Terrarium Drainage Layer Calculator
Calculate drainage layer depth and volume for your terrarium from footprint dimensions.
Returns LECA, gravel, or foam volume to prevent root rot.
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
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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