Terrarium Plant Count Calculator
Calculate plant count for your terrarium from dimensions and type.
Returns count for mosses, ferns, succulents, and tropical plants with spacing guidelines.
Calculating the right number of plants for a terrarium prevents overcrowding (which causes mold and poor airflow) and underfilling (which looks sparse and lets weeds take hold in bioactive setups).
Plant Spacing Formula
Plants per terrarium = Plantable Area / Space per Plant
Plantable Area = Total Base Area x Coverage Factor (typically 0.70–0.85, accounting for hardscape like rocks and wood).
Space per Plant depends on the mature spread of the species.
Plant Size Categories and Spacing
| Category | Examples | Mature Spread | Space Needed |
|---|---|---|---|
| Miniature | Fittonia (nerve plant), baby tears, moss | 5–8 cm | ~25 cm^2 |
| Small | Peperomia, Pilea, small ferns | 10–15 cm | ~80 cm^2 |
| Medium | Calathea, begonia, larger ferns | 15–25 cm | ~200 cm^2 |
| Large / Focal | Small palms, dracaena, large bromeliads | 25–40 cm | ~500 cm^2 |
Worked Example — 40cm x 25cm Closed Tropical Terrarium
Total base area = 40 x 25 = 1,000 cm^2. Plantable area (80% coverage factor, 20% hardscape) = 1,000 x 0.80 = 800 cm^2.
If using small plants (80 cm^2 each): 800 / 80 = 10 plants. If mixing sizes: 1 medium focal (200 cm^2) + remaining 600 cm^2 / 80 = 7 small plants = 8 plants total.
Design Rule of Thirds
Professional terrarium designers use a layered planting approach:
- Background (tall): 1–2 plants, placed at the rear — 15–20% of planting area
- Midground (medium): 2–4 plants — 30–40% of planting area
- Foreground (small/ground cover): 3–6 plants or moss — 40–50% of planting area
This creates visual depth even in a small container.
Overcrowding Warning Signs
If plants touch the glass walls on more than one side, you have too many. Leaves overlapping by more than 30% block light from lower plants. In closed terrariums, overcrowding traps moisture against leaves and causes fungal problems. A good rule: fill 60–70% of the visual space at planting time, and let plants grow into the remaining 30–40% over the following months.
Cylindrical Containers
For round jars and bowls: Plantable area = pi x (radius - 2cm)^2. Subtracting 2 cm from the radius accounts for the curved glass edge where roots cannot reach.