bags dry out faster than plastic pots, so a mix with coir or compost that holds water is worth the extra.