Terrarium substrate calculator
Give the shape and dimensions of your container. The tool works out the depth and volume of each layer, in centimetres and litres. It is free, with no sign-up.
Your layers
A 6.36 L container: 1.62 L of layers, 6.4 cm deep. That leaves 18.6 cm of height for the plants.
| Layer | Depth | Volume |
|---|---|---|
| SubstrateLight, low-nutrient potting mix | 3.9 cm | 0.99 L |
| CharcoalHorticultural or activated, in a thin layer | 0.5 cm | 0.13 L |
| DrainageClay pebbles, gravel or lava rock | 2 cm | 0.5 L |
| All layers | 6.4 cm | 1.62 L |
- Free height for the plants
- 18.6 cm
- Container volume
- 6.36 L
- Suggested plants
- about 4
- Weight once planted
- about 6.4 kg
- Check that the substrate is at least as deep as the largest root ball of your plants.
- Moss and decor go on top. Our guides give them no set depth, so the tool does not count them.
How we calculate
No figure is made up. The rules of thumb come from our guides, which cite their sources.
- The volume of the containerThe tool works out the inside volume with the formula for the chosen shape: cylinder, box or sphere.
- A quarter for the layersDrainage, charcoal and substrate together take up about a quarter of the volume. This is the University of Illinois rule of thumb, used in our substrate guide. The rest is for the plants.
- How the layers share itThe layers keep the ratio of the classic build: 2.5 cm of drainage, 0.5 cm of charcoal, 5 cm of substrate. Drainage stays between 1.5 and 3 cm, and charcoal between 0.5 and 1 cm. In a large container, anything extra goes to the substrate; in a small jar, the layers take up a little more than a quarter.
- Open or closedAn open succulent terrarium has no charcoal layer, and its substrate is a mineral mix. The rest of the calculation is the same.
What the tool cannot know
- It is a guide, not a rule. Our guide to layers says so: a fixed depth in centimetres would be misleading. The size of the root balls decides.
- Drainage and charcoal are optional. If you leave them out, give their height to the substrate.
- For a sphere, the calculation follows the curve of the glass. A demijohn has a neck and a flat base: for an exact figure, measure its volume with water.
- Results are rounded to the millimetre and the centilitre.
The guides behind the numbers
- The layers of a terrarium explained
- Which substrate should you use for a terrarium?
- Charcoal in a terrarium: useful or not?
- What size of terrarium should you choose to start with
Would you rather not measure anything?Have a look at the build-your-own terrarium kits.See the kits
Cite or link to this tool
Do you write about terrariums? You are free to cite this tool. Copy the page address, or the link ready to paste into your site.
FAQ
Questions about substrate and layers.
How deep should the substrate be in a terrarium?
There is no single figure. The rule of thumb: all the layers together take up about a quarter of the container's volume. The substrate should also be at least as deep as the largest root ball of your plants.
How many litres of substrate for a 5-litre jar?
About 1.27 litres of layers in all, for a straight-sided jar: nearly 0.4 litres of drainage, 0.1 litres of charcoal and nearly 0.8 litres of substrate.
Do you need charcoal in a terrarium?
It is not essential. In a closed terrarium, a thin layer of 0.5 to 1 cm limits smells. In an open succulent terrarium it adds little: the tool leaves it out.
Does the calculator work for a demijohn?
Yes, with the “sphere” shape. The calculation follows the curve of the glass: a layer at the bottom holds less material than a layer in the middle. A demijohn has a neck and a flat base, so treat the result as a rough guide.
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