How to Use the Terrarium Soil & Gravel Ratio Calculator
Get the layers wrong in a terrarium and you'll either end up with poor drainage that rots roots, or soil so thin that plants can't establish themselves. This calculator takes your container's diameter and height plus your plant type (moss, succulent, or tropical) and calculates the thickness and material amount needed for the drainage (gravel), charcoal, and soil layers.
Succulents are prone to root rot, so they get the largest drainage share (40%) with a thinner soil layer on top. Tropical plants and moss, on the other hand, prefer humidity, so they get a thicker soil layer around 70% and a smaller drainage layer. The charcoal layer stays thin either way, but it helps control odor and mold, which matters most in closed containers.
The calculation fills roughly 35% of the total container height with drainage, charcoal, and soil, leaving the rest as growing room and visual space above. If your container is narrow and tall, lean toward a slightly thicker soil layer; if it's wide and shallow, pay extra attention to the drainage layer.
Frequently Asked Questions
A charcoal (activated carbon) layer matters most in closed terrariums, where it helps control odor and mold. Open terrariums you air out often can skip it, but it's still recommended.
Succulents get a thicker drainage layer, but they're very prone to root rot from overwatering. Water lightly only after the soil surface is fully dry, and keep the drainage layer generous so water never pools.