Terrarium Drainage Layer Calculator

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:

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:

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)
• Charcoal on top of the LECA is a flat 1 cm regardless of vessel height: π × 4² × 0.394 in = 19.8 cubic inches, about 0.32 L

The full terrarium layer stack (bottom to top):

1. Drainage layer (LECA/gravel), calculated above
2. Mesh screen or landscape fabric, prevents soil from sinking into drainage
3. Activated charcoal, a thin 1 cm layer for filtration
4. Substrate/soil mix, around 1.5× the drainage depth
5. Decorative top layer, moss, stones, leaf litter

Two limits worth knowing about, because a pure percentage rule breaks on tall vessels.

The percentage-of-height rule works on the jars and tanks most people build in. Push it to a 60 cm vertical vivarium and 25% of the height is 15 cm of expanded clay, which is a great deal of weight and wasted growing space for no extra benefit. A drainage layer only has to hold the water the substrate sheds; past about 10 cm (4 in) it is doing nothing the first 10 cm did not already do. This calculator caps the depth there and tells you when the cap has bitten.

The second limit is the total fill. Drainage plus charcoal plus substrate should not swallow more than roughly 70% of the container, or there is nowhere for the plants to grow. A tall arid build hits that ceiling easily, since the arid drainage ratio is 1/3. When the stack overruns, the substrate is trimmed back rather than the drainage, because the drainage depth is the part doing the structural job.