Terrarium Drainage Layer Calculator
Calculate the ideal drainage layer depth and material volume for your terrarium.
Prevent root rot with proper layering.
Why terrariums need a drainage layer:
Unlike regular pots, most terrariums have no drainage holes. Excess water pools at the bottom, and without a drainage layer, plant roots sit in stagnant water and rot. The drainage layer creates a reservoir that separates the soil from standing water, letting gravity pull moisture away from the root zone.
Drainage layer depth formula: The standard guideline is that the drainage layer should be 1/5 to 1/4 of the total container height for closed terrariums, and 1/6 to 1/5 for open terrariums (which lose moisture to evaporation and are less prone to waterlogging).
Drainage depth = Container height × Layer ratio
Layer ratios by terrarium type:
| Type | Drainage Ratio | Example (12 in tall) |
|---|---|---|
| Closed tropical | 1/4 (25%) | 3 in |
| Open tropical | 1/5 (20%) | 2.4 in |
| Succulent / arid | 1/3 (33%) | 4 in |
| Bioactive vivarium | 1/4 (25%) | 3 in |
Volume calculation: For a rectangular container: Volume = Length × Width × Drainage depth. For a cylindrical container: Volume = π × (Diameter/2)² × Drainage depth. For a bowl/sphere (approximated): Volume ≈ 0.65 × π × (Diameter/2)² × Drainage depth.
Drainage material options:
| Material | Particle Size | Best For | Notes |
|---|---|---|---|
| LECA (expanded clay) | 8–16 mm | All terrariums | Lightweight, pH neutral, reusable |
| Pea gravel | 4–8 mm | Open terrariums | Heavy, cheap, widely available |
| Lava rock | 10–25 mm | Bioactive vivariums | Very porous, excellent drainage |
| Activated charcoal | 2–5 mm | Added as thin sub-layer | Absorbs toxins, prevents odor |
Worked example: A cylindrical closed terrarium, 8 in diameter × 14 in tall:
- Drainage ratio: 1/4 (closed)
- Drainage depth: 14 × 0.25 = 3.5 in (89 mm)
- Volume: π × 4² × 3.5 = 176 cubic inches = 2.88 liters
- LECA weighs ~300 g/L, so you need ~865 g (1.9 lb)
- Add a 0.5 in layer of activated charcoal on top of LECA (~0.4 L)
The full terrarium layer stack (bottom to top):
- Drainage layer (LECA/gravel) — calculated above
- Mesh screen or landscape fabric — prevents soil from sinking into drainage
- Activated charcoal — thin 1 cm layer for filtration
- Substrate/soil mix — typically 2–3× the drainage depth
- Decorative top layer — moss, stones, leaf litter