Calculator
Choose a method, enter dimensions, and estimate how many bags you should buy.
Example data
Use these sample scenarios to sanity-check your results.
| Scenario | Dimensions | Total volume | Bag size | Bags |
|---|---|---|---|---|
| Raised bed | 1.2 m x 0.6 m x 0.25 m | 180 L | 40 L | 5 |
| Round pot | 35 cm diameter x 30 cm fill height | 28.9 L | 20 L | 2 |
| Tapered pot | 40/28 cm diameters x 32 cm fill height | 27.0 L | 25 L | 2 |
| Custom total | 120 L total volume + 10% waste | 132 L | 50 L | 3 |
Formula used
- Rectangular:
V = L x W x D - Cylinder:
V = pi x r^2 x h - Frustum:
V = (pi*h/3) x (R1^2 + R1*R2 + R2^2) - Waste/compaction:
Vfinal = V x (1 + p/100)
- Bags needed:
ceil(Vfinal / Vbag) - Leftover:
(bags x Vbag) - Vfinal - Conversions:
1 cu ft = 28.3168 L
For best accuracy, measure the soil fill height, not pot height.
How to use this calculator
- Select a volume method that matches your container shape.
- Enter dimensions and confirm the dimension unit.
- Set the container count and your waste/compaction percentage.
- Choose a bag unit and type the bag size sold locally.
- Press Calculate to view totals above the form.
- Use Download CSV or Download PDF for records.
Potting mix volume planning
Accurate volume planning reduces emergency trips and prevents excess stock. This calculator estimates total mix needed for beds and containers, then converts that volume into purchase-ready bag counts. Use it when preparing seasonal repots, new raised beds, or container groupings with consistent fill depths. It supports CSV and PDF exports for record keeping.
Understanding container geometry
Different shapes hold different volumes even when they look similar. Rectangular planters use length × width × depth. Round pots use π × r² × height. Tapered nursery pots are best modeled as a frustum, using the top and bottom diameters plus fill height for a more realistic estimate. Measure the internal opening and the soil fill height.
Waste, compaction, and settling factors
Potting mixes settle after watering and handling. A practical allowance is 5–15% for waste and compaction, depending on texture and handling. Fine mixes and heavily watered setups settle more. If you add perlite or bark, settling can be lower, but spillage during filling can increase losses. Dry mixes may pack differently after watering.
Bag sizing, unit conversion, and leftovers
Suppliers often sell bags in liters (20–50 L) or cubic feet (1–2 cu ft). The calculator uses 1 cu ft = 28.3168 L to standardize totals. Bags are rounded up with a ceiling rule, so you always buy enough. The leftover field shows the expected extra volume after purchasing whole bags. If you have multiple bag sizes available, run the tool twice to compare leftover volume and total cost.
Cost control and purchase strategy
When you enter a price per bag, the tool provides an estimated cost so you can compare brands and bag sizes quickly. If storage space is limited, choose a slightly larger waste factor and a smaller bag size to reduce leftover volume. For large projects, compare two bag sizes to minimize total cost per liter. Save the summary using the CSV or PDF export so you can share quantities with suppliers or crew members.
FAQs
How do I choose the right volume method?
Pick the shape that matches your container: rectangular for beds and boxes, cylinder for round pots, frustum for tapered nursery pots, or custom when you already know the total volume.
What waste or compaction percentage should I use?
Start with 10% for most home projects. Use 5% for careful filling and coarse mixes, and 15% when you expect spillage, heavy watering, or noticeable settling after installation.
My bags are labeled in cubic feet. Can I still use liters?
Yes. Select “Cubic feet” under Bag unit, enter the bag size (for example 1.5 or 2), and the calculator converts everything using 1 cu ft = 28.3168 liters.
Why does the calculator round bags up?
Bags are sold as whole units, so the tool uses a ceiling rule to ensure you buy enough mix. The leftover value estimates how much volume remains after purchase.
How is the estimated cost calculated?
Enter an optional price per bag. The tool multiplies the rounded bag count by that price and shows an estimated total, helping you compare brands and bag sizes quickly.
Can I plan multiple pots of the same size at once?
Yes. Set the Count field to the number of identical containers, then enter one set of dimensions. The calculator multiplies the single-container volume, applies waste, and returns the total bags needed.