Grow Bag Calculator
Example Data Table
These examples help compare common grow bag styles and filling plans.
| Bag Type | Dimensions | Shape | Gross Volume | Suggested Fill | Approximate Mix Needed |
|---|---|---|---|---|---|
| Tomato Bag | 35 cm diameter × 30 cm height | Cylinder | 28.86 L | 90% | 29.09 L with 12% settling |
| Herb Bed Bag | 40 × 40 × 30 cm | Rectangular | 48.00 L | 85% | 45.70 L with 12% settling |
| Potato Bag | 40 cm top, 30 cm bottom, 45 cm high | Tapered Round | 42.02 L | 92% | 43.30 L with 12% settling |
| Raised Fabric Planter | 45×45 top, 35×35 bottom, 30 cm high | Tapered Rectangular | 48.25 L | 88% | 47.58 L with 12% settling |
Formula Used
Volume = π × (Diameter ÷ 2)2 × Height
Volume = Length × Width × Height
Volume = (π × Height ÷ 3) × (rtop2 + rtoprbottom + rbottom2)
Volume = (Height ÷ 3) × (Top Area + Bottom Area + √(Top Area × Bottom Area))
Usable Volume = Gross Volume × Fill Percentage
Mix Needed = Usable Volume × (1 + Settling Allowance)
The calculator converts all dimensions into meters first, computes the raw volume, then converts final results into liters, gallons, and cubic feet.
How to Use This Calculator
- Select the bag shape that best matches your container.
- Choose the dimension unit you measured with.
- Enter the bag dimensions carefully.
- Set the number of bags you want to fill.
- Choose a fill percentage if you want headspace.
- Add settling allowance for compost and potting mix shrinkage.
- Enter bulk density to estimate carrying weight.
- Enter cost per liter to budget your growing media purchase.
- Click the calculate button.
- Review the results, graph, and download CSV or PDF if needed.
Frequently Asked Questions
1. Why does fill volume differ from gross volume?
Gross volume is the full internal capacity. Fill volume reflects the percentage you actually plan to fill, leaving headspace for watering, mulch, or easier planting.
2. Why add a settling allowance?
Fresh potting mixes often compress after watering. A settling allowance helps you buy enough media so the bag reaches the intended soil level after the first few irrigations.
3. Which shape should I choose for fabric grow bags?
Use cylinder for most standard round bags. Choose rectangular prism for boxy planters. Use tapered options when the base is narrower than the opening.
4. Can I estimate how heavy the filled bag will be?
Yes. Enter the bulk density of your potting mix in kilograms per liter. The calculator multiplies total mix volume by density to estimate total media weight.
5. Is liters or gallons better for gardening purchases?
Liters are often easier for bagged media comparisons, while gallons help if you shop where nursery containers are labeled in gallons. This calculator shows both.
6. How accurate is the cost estimate?
It is a planning estimate based on your cost per liter. Real costs may differ because of taxes, bag size rounding, premium blends, or delivery charges.
7. What fill percentage is common for vegetables?
Many gardeners use about 85% to 95% fill. Taller crops may need a little headspace for mulch and watering, while shallow herbs can often be filled higher.
8. Can I use this calculator for raised fabric beds too?
Yes. Rectangular and tapered rectangular modes work well for fabric raised beds, root pouches, and soft-sided planters with measurable top and bottom dimensions.