Grow bags are popular for tomatoes, potatoes, peppers and herbs, but their labels often round or overstate the size. This grow bag volume calculator measures it for you. Enter the shape and the dimensions of the bag, how high the soil sits and how full you fill it, and it returns the volume in gallons, liters and cubic feet, and how many bags of potting mix you will need.
How to use the grow bag volume calculator
- Choose the shape, round or rectangular.
- Measure the bag when it is open and standing. For a round bag, measure across the middle. For a rectangular bag, measure length and width.
- Enter the soil height, the depth the potting mix will reach, not the height of the empty bag.
- Set how full you fill it. Ninety percent leaves a little room for watering.
How the volume is calculated
For a round bag, the base area is pi times the radius squared. For a rectangular bag, it is length times width. Multiply by the soil height and the fill share to get cubic inches. Dividing by 231 gives US gallons, dividing by 1,728 gives cubic feet, and multiplying gallons by 3.785 gives liters.
Bag size and crop
| Crop | What to look for |
|---|---|
| Herbs and lettuce | A small bag, often 1 to 3 gallons, is enough |
| Peppers and bush beans | Medium bags, often 3 to 5 gallons per plant |
| Tomatoes | Many extension guides suggest at least 5 gallons per full-size plant |
| Potatoes | Larger bags that you can add soil to as the plants grow |
These are common guidelines, not strict rules. Check your seed packet or plant tag, and choose bigger bags for hot, dry sites, where small bags dry out fast.
Getting the most from grow bags
- Use potting mix, not garden soil, which compacts in containers and drains poorly.
- Check drainage. Good bags have drainage holes or are made of breathable fabric.
- Water often. Bags dry quicker than beds, sometimes daily in summer.
- Feed regularly. Nutrients wash out of containers, so follow your fertilizer label.
- Work out how much mix to buy with the soil calculator, and add it to your list in the garden shopping list generator.
- Plan a mixed container garden with the container garden planner, or fit plants in a small box with the window box plant count calculator.
Limits of this tool
Fabric bags change shape when filled and watered, so the real volume can differ from the measurement. Bag labels may be rounded. For best results, measure your own bag and add a little extra mix.
Frequently asked questions
How many gallons is my grow bag?
Measure it and enter the diameter or length and width, plus the soil height. The calculator gives gallons, liters and cubic feet.
How big a bag do tomatoes need?
Many extension guides suggest at least 5 gallons for a full-size plant, and more for large or vining types.
How do I convert gallons to liters?
Multiply gallons by 3.785. The calculator shows both.
How much potting mix is in a 1 cubic foot bag?
About 7.5 gallons, since one cubic foot is 1,728 cubic inches and a gallon is 231 cubic inches.
Do grow bags dry out faster?
Yes, especially fabric bags in hot weather. Check them every day and water when the top of the mix feels dry.