Substrate Layer Ratio Calculator
Enter your container’s height and whether it’s open or closed, and get the drainage, charcoal, substrate, and top-dressing depths for a healthy planted terrarium stack.
Layer-by-layer breakdown
How the ratio is built
The stack is measured from the base of the container upward: drainage material first, then a thin charcoal filter, then the growing substrate, then a top dressing. The percentages are standard hobbyist starting ratios for a planted glass vessel — a small jar and a large tank both scale the same way, so double the height and every layer roughly doubles too.
Treat the numbers as a starting layout, not a rule cast in stone. A chunkier drainage material (like large LECA balls) may want a touch more depth than fine aquarium gravel, and very shallow dishes sometimes skip the charcoal layer entirely if they’re opened and refreshed often.
Frequently Asked Questions
Why does a terrarium need layers at all?
A planted glass container has no drainage hole, so the layers do the job a plant pot's hole normally does. A drainage layer at the bottom gives excess water somewhere to go so roots don't sit in it, a thin charcoal layer helps keep that trapped water from turning stagnant, and the substrate on top is where roots actually grow.
Why does a closed terrarium get a deeper drainage layer?
A sealed or tight-lidded terrarium recycles its own moisture in a mini water cycle — condensation drips back down constantly. That means more water ends up pooling at the bottom over time, so the calculator gives closed terrariums a deeper reservoir (about 15% of the container height) than open ones (about 10%).
Do I really need the charcoal layer?
It's a thin layer (about 3% of the height) but it earns its place: activated charcoal helps filter the water sitting in the drainage layer and reduces odour and mould risk in a sealed container. It sits directly on top of the drainage material, before the substrate.
What counts as the top-dressing layer?
That's whatever sits on the visible surface — a moss carpet, decorative sand, or fine gravel. It's mostly cosmetic and helps hold moisture near the surface, but it isn't load-bearing, so the calculator reserves a modest 10% of the height for it.
Hobby and decor guidance only — a starting layout for planning a terrarium build, not a horticultural guarantee. This tool makes no claims about plant safety or toxicity; always research a specific plant before adding it to a container shared with pets or children.