Formula Used
- Round cylinder: V = π × r² × h
- Tapered pot: V = πh(D₁² + D₁D₂ + D₂²) ÷ 12
- Rectangular planter: V = length × width × depth
- Recommended diameter: maximum of root, repotting, spread, and stability requirements
- Soil required: usable volume minus layers and root-ball displacement, plus allowances
- Bag count: soil required ÷ bag size, rounded upward
How to Use
- Select the calculation mode and measurement units.
- Enter plant, root-ball, and current pot details.
- Choose the container shape and enter internal dimensions.
- Add soil layers, settling, reserve, and bag size.
- Enter a second pot when comparison is required.
- Press calculate and review all warnings carefully.
Example Data
| Example | Plant | Root Ball | Suggested Pot | Typical Use |
|---|---|---|---|---|
| 1 | Small herb | 10 × 9 cm | 14–16 cm | Kitchen container |
| 2 | Young houseplant | 20 × 18 cm | 24–28 cm | Indoor decorative pot |
| 3 | Tomato plant | 24 × 25 cm | 32–40 cm | Outdoor vegetable pot |
| 4 | Small shrub | 35 × 32 cm | 45–55 cm | Patio planter |
| 5 | Taproot plant | 18 × 30 cm | 24–30 cm deep | Deep container |
Frequently Asked Questions
How much larger should a new pot be?
Most plants need a modest increase during repotting. Large jumps can hold excess water around roots. Increase diameter gradually while checking drainage and root health.
Should pot measurements be internal or external?
Internal measurements give the most accurate usable volume. External measurements include walls and may overstate capacity. Enter wall thickness when only outside dimensions are available.
Why subtract the root-ball volume?
The root ball already occupies part of the container. New potting mix only fills the remaining free space. This prevents buying more soil than the project needs.
Does a drainage layer always help?
Drainage holes matter more than a deep gravel layer. Some plants need only a thin base layer. Use the setting to match your actual planting method.
Can this calculate tapered nursery pots?
Yes, tapered and truncated-cone formulas are included. Enter both top and bottom internal diameters. The calculator then estimates practical container volume accurately.
How are tall plants handled?
Tall plants need enough width for container stability. Wind exposure can increase the recommended base diameter. Heavy pots may still be required outdoors.
What does the custom shape factor mean?
The factor adjusts a rectangular bounding-box estimate. Use values below one for irregular empty space. Use one when the container fills the measured box.
Why include soil settling allowance?
Fresh potting mix can settle after watering. The allowance adds extra material for later topping. It reduces shortages during large planting projects.
Can I compare pots with different shapes?
Yes, each pot can use a different formula. The result shows both volume and percentage difference. This helps compare round, square, and tapered containers.
Are commercial pot sizes standardised?
Labelled sizes vary between manufacturers and regions. Actual internal volume may differ from advertised capacity. Measure the pot whenever precision matters.
Is the recommendation suitable for every species?
No calculator replaces species-specific horticultural guidance. Root habits and moisture needs can vary widely. Confirm unusual plants with reliable growing instructions.
Gardening Disclaimer
This calculator provides practical estimates for planning containers. Plant needs vary with species, climate, and care. Confirm sensitive plants with trusted horticultural guidance before repotting.