A terrarium build calculator tells you exactly how deep each substrate layer should be and how much material to buy based on your container's dimensions. Whether you are building an open desert terrarium for succulents or a closed tropical terrarium for ferns and mosses, the layer depths and plant count will vary — this tool handles both.
Container Details
Enter your container dimensions and click Calculate Terrarium to see layer depths and material quantities.
How to Build a Terrarium: Layer Guide
The terrarium build calculator takes the guesswork out of substrate layering. Enter your container shape and dimensions, choose between a closed tropical or open desert build, and the calculator tells you exactly how deep each layer should be and how much material to buy.
Step 1: Measure Your Container
Select your container shape (cylinder, rectangle, or bowl/dome) and enter the dimensions in inches or centimeters. The total height is the most critical measurement — it determines how much vertical space is available for layers and planting room. Aim for at least 8-10 inches of total height for a functional terrarium.
Step 2: Choose Your Terrarium Type
Closed tropical terrariums need more soil depth and higher moisture retention, so they use heavier substrate ratios. Open desert terrariums need excellent drainage with more perlite and grit in the mix. The layer proportions differ: tropical builds use 25% substrate while desert builds use only 20% soil (with more drainage medium).
Step 3: Build the Layers
Layer 1 — Drainage (pebbles/LECA): At least 1 inch or 15% of container height. Stores excess water away from roots. Use pea gravel, LECA balls, or aquarium pebbles.
Layer 2 — Activated charcoal: A thin 0.5-inch layer. Prevents bacterial growth and odors. Use horticultural activated charcoal only.
Layer 3 — Mesh screen: Optional barrier between drainage and soil. Prevents soil from migrating into drainage layer over time.
Layer 4 — Substrate: 25-30% of container height. Use terrarium-specific soil for closed builds or cactus/succulent mix for open builds.
Layer 5 — Decorative top layer: 5% of height. Moss, pebbles, sand, or decorative bark for visual appeal and moisture retention.
Plant Spacing and Count
The calculator estimates plant count using approximately 9 square inches of planting area per small plant (roughly a 3-inch pot footprint). For a terrarium with 50 square inches of planting area, that suggests about 5-6 small plants. Leave more space between plants if you are using species that will spread or trail. Always leave the center slightly more open — plants fill in from the edges.
FAQ
Is this terrarium calculator free?
Yes, completely free with no signup required. Enter your container dimensions and get instant layer depths, material quantities, plant count, and a shopping list. Imperial and metric both supported.
Is my data private?
All calculations happen locally in your browser. Nothing is sent to any server.
What is the difference between an open and closed terrarium?
Open terrariums have exposed tops and are best for succulents, cacti, and air plants that need low humidity and good airflow. Closed terrariums have lids and create a self-contained humid environment ideal for tropical plants like ferns, mosses, and miniature aroids. The substrate layers and plant recommendations differ between the two types.
Why does a terrarium need a drainage layer?
Since most terrariums have no drainage holes, excess water must be managed within the container. A bottom layer of pebbles, LECA (lightweight expanded clay aggregate), or gravel stores water away from roots, preventing root rot. The activated charcoal layer above it helps prevent bacterial growth and keeps the terrarium fresh-smelling.
What is the activated charcoal layer for?
Activated charcoal (also called activated carbon) has natural antimicrobial properties that reduce bacterial and fungal growth inside the terrarium. It helps control odors and keeps the substrate healthy. Use horticultural activated charcoal, not aquarium or grill charcoal. A thin layer of 0.5 inches is sufficient.
How many plants fit in a terrarium?
The calculator estimates plant count based on planting area divided by 9 square inches per small plant — a reasonable spacing for miniature varieties. In practice, you can plant more densely with very small plants (under 2 inches diameter) or less densely with plants that will grow larger. Leave at least 1-2 inches between plants to allow for growth and airflow.
What plants work best in closed terrariums?
Closed terrariums excel with moisture-loving plants: ferns, mosses, miniature Ficus pumila, nerve plants (Fittonia), small Pilea species, and miniature Begonias. Avoid succulents and cacti in closed environments — the high humidity causes root rot. The tool filters plant suggestions from the database to match your terrarium type.
How do I convert cups to quarts for terrarium materials?
4 cups = 1 quart = roughly 1 liter. The calculator shows both volume and practical purchasing units (bags of gravel, pots of soil) to make shopping easier. For small terrariums, buy loose materials by volume. For larger projects, bags of substrate are more economical.