Closed Terrarium Layers by Jar Size
There is no single “correct” number of inches for a terrarium’s layers, because there is no single size of jar. The rule that actually works is a ratio: drainage takes up roughly 10 to 15 percent of the container’s total height, a thin charcoal band sits above it, and substrate fills roughly 20 to 30 percent. Apply that ratio to a small mason jar, a medium cookie jar, or a large glass bowl, and the same build method holds up regardless of what container is sitting on the table.
Most layer guides give a single answer, “add an inch of gravel,” as if every closed terrarium were built in the same six-inch fishbowl. It is not. A recycled pickle jar, a cookie jar, and a repurposed carboy each need a different literal depth even though they follow the exact same underlying logic. This piece is the depth reference that the full build method points to, so bookmark it and come back whenever the container changes.
The one rule that scales to any jar
Instead of memorizing a table of fixed numbers, work from a ratio of total container height:
- Drainage layer: 10 to 15 percent of height
- Charcoal layer: thin and flat, not scaled by percentage, since its job is surface area, not volume
- Substrate layer: 20 to 30 percent of height
A 6-inch-tall jar gets roughly 0.6 to 0.9 inches of drainage and 1.2 to 1.8 inches of substrate. A 12-inch container roughly doubles both. The ratio holds because the physics it is solving for, a reservoir for excess water below the roots and enough substrate depth for a root system to establish, scales with the size of the root zone and the volume of water the container will ever hold.
Drainage layer depth by container size
Three worked examples, using the 10 to 15 percent rule:
| Container | Approx. height | Drainage depth (10-15%) |
|---|---|---|
| Small jar (roughly 0.5 liter, a spice jar or small mason jar) | 4-5 inches | 0.4-0.75 inches |
| Medium jar (roughly 2-3 liters, a cookie jar or large mason jar) | 8-10 inches | 0.8-1.5 inches |
| Large bowl or carboy (5+ liters) | 12-14 inches | 1.2-2.1 inches |
Round up rather than down when in doubt. A slightly deeper drainage reservoir costs nothing but glass space; a shallow one lets water reach the substrate faster than it should. Container shape matters too: a tall, narrow jar holds less total volume per inch of drainage than a wide bowl, so the percentage rule tracks the physical situation better than a flat “one inch for everything” instruction ever could.
The charcoal layer: why it stays thin regardless of jar size
Charcoal does not scale with container height, because its job is surface area for odor and gas adsorption, not water storage. A quarter to half an inch is enough in a small jar or a large one. Piling on more charcoal does not add more benefit, it just eats into space that should go to substrate. The full case for whether this layer earns its place at all lives in do you actually need activated charcoal; the short version for this piece is: keep it thin, and do not treat a thicker layer as extra insurance.
The barrier layer: full coverage, no measurement needed
Between drainage and charcoal, a barrier (plastic window mesh or a thin pad of sphagnum moss) needs full edge-to-edge coverage, not a specific thickness. Its only job is stopping substrate from washing down into the drainage reservoir over time. Cut it to the container’s exact shape, including the corners of a square jar, since a gap at the edge defeats the point.
Substrate depth by container size
Using the same three examples, at 20 to 30 percent of total height:
| Container | Approx. height | Substrate depth (20-30%) |
|---|---|---|
| Small jar | 4-5 inches | 0.8-1.5 inches |
| Medium jar | 8-10 inches | 1.6-3 inches |
| Large bowl or carboy | 12-14 inches | 2.4-4.2 inches |
Root depth is the constraint here, not moisture capacity alone. Small ferns and mosses root shallow and are fine near the lower end of the range; a plant with a taller root ball needs the fuller depth. The substrate mix itself matters as much as the depth, since a compacting mix defeats even a generous layer.
Quick-reference table
| Jar size | Drainage | Charcoal | Substrate |
|---|---|---|---|
| Small (0.5L, ~4-5 in tall) | 0.4-0.75 in | ~0.25 in | 0.8-1.5 in |
| Medium (2-3L, ~8-10 in tall) | 0.8-1.5 in | ~0.25-0.5 in | 1.6-3 in |
| Large (5L+, ~12-14 in tall) | 1.2-2.1 in | ~0.5 in | 2.4-4.2 in |
A worked example, start to finish
Take a medium cookie jar, roughly 9 inches tall. Applying the ratios above:
- Drainage: 9 inches x 10-15 percent = roughly 0.9 to 1.35 inches of LECA or gravel.
- Charcoal: a flat quarter to half inch, regardless of the jar’s height.
- Barrier layer: full coverage, cut to the jar’s exact footprint, no measurement needed.
- Substrate: 9 inches x 20-30 percent = roughly 1.8 to 2.7 inches.
That leaves roughly 4 to 5 inches of open space above the substrate line for plants, hardscape, and the airspace the water cycle actually needs to function. Running the same math on a small spice jar or a large carboy, using their own heights, produces the same proportions scaled down or up, which is the entire point of working from a ratio instead of a fixed number.
What happens if you get a layer wrong
Each layer has a specific failure mode when it is undersized:
- Drainage too shallow: excess water reaches the substrate faster, and roots sit closer to standing water. See the drainage layer deep-dive for what standing water actually does over weeks.
- Charcoal skipped or too thin: a sealed jar can turn stale-smelling faster, especially if something starts to decay, though this is a minor factor next to watering discipline.
- Substrate too shallow: roots cannot establish, and the whole build reads as cramped and prone to tipping once plants grow.
- Substrate too deep with too little drainage underneath: the classic beginner mistake. Extra substrate does not fix a drainage problem, it just gives excess water more places to pool.
None of these are catastrophic on their own. A jar rebuilt with corrected ratios recovers fine; this is a depth reference, not a one-shot decision.
FAQ
Does layer depth change for a bigger jar, or just the ratio?
Just the ratio, applied to a bigger number. A 12-inch jar and a 6-inch jar use the same 10-15 percent drainage and 20-30 percent substrate math; the 12-inch jar simply gets roughly double the literal depth.
What happens if my drainage layer is too shallow?
Excess water has less room to collect before it reaches the substrate, so roots sit closer to standing moisture sooner than they should. It is not an immediate failure, but it raises the odds of root rot over the following weeks, especially with generous watering.
Can I skip the barrier layer if I am careful with the charcoal amount?
No. The barrier layer’s job is stopping substrate from washing down into the drainage reservoir over months of moisture cycling, which charcoal placement does not address at all. Skipping it means the layers slowly merge into wet mud at the base.
Is there a maximum jar size where this ratio stops working?
Not in any practical home-terrarium range. Very large containers (aquarium-sized and up) start to need engineering considerations beyond a simple ratio, but anything from a spice jar to a large carboy fits comfortably inside this method.