Don’t guess your way through it: here’s what you actually need to know before buying!
So you’ve decided to build a terrarium? Amazing! But before you start picking out moss and ferns, there’s one decision that’ll shape everything else: the terrarium glass size. Getting this right from the start means healthier plants, a better-looking display, and a whole lot less frustration.
It’s one of those things that seems simple until you’re standing in a shop holding two containers, unsure which one to take home. Too small and your plants will feel cramped before you know it; too large and the whole thing can look a little lost, or become harder to manage than you bargained for.
The good news is that once you understand a few key factors the decision gets a lot clearer. There’s no single “right” size that works for everyone, but there’s definitely a right size for you and the terrarium you have in mind.
So let’s break it all down so you can feel confident walking into any shop (or scrolling any website) knowing exactly what you need. By the end of this, picking a container will feel like the exciting first step it actually is.
Why size matters more than you think
It might be tempting to just grab the prettiest glass container you can find, but the terrarium glass size affects way more than aesthetics. It determines how much soil you can use, how much moisture builds up, and even how easy the whole thing is to maintain.
Think of it this way: a tiny container with lush tropical plants is a recipe for overcrowding and rot, while a massive tank with just a couple of small succulents might look oddly empty. The goal is balance, finding a size that suits both your plants and your space.
Before you even look at containers, take a moment to think about where this terrarium is going to live. A small shelf calls for something compact, while a coffee table centrepiece can handle something more generous.
Also consider how much time you want to spend on upkeep! Smaller terrariums need more frequent attention, while larger ones tend to be a little more forgiving.
When it comes to terrarium glass size, the golden rule is to always think about the plants first. What are their mature sizes? How deep do their roots grow? Do they need a lot of vertical space? These questions will guide you toward the right volume and shape before anything else.
You can find a whole new content about the best closed terrarium plants and how to choose them here! Check it out and learn how to pick the perfect ones for your next project.
How to measure what you actually need
This is where things get a little more practical. Measuring what you need doesn’t require any fancy tools or expertise. It’s really just about asking the right questions before you commit to a container.
There are three main things to think through: how tall your plants will get, how deep their roots go, and how many plants you’re planning to include. Work through each one and you’ll have a really solid picture of the size you’re after.
Starting with plant height
One of the easiest starting points is to look at how tall your plants will eventually grow. A good rule of thumb is to choose a terrarium glass size that’s at least twice the mature height of your tallest plant. This gives them room to breathe and keeps the arrangement looking intentional rather than cramped.
For shorter, ground-hugging plants like baby tears or certain mosses, this measurement becomes less critical. They’re more concerned with spreading sideways than growing upward, so you’ll want to focus on the diameter of the base instead, giving them a bit of horizontal territory to claim as their own.
Thinking about root depth
Root depth is just as important as plant height, and it’s often overlooked. Even small-looking plants can have surprisingly deep root systems that need room. Always research your specific plants before deciding on a container. Some that look tiny above the soil line need a solid 15 cm of depth below it.
As a starting point, most common terrarium plants are happy with around 10–15 cm of substrate. That means your container needs to be deep enough to hold that much soil plus a drainage layer (usually around 3–5 cm of gravel or leca) at the bottom, with some visual space above the soil line too.
Matching the terrarium glass size to your plant count
The number of plants you’re planning to include is another big factor in choosing your terrarium glass size. A good starting guideline is to allow roughly 5–8 cm of space between each plant at the time of planting. They’ll fill in over time, but you want some breathing room at the start so they can establish properly.
If you’re planning a three-plant setup, you’ll need a very different container than if you’re building a dense, layered scene with eight or ten varieties. It helps to sketch out a rough top-down plan of your arrangement before committing to a container. This way, you can visualise the spacing and get a better sense of the volume you need.
Container shapes and what they’re good for
Not all glass containers are created equal, and the shape matters almost as much as the size. Here’s a quick breakdown of common shapes and their best uses:
| Shape | Best for | Watch out for |
|---|---|---|
| Cylinder / jar | Ferns, mosses, tropical plants | Can limit horizontal spread |
| Geometric / angular | Display pieces, single focal plants | Awkward corners are hard to plant into |
| Bowl / dish | Succulents, cacti, open terrariums | Shallow depth limits root room |
| Tall tank / aquarium | Layered jungle scenes, tall plants | Needs strong light source from above |
| Bottle / carboy | Self-sustaining closed ecosystems | Tricky to maintain long-term |
Choosing a shape that complements your plant selection makes the whole process way easier. Cylindrical jars are forgiving and versatile. Geometric shapes look stunning but need a bit more planning at the planting stage.
Common sizing mistakes (and how to avoid them)
Even experienced terrarium builders get tripped up by a few recurring mistakes when choosing their terrarium glass size. Here’s what to watch for:
- Going too small too soon. It’s easy to underestimate how much space plants need once they start establishing. When in doubt, size up.
- Ignoring the drainage layer. Remember, that gravel or leca at the bottom takes up space. Account for it in your total container depth.
- Choosing a container before choosing plants. Always start with your plant list, then find the container — not the other way around.
- Forgetting about your tools. If you can’t fit your hand or a long pair of tweezers inside to arrange plants, maintenance becomes a real headache.
Additionally, don’t underestimate the importance of the opening width. A narrow-necked bottle looks beautiful, but if you can’t get your hands in to tend the plants, you’ll regret it. For beginners especially, wider openings make planting and maintenance so much easier.
A quick size guide to get you started
Here’s a simple reference to match container volume with plant type and number — just to give you a starting point:
- Under 5 litres: Best for one to two very small plants. Think mini ferns, delicate moss patches, or compact peperomias. These tiny setups are great for desks or windowsills, but they need a little extra attention since the environment can shift quickly.
- 5–10 litres: Great for two to four small plants in an open or closed setup. You’ve got enough room to create a simple but lovely composition, and maintenance is still pretty manageable even if you’re just starting out.
- 10–20 litres: Ideal for more complex arrangements with five or more plants and properly layered substrates. This is the sweet spot for most hobbyists. Enough space to get creative, but not so big that it becomes overwhelming to care for.
- 20+ litres: Perfect for lush, statement pieces with tall or spreading plants. If you want to build a real showstopper with height, texture, and layers, this is your range. Just make sure you’ve got a spot with enough light to match.
Of course, these are guidelines, not rules. Your specific plants, container shape, and creative vision all play into the final decision. Trust the process!
Even after reading all the guides in the world, the terrarium glass size question sometimes only gets answered by experimenting a little. Start with a medium-sized container if you’re unsure, something in the 10-litre range, and you’ll have enough flexibility to make design choices without feeling too constrained.
Keep going: a read we think you’ll love
If you’ve made it this far, you’re clearly serious about building something beautiful and we love that! There’s another article waiting for you that digs into a question that comes up for almost every terrarium builder at some point, and it’s one worth thinking through carefully before you make your next move.
We’re talking about the open vs closed terrarium debate, and it’s more nuanced than you might expect. Both styles have their strengths, and the right choice really does depend on your lifestyle, your plants, and honestly, how much you enjoy tinkering with your setup over time.
The article “Open vs closed terrarium: which is for you?” walks you through everything. It’s practical, it’s honest, and it’ll save you from making a choice you’ll want to undo later.
So go ahead and give it a read! It pairs perfectly with what you’ve just learned here about sizing. The more you understand about the kind of terrarium you want to build, the easier every single decision after that becomes. You’ve got this!