Terrarium Substrate Layer Calculator
Calculate the depth and litres of every terrarium layer: drainage, charcoal, growing medium and top dressing, from your container size and build type.
A properly layered terrarium substrate is essential for plant health, drainage, and preventing root rot. Each layer serves a specific purpose, and the proportions depend on your container size and terrarium type (open vs. closed).
Standard Terrarium Layer Structure (bottom to top)
| Layer | Material | Purpose | Share of Total Height |
|---|---|---|---|
| 1. Drainage | Pebbles, LECA, gravel | Prevents waterlogging | 15–20% |
| 2. Separation | Mesh screen or charcoal | Stops soil from sinking into drainage | Negligible (a sheet) |
| 3. Activated Charcoal | Horticultural charcoal | Filters water, prevents odors | 3–5% |
| 4. Growing Medium | ABG mix, potting soil | Where roots grow | 25–35% |
| 5. Top Dressing | Sphagnum moss, leaf litter | Retains moisture, aesthetics | 5–10% |
How the shares are applied. The whole stack fills 60% of the container, leaving 40% as air and planting space. Those four percentages are its internal split. For a closed tropical build they add up to exactly 60, so each one is also a straight percentage of the container height. The arid and bioactive builds use a different split with a different total, so the calculator rescales them to the same 60% fill. That is why an arid drainage layer comes out slightly deeper than the 12% weight suggests.
Volume Formula
For a rectangular container: Volume (liters) = Length (cm) x Width (cm) x Depth (cm) / 1000
For a cylindrical container: Volume (liters) = pi x Radius^2 (cm) x Depth (cm) / 1000
Each layer’s volume = Total volume x (layer percentage / 100).
Worked Example: 30cm x 20cm x 25cm Closed Tropical Terrarium
Total interior volume = 30 x 20 x 25 = 15,000 cm^3 = 15 liters. Base area = 30 x 20 = 600 cm^2. The stack fills 60% of the 25 cm height, so 15 cm of it.
- Drainage layer (20% of 25 cm): 5 cm deep = 3 liters of LECA
- Separation mesh: thin sheet, negligible volume
- Charcoal layer (5%): 1.25 cm = 0.75 liters of charcoal
- Growing medium (30%): 7.5 cm = 4.5 liters of ABG mix
- Top dressing (5%): 1.25 cm = 0.75 liters of sphagnum moss
- Total stack: 15 cm, 9 liters of material
- Remaining 10 cm (40% of the height) = open air space for plant growth
Nine litres of substrate sounds like a lot for a 15-litre tank, and it is worth pausing on. Terrarium builders consistently underestimate this: a bag of ABG mix that looks enormous in the shop disappears into a medium tank. Buy more than you think, because topping up a half-built stack means disturbing what you already laid.
Terrarium Type Adjustments
Closed tropical terrariums need thicker drainage layers (20%) because excess moisture cannot evaporate. Open arid terrariums need thinner drainage (10–15%) but deeper growing medium with added perlite or sand for faster drainage. Bioactive vivariums require a deeper substrate overall (at least 7–8 cm of growing medium) to support microfauna like springtails and isopods.
Where the percentage rule stops working. A 60% fill is right for the jars and tanks most builds use. Apply it to a 60 cm tall vertical vivarium and it asks for 36 cm of material, which is a great deal of weight and no benefit: roots do not go that deep, and drainage past about 10 cm holds no water the first 10 cm did not. The calculator caps each layer (10 cm drainage, 2 cm charcoal, 12 cm growing medium, 3 cm top dressing) and says so when a cap has bitten. Below roughly 45 cm of height nothing is capped and the percentages run as written.
Material Quantities by Weight
| Material | Approximate Density | 1 Liter Weighs |
|---|---|---|
| LECA (expanded clay) | 0.3–0.5 kg/L | ~400g |
| Activated charcoal | 0.2–0.4 kg/L | ~300g |
| ABG mix | 0.4–0.6 kg/L | ~500g |
| Sphagnum moss (dry) | 0.03–0.05 kg/L | ~40g |
| Pea gravel | 1.5–1.7 kg/L | ~1,600g |
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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