Terrarium Plant Count Calculator
Calculate how many plants fit in your terrarium based on container size, plant species, and growth habit.
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.