---
title: "Terrarium Calculator: Layers, Litres & Quarts | Diorama Fern"
description: "Work out a terrarium or bottle garden's volume and how much drainage, charcoal and potting mix each layer needs, in litres and US quarts, with bags to buy."
url: https://dioramafern.me/terrarium-calculator/
---

Litres · US quarts · bags

# Terrarium calculator

How much gravel, charcoal and potting mix your jar, tank, globe or bowl needs, layer by layer.

## 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](https://dioramafern.me/terrarium-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](https://doi.org/10.32473/edis-mg356-2017), 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](https://hgic.clemson.edu/factsheet/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](https://www.nist.gov/pml/owm/nist-handbook-44-current-edition), Appendix C. Volumes of the box, cylinder, sphere and spherical cap are standard geometry.