Litres · US quarts · bags
Terrarium calculator
How much gravel, charcoal and potting mix your jar, tank, globe or bowl needs, layer by layer.
Layers
- Container holds3.53 L3.73 US qt (liquid) · 20 cm (7.87 in) tall inside
- Potting mix0.884 L0.8 US dry qt · 1 bag of 5 L
- Room left for plants11.9 cmabove 8.1 cm (3.19 in) of layers
| Layer | Depth | Volume | US dry qt | Bags of 5 L |
|---|---|---|---|---|
| Drainage (gravel or sand) | 2.5 cm (0.98 in) | 0.442 L | 0.4 | 1 0.09 of a bag |
| Horticultural charcoal | 0.6 cm (0.24 in) | 0.106 L | 0.1 | 1 0.02 of a bag |
| Barrier fabric | a single sheet | – | – | cut a circle 15 cm (5.91 in) across |
| Potting mix | 5 cm (1.97 in) | 0.884 L | 0.8 | 1 0.18 of a bag |
Lay the barrier (weed cloth, cheesecloth or old nylon) over the drainage so the potting mix cannot sift into it.
The potting mix fills 25% of the container; UF/IFAS suggests about a fifth, and roughly as deep as the plants' root balls. Keep a closed terrarium in bright, indirect light: direct sun heats the glass and burns the plants.
How the layers are worked out
Measure the inside of the container. The calculator finds its volume by shape: length × width × height for a box, a cylinder for a jar, a sphere for a globe or bottle garden, and for a bowl or fishbowl the part of a sphere set by its opening and height. Each layer's volume is the slice of the container it fills, so a round bowl needs less gravel at the bottom than its width suggests, and the layers can never add up to more than the container holds.
The order follows UF/IFAS Extension: pea gravel or sand for drainage, a thin layer of horticultural charcoal in closed terrariums, a sheet of fabric such as weed cloth so the soil cannot sift down, then potting mix to roughly a fifth of the container. The depths are inputs, so you can follow Clemson's figures instead (½ in of charcoal and at least 1½ in of soil).
Open or closed?
Closed terrariums keep moisture in and suit tropical, humidity-loving plants; open ones let air through and suit succulents and plants that like it dry. Keep either in bright, indirect light, and a closed one out of direct sun, which heats the glass. The plant picker sorts plants by which kind they suit.
Questions
How much potting mix does a 1 gallon jar need?
A US gallon is 3.785 L, so about a fifth of it, roughly 0.75 L (0.7 US dry qt), following the UF/IFAS rule of thumb. Enter the jar's size for the exact figure at your depth.
Is a US quart on a soil bag the same as a liquid quart?
Not always. A US dry quart is 1.101 L and a liquid quart is 0.946 L. A bag marked “8 qt (8.8 L)” is using dry quarts; choose US dry quarts as the bag unit for those.
Do I need charcoal in an open terrarium?
UF/IFAS puts charcoal only in closed terrariums, to stop a sour smell developing, and says an open one can skip it. The calculator unticks it when you choose Open.
Sources
- Layers and amounts (drainage at least ½ in, 1.3 cm, up to 1½ in, 3.8 cm; ¼ in of horticultural charcoal for closed terrariums only; synthetic fabric over the drainage; potting soil to about a fifth of the container; open containers for succulents, closed for tropical plants; bright indirect light, no direct sun): Chairez, Vu and Park Brown, Plants and Youth: Designing and Building a Terrarium, UF/IFAS Extension ENH121 (revised April 2026).
- Alternative figures (½ in of charcoal; soil at least 1½ in; drainage and soil together about a quarter of the container): Clemson Cooperative Extension, HGIC 1457, Indoor Plants – Terrariums.
- US liquid quart (0.946352946 L), US dry quart (1.101220942715 L), US gallon (3.785 L) and cubic foot (28.317 L): NIST Handbook 44, Appendix C. Volumes of the box, cylinder, sphere and spherical cap are standard geometry.