A closed or open plant terrarium is built in layers, each with a job. Getting the depth of each layer right keeps roots out of standing water and leaves enough room above for the plants to grow. This calculator turns layer depths into volumes so you buy the right amount of each material.
The classic layers
From bottom to top:
- Drainage layer, usually 1 to 2 in of small stones, gravel or expanded clay balls. Terrariums have no hole in the bottom, so excess water collects here instead of drowning the roots.
- Barrier, a thin mesh or a layer of moss that stops soil from sifting into the drainage.
- Activated charcoal, about ¼ to ½ in. It is often added to limit odors and the growth of molds in closed jars; its real effect is debated, but it does no harm.
- Soil, 2 to 4 in of a light, well-drained mix suited to your plants: a tropical mix for ferns and mosses, a gritty cactus mix for succulents (which belong in open containers).
- Decoration: moss, pebbles, bark or small figures.
How the volumes are computed
Each layer is a slab with the floor area of the container:
volume = floor area × layer depth
For an 18 × 12 in tank, the floor is 216 sq in. A 1.5 in drainage layer is 324 cubic inches, or about 5.6 quarts (57.75 cubic inches per quart). The charcoal takes about 0.9 qt and 3 in of soil about 11.2 qt.
For a round jar or bowl, enter the inside diameter at the level of the layers. For a bowl that narrows at the bottom, the result is a slight overestimate, which gives you a little spare.
Leave room for the plants
The layers should take no more than about a third of the container height. The calculator warns you above 40%. In a 12 in tall tank with 4.75 in of layers, about 7 in remain for foliage, which suits small ferns, fittonias and mosses.
Tips
- Rinse gravel and clay balls before use to avoid cloudy water.
- Water sparingly: a closed terrarium may need water only every few weeks.
- Keep it in bright, indirect light; direct sun overheats glass.
- Remove dead leaves quickly to prevent mold.