Calculator
Example data table
| Example | Shape | Dimensions | Approx. volume (L) |
|---|---|---|---|
| A | Round Pot | Diameter 30 cm, Height 28 cm | 19.792 |
| B | Rectangular Planter | Length 60 cm, Width 25 cm, Height 22 cm | 33.000 |
| C | Tapered Pot | Top 34 cm, Bottom 24 cm, Height 30 cm | 22.210 |
| D | Round Bowl | Diameter 40 cm (hemisphere) | 16.755 |
Formula used
- Round pot (cylinder): V = π × r² × h
- Rectangular planter: V = L × W × H
- Tapered pot (frustum): V = (π × h / 3) × (r₁² + r₁r₂ + r₂²)
- Round bowl (hemisphere): V = (2/3) × π × r³
- Net fill volume: (Gross − Drainage) × Fill%
- Soil needed: Net × (1 + Compaction%) × Containers
How to use this calculator
- Select the container shape that matches your pot or planter.
- Choose units and enter the required dimensions carefully.
- Set fill percentage for rim space and watering headroom.
- Add a drainage layer height if you reserve the bottom space.
- Use compaction allowance if soil will settle after planting.
- Press Calculate and download the CSV or PDF report.
Practical volume planning for healthier container plants
Accurate container volume helps you match root space, watering frequency, and fertilizer dose to the plant’s growth stage. Undersized pots dry quickly and restrict roots, while oversized pots can stay wet and reduce oxygen near the root zone. Using geometric volume avoids guesswork and gives repeatable numbers for planning soil purchases and container upgrades.
Shape selection and measurement habits
Most nursery pots behave like cylinders, while window boxes behave like rectangular planters. Tapered pots are common for decorative planters, so the frustum formula prevents overestimating soil needs. Measure internal dimensions when possible, because thick walls and rims reduce usable volume compared with external measurements.
Drainage layer and usable fill space
Many growers reserve a bottom layer for drainage materials or for keeping the root zone above standing water. This tool subtracts that reserved height for supported shapes, then applies a fill percentage for rim clearance. Together, these options represent real planting conditions instead of an ideal “filled to the brim” container.
Soil settling and procurement strategy
Potting mixes compress after watering and root growth. A compaction allowance adds extra volume so you can top up later without running short. For bulk buying, use the total soil in liters, then compare to bag sizes. Conversions to US gallons and cubic feet help when suppliers quote different units.
Example data for a quick decision
Example: Round pot, 30 cm diameter, 28 cm height, fill 95%, drainage layer 3 cm, compaction 8%, containers 6. The calculator estimates net fill and adds settling allowance, producing a realistic total soil requirement for purchasing and staging.
- Suggested approach: verify dimensions twice, then keep the report for refills.
- Best practice: standardize containers in a bed to simplify irrigation tuning.
- Maintenance: re-check volume after repotting into a new shape or size.
FAQs
1) Should I measure inside or outside dimensions?
Inside dimensions are more accurate because walls and rims reduce usable space. If you only have outside measurements, expect actual planting volume to be slightly lower, especially for thick plastic or ceramic containers.
2) What fill percentage should I use for most pots?
Use 90–98% to leave rim clearance for watering. Small pots usually need more headroom to prevent overflow, while larger planters can be closer to full if watering is controlled.
3) When does a drainage layer height matter?
Use it when you intentionally reserve the bottom space for gravel, coarse material, or to elevate the root zone. It reduces the soil volume used for planting, which helps avoid buying excess mix.
4) Why add a compaction allowance?
Potting mixes settle after watering and over time. Adding 5–15% is common for fluffy mixes. It reduces the chance you run short when topping up containers during the first few weeks.
5) Which shape should I choose for tapered decorative pots?
Select the tapered option and enter top and bottom diameters plus height. This better matches common decorative planters and prevents overestimating soil compared with a simple cylinder assumption.
6) How accurate are the conversions to gallons and cubic feet?
The conversions use standard unit factors from liters to US gallons and cubic feet. Minor differences can occur if suppliers round bag volumes, so consider adding a small buffer for large purchases.
7) Can I use this for raised beds?
Yes, use the rectangular planter shape with internal length, width, and height. For beds filled below the top, adjust the fill percentage to match the planned soil depth.