Choose a pot shape, enter dimensions, and apply drainage and fill settings.
| Example pot | Dimensions | Drainage | Fill | Usable volume |
|---|---|---|---|---|
| Cylindrical | Diameter 30 cm, Height 25 cm | 2 cm | 95% | ~16.0 L |
| Rectangular | 40 × 25 × 20 cm | 2 cm | 90% | ~16.2 L |
| Tapered round | Top 35 cm, Bottom 22 cm, Height 28 cm | 3 cm | 95% | ~18.3 L |
| Tapered square | Top 30 cm, Bottom 18 cm, Height 26 cm | 2 cm | 95% | ~14.1 L |
- Cylinder: V = π × (d/2)² × h
- Rectangular: V = L × W × h
- Tapered round (frustum): V = (π × h × (R² + Rr + r²)) / 3
- Tapered square (frustum): V = (h × (A1 + √(A1A2) + A2)) / 3, where A1 = top², A2 = bottom²
- Select the pot shape that matches your container.
- Choose centimeters or inches, then enter the dimensions.
- Set a drainage layer depth if you use stones or grit.
- Adjust fill percentage to leave headspace for watering.
- Press Calculate to see usable liters and conversions.
Soil volume planning for healthy roots
Container gardening depends on reliable soil volume estimates. When volume is known, you can purchase the right amount of potting mix, keep nutrient levels consistent, and avoid compacting roots by underfilling. This calculator reports usable volume after accounting for a drainage layer and optional headspace, helping you standardize planting across different pot styles.
Comparing common pot shapes
Cylindrical pots are easy to measure and provide uniform wall spacing. Rectangular planters suit balconies and raise bed edges, but need careful width measurement. Tapered pots hold less soil than their top opening suggests, so frustum geometry is used to capture the true internal capacity for both round and square designs.
Drainage layers and fill percentage
A gravel or stone layer reduces soil volume and can change moisture behavior. Entering a realistic drainage depth prevents overbuying mix and improves repeatability between pots. Fill percentage models watering headspace: many growers target 90–95% fill so water stays inside the rim and fertilizer solutions distribute evenly.
Example data for quick checking
Use these values to validate inputs and understand typical results (rounded):
- Cylinder: 30 cm diameter × 25 cm height, 2 cm drainage, 95% fill → ~16.0 L
- Rectangular: 40 × 25 × 20 cm, 2 cm drainage, 90% fill → ~16.2 L
- Tapered round: 35 cm top, 22 cm bottom, 28 cm height, 3 cm drainage, 95% fill → ~18.3 L
Ordering mix and standardizing containers
The liters output aligns with bag labeling, while gallons and cubic inches support mixed-unit planning. If you manage many pots, record your favorite dimensions and aim for consistent usable volume per crop group. This helps unify irrigation schedules, reduces nutrient variability, and makes seasonal replanting faster and more predictable.
1) Which measurement point should I use for pot height?
Measure the internal height from the base to where soil can sit, not the decorative rim. If the base is raised, measure to the lowest soil line so capacity is not overstated.
2) Why does a tapered pot hold less soil than expected?
Tapered walls reduce cross-sectional area toward the bottom. Even with a wide opening, the average area is smaller, so frustum volume gives a more accurate capacity than using only the top diameter.
3) Should I always add a drainage layer?
Not always. Many modern mixes drain well without stones. If you use a drainage layer for stability or airflow, enter its depth so the soil volume reflects what plants actually receive.
4) What fill percentage is recommended for most pots?
A common target is 90–95%. This leaves headspace for watering, mulch, and top dressing. For seedlings or low-water plants, you may prefer slightly more headspace to avoid overflow.
5) How do I estimate soil bags needed from liters?
Divide usable liters by the bag size in liters. For example, a 25 L bag covers about one pot needing 16 L, with extra for top-ups. Add a small buffer for spillage and settling.
6) Can I use outside dimensions if I cannot measure inside?
You can, but subtract wall thickness. If thickness is unknown, results may be high. For accuracy, measure internal diameters or sides with a flexible tape and use internal height to the soil line.
7) Why are there multiple unit conversions in the result?
Liters match most potting mix labels, gallons help when buying bulk in some regions, and cubic inches support legacy dimensions. Conversions let you compare containers and purchases without manual math.