Substrate Layer Ratio Calculator
Enter your container’s height, the share of it to fill with layers, and whether it’s open or closed, and get the drainage, charcoal, substrate, and top-dressing depths, with the room left for plants.
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 layers fill only part of the container, a third by default (a rule of thumb), so the plants have room above them; the fill is split into charcoal, soil and top dressing by standard hobbyist shares, and the drainage reservoir is sized from the soil above it. Until October 2026 this calculator split the full height to the rim, so its layers filled the whole container; it now never stacks them to the rim.
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.
How deep is the drainage layer, and why does it match the Drainage Layer calculator?
The reservoir is our Drainage Layer calculator's answer for the soil depth above it: 18% of that depth, and never less than 2.5 cm in a container with no drainage hole. Until October 2026 this calculator used its own share of the height (15% closed, 10% open), which gave a different depth from the Drainage Layer calculator for the same jar (4.5 cm against 3.9 cm for a 30 cm jar). One jar now gets one reservoir depth from both.
Do I really need the charcoal layer?
It's a thin layer (about 3% of the fill 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 fill 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.