| Growth form | Diameter | Height | Material | Environment | Recommended pot diameter | Recommended depth | Approx. soil volume |
|---|---|---|---|---|---|---|---|
| Globular | 8 cm | 10 cm | Terracotta | Indoor | 11–12 cm | 11–12 cm | 0.9–1.1 L |
| Columnar | 6 cm | 25 cm | Ceramic | Indoor | 9–10 cm | 14–16 cm | 0.8–1.2 L |
| Clustering | 16 cm | 18 cm | Plastic | Humid | 19–20 cm | 16–18 cm | 4.0–5.0 L |
This calculator estimates internal pot diameter from the rootball (or plant) diameter, then limits upsizing to reduce overwatering risk. Depth is based on root depth (or a growth-form estimate) plus drainage and headspace.
Dtarget = min(Dtarget × Adjustment, Dref + Limit)
SoilDepth = Depth − Headspace
- Measure cactus width (widest point) and height from soil line.
- If possible, measure the rootball after removing the plant.
- Choose growth form and set a realistic moisture risk score.
- Set drainage layer and headspace based on your pot style.
- Press Calculate Pot Size and review results.
- Use the recommended internal size when selecting a pot.
- Download CSV or PDF to save your repot notes.
- Use a gritty, fast-draining mix for safer drying.
- After repotting, wait before watering to heal roots.
- If you must use a larger pot, water less often.
Pot sizing for healthy roots
Pot size affects drying speed, root oxygen, and stability. This calculator recommends internal dimensions based on the rootball (or plant diameter when roots are unknown), then limits the jump to reduce soggy soil. A modest increase usually dries faster and lowers rot risk, especially indoors or in humid rooms.
Clearance and safe upsizing ranges
Clearance is the space between the rootball and the pot wall. Small cacti often do best with 1.5–3 cm (or 0.6–1.2 in) per side, while larger specimens may use 3–4 cm. Clustering forms typically need slightly more lateral room. Overpotting occurs when the pot is much wider than the root mass, increasing wet time after watering.
Depth, drainage, and stability
Depth is tied to root depth, plus a drainage layer and headspace. Columnar cacti may need extra depth for stability, but very deep pots can trap moisture at the bottom. Terracotta usually dries faster than plastic, which can justify a slightly larger diameter under the same watering routine.
Soil volume planning and mix strategy
Soil volume is estimated from the internal dimensions and pot shape. Round pots hold less soil than square pots at the same width. Use the volume output to plan a gritty blend (pumice/perlite, coarse sand, and mineral components) so the pot dries evenly. In humid settings, reduce organic content and increase airflow.
Example data and practical decisions
Example inputs and outputs help validate your measurements. If a cactus is firm and not rootbound, stay near the lower end of the range. If roots circle tightly, move toward the upper end. For rot recovery, keep the pot closer to the current rootball size and water lightly.
| Input example | Setup | Typical recommendation |
|---|---|---|
| Diameter 5 cm, height 9 cm | Terracotta, indoor, risk 5/10 | Pot Ø 8–9 cm, depth 10–11 cm, ~0.5 L soil |
| Diameter 10 cm, height 20 cm | Ceramic, indoor, risk 6/10 | Pot Ø 13–14 cm, depth 15–17 cm, ~2.0 L soil |
| Diameter 15 cm, height 18 cm | Plastic, humid, risk 8/10 | Pot Ø 17–18 cm, depth 16–18 cm, ~3.5 L soil |
1) Should I measure the pot’s inner or outer diameter?
Use the inner diameter. Outer measurements vary by wall thickness and can mislead sizing, especially with thick ceramic pots.
2) Why does a bigger pot increase rot risk?
More soil holds more water and dries slower. If roots cannot use that moisture quickly, the root zone stays damp and oxygen-poor.
3) How much bigger should the new pot be?
A modest upgrade is safer: usually one size up or within the recommended range. Large jumps often prolong drying and slow growth.
4) Does pot shape change watering needs?
Yes. Square pots typically hold more soil than round pots at the same width, so they may dry slower and need more conservative watering.
5) What moisture risk score should I choose?
Use higher scores for humid rooms, low light, cool seasons, or plastic pots. Use lower scores for hot, bright, airy setups with gritty mixes.
6) Can I reuse the same pot after trimming roots?
Often, yes. After pruning or rot removal, a snug pot that matches the reduced rootball can dry faster and help roots recover.
7) How soon can I water after repotting?
Many growers wait several days so small root injuries can callus. The exact timing depends on warmth, airflow, and how dry the mix is.