Terrarium Size Calculator
Calculate terrarium dimensions and plant capacity from container volume and plant types. Enter values for instant results with step-by-step formulas.
Reviewed for accuracy by Daniel Agrici, Founder & Lead Developer
Terrarium Size Calculator
Calculator
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Formula: Volume = L x W x H (rectangular) | Volume = Pi x r^2 x H (cylindrical) | Plants = Base Area / Spacing
Worked example — 7 small plants | 4.2 gal volume | 5.0 qt soil | 6.7 cups drainage
Formula
Volume = L x W x H (rectangular) | Volume = Pi x r^2 x H (cylindrical) | Plants = Base Area / Spacing
Calculate container volume based on shape, then subtract drainage and soil layer volumes to determine usable growing space. Divide the base area by the spacing requirement per plant size to estimate maximum plant capacity. Use 70% of maximum for recommended planting density.
Worked Examples
Example 1: Rectangular Glass Box Terrarium
Problem:A rectangular glass container measuring 12 x 8 x 10 inches. Plan for 1 inch drainage, 3 inches of soil, and small tropical plants (2-3 inch spacing).
Solution:Total volume = 12 x 8 x 10 = 960 cu in (4.2 gal / 15.7 liters) Base area = 12 x 8 = 96 sq in Usable growing height = 10 - 1 - 3 = 6 inches Drainage layer = 96 x 1 = 96 cu in (6.7 cups pebbles) Soil layer = 96 x 3 = 288 cu in (20.0 cups / 5.0 quarts) Charcoal = 96 x 0.5 = 48 cu in (3.3 cups) Max small plants = 96 / 9 = 10 plants Recommended = 10 x 0.7 = 7 plants Watering amount = 15.7 x 5 = 79 ml per watering
Result:7 small plants | 4.2 gal volume | 5.0 qt soil | 6.7 cups drainage
Example 2: Large Spherical Glass Globe
Problem:A spherical glass globe with 14-inch diameter. Plan for 2 inches drainage, 3 inches of soil, and medium plants.
Solution:Total volume = (4/3) x pi x 7^3 = 1,437 cu in (6.2 gal / 23.5 liters) Base area = pi x 7^2 = 154 sq in Usable height = 14 - 2 - 3 = 9 inches Drainage = 154 x 2 = 308 cu in (21.3 cups) Soil = 154 x 3 = 462 cu in (32.0 cups / 8.0 quarts) Charcoal = 154 x 0.5 = 77 cu in (5.3 cups) Max medium plants = 154 / 16 = 9 plants Recommended = 9 x 0.7 = 6 plants Watering = 23.5 x 5 = 118 ml per watering
Result:6 medium plants | 6.2 gal volume | 8.0 qt soil | 21.3 cups drainage
Frequently Asked Questions
What size container do I need for a terrarium?
The ideal terrarium container size depends on the number and type of plants you want to include. For a small desktop terrarium with two to three plants, a container with a volume of one to two gallons works well. Medium terrariums suitable for five to eight plants need three to five gallons. Large display terrariums for ten or more plants require five to ten gallons or more. The container should be at least six inches tall to accommodate drainage, soil, and plant growth layers. Width matters more than height for most terrarium plants since they tend to spread horizontally. Glass containers are preferred because they allow maximum light penetration and create the greenhouse effect needed for closed terrariums. Consider the opening size as well, since narrow-necked bottles are visually striking but extremely difficult to plant and maintain.
How much drainage material do I need in a terrarium?
The drainage layer should be approximately one to two inches deep, regardless of the overall container size. This layer prevents water from pooling around plant roots, which causes root rot in the enclosed environment of a terrarium. Use small pebbles, gravel, or LECA (Lightweight Expanded Clay Aggregate) for the drainage layer. On top of the drainage material, add a thin layer of activated charcoal about one-quarter to one-half inch thick, which filters the water and prevents mold and bacterial growth. A piece of mesh or landscape fabric between the drainage layer and soil prevents soil from sifting down into the gravel and clogging the drainage system. For spherical containers, the drainage layer should be slightly thicker at two inches because water tends to collect at the lowest point of the curved bottom.
What type of soil is best for terrariums?
Use a well-draining potting mix specifically formulated for terrariums or indoor tropical plants. A good terrarium soil blend consists of two parts regular potting soil, one part perlite or pumice for drainage, and one part peat moss or coco coir for moisture retention. Avoid using garden soil or topsoil, which is too heavy and compact for container gardening, and often contains pathogens, insects, or weed seeds that thrive in the warm, humid terrarium environment. For succulent and cactus terrariums (open type), use a fast-draining mix with fifty percent perlite or coarse sand. The soil layer depth should be two to four inches to give roots adequate space to establish. Pre-moisten the soil before adding it to the terrarium so it is damp but not soggy, as over-saturated soil is the most common cause of terrarium failure.
How many plants can I fit in my terrarium?
The number of plants depends on the base area of your container and the mature size of each plant. A general rule is to allow nine square inches per small plant (two to three inch spread), sixteen square inches per medium plant (four to five inch spread), and thirty-six square inches per large plant (six inches or wider spread). Fill only about sixty to seventy percent of the available space initially to allow room for growth. Overcrowding is the most common mistake beginners make, leading to poor air circulation, mold growth, and plants competing for light and nutrients. Start with fewer plants than you think you need, as most terrarium plants grow to fill the space within a few months. Include a variety of heights using tall background plants, medium middle-ground plants, and low spreading ground-cover plants for visual depth.
What plants work best in closed terrariums?
Closed terrariums create a humid, tropical microclimate ideal for moisture-loving plants. The best choices include ferns such as maidenhair fern, button fern, and bird's nest fern, which thrive in consistently high humidity. Fittonias (nerve plants) are terrarium favorites because they stay compact, come in attractive pink and white varieties, and love humid conditions. Selaginella (spikemoss) creates a beautiful moss-like carpet. Peperomias offer varied foliage textures and stay relatively small. Small-leafed ivy works well as a trailing element. Pilea species like the friendship plant and aluminum plant adapt well to enclosed environments. Avoid succulents, cacti, and air plants in closed terrariums because they require dry conditions and good airflow. Also avoid fast-growing plants like pothos that will quickly outgrow the container and require frequent trimming that disrupts the ecosystem.
How do I water a terrarium properly?
Closed terrariums are essentially self-watering ecosystems that rarely need additional water once properly established. After initial planting, water lightly and observe the moisture cycle for one to two weeks. Condensation on the glass walls is normal and indicates proper humidity. If heavy condensation obscures visibility, open the lid for a few hours to release excess moisture. Add water only when the soil surface appears dry and condensation has stopped forming, typically every two to four weeks depending on the container size and ambient temperature. Use a spray bottle to mist lightly rather than pouring water directly, as overwatering is the leading cause of terrarium failure. A good guideline is to add about five milliliters of water per liter of container volume at each watering. Open terrariums with succulents need watering every one to two weeks using a small watering can directed at the soil base.
Where should I place my terrarium for optimal light?
Place terrariums in bright indirect light, which means near a window but not in direct sunlight. Direct sunlight through glass creates a greenhouse effect that can raise the internal temperature to lethal levels for plants, essentially cooking them inside the enclosed container. East-facing windows provide gentle morning light ideal for most terrarium plants. North-facing windows offer consistent indirect light suitable for shade-tolerant species like ferns and mosses. West and south-facing windows work if the terrarium is positioned three to five feet back from the glass or filtered through a sheer curtain. If natural light is insufficient, LED grow lights positioned twelve to eighteen inches above the terrarium for eight to twelve hours daily provide excellent supplemental lighting. Rotate the terrarium one-quarter turn weekly to ensure all plants receive even light exposure and grow symmetrically.
What is the difference between open and closed terrariums?
Closed terrariums have sealed or lidded containers that trap moisture, creating a self-sustaining water cycle through evaporation and condensation. They maintain high humidity levels of seventy to one hundred percent and require very infrequent watering, making them ideal for tropical plants like ferns, fittonias, and mosses. Open terrariums have no lid and allow air circulation, creating conditions similar to normal indoor environments with lower humidity. Open terrariums are suitable for plants that prefer drier conditions, including succulents, air plants, and cacti. Closed terrariums are more forgiving for beginners because the moisture cycle is largely self-regulating, but they are less tolerant of overwatering mistakes since excess moisture has nowhere to escape. Open terrariums require more frequent watering but are less prone to mold and rot issues that plague improperly managed closed systems.
How do I prevent mold in my terrarium?
Mold prevention starts with proper construction using sterile potting mix, an activated charcoal layer, and clean containers. If white fuzzy mold appears on the soil surface or on decaying plant matter, it usually indicates excessive moisture and poor air circulation. Remove any dead leaves or decaying plant material immediately, as these are the primary food source for mold. Open the lid of closed terrariums for several hours daily until the mold subsides and moisture levels decrease. Spraying the affected area with a diluted hydrogen peroxide solution (one part three percent hydrogen peroxide to four parts water) kills mold without harming plants. Springtails, tiny beneficial insects, are a popular biological mold control that many terrarium enthusiasts add to their builds. These organisms feed on mold and decaying matter, keeping the ecosystem clean while being virtually invisible during normal viewing.
How long does a terrarium last with proper care?
A well-constructed and properly maintained terrarium can last for years or even decades. The oldest known terrarium was sealed by David Latimer in 1960 and remained thriving with only one watering in 1972 before being sealed permanently, continuing to grow for over fifty years as a completely self-sustaining ecosystem. Realistic expectations for a typical hobbyist terrarium are one to five years with minimal intervention. Key factors affecting longevity include choosing slow-growing plants that will not outgrow the container, maintaining proper moisture balance, providing appropriate light, and occasionally pruning plants that become too large. Some plant replacement is normal, as individual plants may decline while others thrive and fill the space. The terrarium ecosystem matures over time, with the microbial community in the soil becoming more stable and the water cycle becoming more consistent as the system reaches biological equilibrium.
References
Reviewed for accuracy by Daniel Agrici, Founder & Lead Developer · Editorial policy
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