Use real internal dimensions. If your pot has feet, assume a small air gap below.
| Container | Media | Exposure | Suggested holes |
|---|---|---|---|
| Round pot: 25 cm × 25 cm | Standard potting mix | Outdoor, covered | 6 holes of 8 mm |
| Rect box: 50 × 25 × 25 cm | Chunky mix | Outdoor, open rain | 8 holes of 12 mm |
| Round pot: 12 cm × 12 cm | Dense mix | Indoors | 3 holes of 8 mm |
These examples assume a screen or mesh over holes.
1) Base area
- Round container: Base area = π × (D/2)²
- Rectangular container: Base area = Length × Width
2) Volume
- Volume = Base area × Height (or Depth)
- Liters = Volume(cm³) ÷ 1000
3) Target drainage area
- Target fraction starts at 1% of base area.
- It is adjusted by media, plant, exposure, and clogging.
- Target area = Base area × Target fraction
4) Hole count
- Area per hole = π × (d/2)²
- Hole count = ceil(Target area ÷ Area per hole)
- Pick the container shape and enter internal dimensions.
- Select your growing media and plant water preference.
- Choose exposure based on rain and placement.
- Set clogging risk based on fines and debris.
- Press Calculate, then review spacing and notes.
- Download CSV or PDF for records and planning.
Drainage performance and root health
Proper drainage keeps pore spaces open so roots exchange oxygen and avoid anaerobic stress. In containers, the base is the bottleneck, so capacity should scale with base area, not volume. Insufficient outlets can leave a perched water layer that stays saturated after irrigation. Use the calculator to align hole area with crop demand and local humidity.
Choosing a target drainage percentage
The calculator begins with a target open‑area percentage of the container base. Drier exposure, gritty mixes, and vigorous plants can use a higher percentage, while sheltered patios and moisture‑retentive blends use less. Increase the target for frequent rainfall, heavy hand‑watering, or saucers that restrict air. Decrease it for seedlings and herbs that prefer steady moisture.
Hole diameter versus hole count
Smaller holes spread flow and reduce media loss, but they clog faster when compost fines accumulate. Larger holes resist clogging and pass debris, yet can create local jets that wash mix away. A practical method is to pick a diameter that your mesh screen can cover, then let the calculator set the count. When using thick-walled planters, step up the diameter to prevent restriction.
Spacing rules to prevent weak spots
Even spacing distributes load and reduces cracking in plastic and thin ceramic bases. Keep the edge offset at least one hole diameter, and keep center‑to‑center spacing at least 1.5× diameter when possible. For rectangular boxes, distribute holes across the full footprint, not just corners. If feet or ribs exist, place holes between supports for free outflow.
Maintenance, clogging, and verification
After the first watering, confirm that water exits within one to two minutes for typical potting mixes. If flow is slow, raise the clogging risk setting and consider a coarser bottom layer or a removable mesh disk. Recheck holes seasonally, especially outdoors where silt and algae build up. Clean with a narrow brush, and retest after repotting or media breakdown. For self‑watering inserts, keep overflow holes slightly larger to prevent lockups. Record your chosen settings in the CSV so future potting batches stay consistent across seasons and container sizes. When in doubt, run a simple soak test before planting.
1) What hole diameter should I start with for most pots?
For typical indoor containers, 6–10 mm works well with a mesh screen. Outdoors or with chunky mixes, 10–16 mm reduces clogging. Use the calculator to fine‑tune the count for your base area.
2) Do I need mesh or screen over the holes?
A screen helps keep media inside while allowing water to pass. It also reduces clogging from roots and debris. If you skip mesh, consider slightly smaller holes and a higher clogging risk setting.
3) What if water pools on the surface after watering?
Pooling can indicate compacted media, blocked outlets, or too little drainage area. Check that holes are clear, reduce saucer restriction, and rerun the calculator with higher clogging risk or wetter exposure to increase total outlet area.
4) Can I add more holes than the result suggests?
Yes, especially for rain‑exposed planters. Keep spacing rules to protect the base and avoid structural weakening. If you add holes, consider using a slightly smaller diameter so the total opening area stays controlled.
5) How do saucers and indoor trays affect drainage?
Trays can trap water and reduce airflow under the base, slowing drainage. Empty standing water promptly and elevate pots with feet or risers. In the calculator, choose a sheltered exposure and monitor the note about air restriction.
6) Why is my soil washing out through the holes?
Washing out usually means holes are large for the particle size, or there is no screen. Add mesh, use a coarser layer near the base, or switch to fewer holes with smaller diameter while keeping the target open area similar.
Tip: If drainage is too fast, use a finer mix or fewer holes.