What Size Formicarium Does My Ant Colony Need?
Picking the right size formicarium is harder than it looks. You don't want a crowded colony, but a huge nest for a small one doesn't help either.
So does a colony of 10 workers need a formicarium? What about 50, 100 or 500? No worker number tells you exactly when to upgrade. Species differ a lot in worker size, brood output, growth rate and how they use their nesting space. A better approach is to work out how much room your colony needs now, and make sure there's a sensible place for it to expand next.
Why size matters
In nature, a colony doesn't start life in a finished nest. A founding queen begins in a small chamber, and as workers arrive the colony gains space by digging or by taking over suitable cavities. Captive keeping works best when it follows the same idea: nesting space grows as the colony grows.
A formicarium should hold the queen, workers and brood without surrounding a tiny colony with a lot of unused chamber.
Can a formicarium be too big?
Yes. It's tempting to buy a large one now so you never need another. For a small colony, though, the extra space is mostly wasted. Ants may use unused chambers to store rubbish or food remains, and a colony scattered across an oversized nest is harder to keep an eye on.
Ask what's right for the colony you have now, not what's the biggest nest you can afford. You can deal with growth when it happens.
Does a queen need a formicarium?
Usually not. For many commonly kept fully claustral species, a correctly prepared test tube is an excellent founding nest. It gives the queen a compact chamber with a water reservoir and suitable humidity, and makes occasional checks easy. She doesn't need a network of chambers because you've started a colony. Species founding strategies differ, so check the needs of the one you're keeping.
What about the first workers?
The first workers, often called nanitics, are exciting, but their arrival doesn't mean an immediate move. What changes is that workers now need to forage. A test tube joined to a small outworld handles that: the tube stays the nest and the outworld is where food goes. That's much easier than feeding an active colony inside a tube, and the colony can stay put until its nesting space becomes limiting or another reason to move comes up.
How many workers before a formicarium?
There's no reliable number. Twenty tiny Lasius workers take up far less room than twenty large Camponotus, and two colonies of the same species can develop very differently depending on brood and growth. Look at the colony itself. Signs that more nesting space is becoming useful include:
- Workers occupying most of the available nesting area
- Brood filling the existing space
- Feeding or managing the colony getting harder
- Large numbers of workers gathering at the tube entrance
- The water reservoir running low
- The founding setup deteriorating
- The colony bumping up against the practical limits of its housing
None of these is an emergency alone. The aim is to notice when the current setup is nearing its limit and prepare the next stage before it becomes a problem.
Worker count is a guide, not a rule
You'll see advice like "move at 20 workers" or "this nest suits 50 ants". Treat those as rough guidance, not biological thresholds. Take two colonies of 30 workers: one has small workers and modest brood, the other much larger workers and a big pile of larvae and pupae. Their space needs differ. Worker count only tells you something once you combine it with species, worker size, queen size, brood quantity, chamber dimensions and how much of the current nest is occupied. That's why the sizes in the Antastic Tom range shouldn't be read as a precise number of ants each nest holds.
When to choose a small formicarium
A small nest is usually the first dedicated formicarium for a young colony outgrowing its founding setup. The goal at this stage is an appropriate next step, not all the space the colony might need next year. A smaller nest makes it easier to watch the colony, monitor brood, keep hydration right, avoid empty chambers and keep the setup compact, and you can expand again when the time comes. For most colonies, going from a founding setup to a small formicarium is a far more sensible step than jumping to a large one.
When to choose a medium formicarium
A medium nest makes sense once an established colony is filling more of its smaller nest and needs more chamber space. By then there's usually more brood as well as more workers, and food consumption and foraging have risen. Again, skip the exact worker count and watch how the ants use the current nest. If they still occupy only a small part of it, more space achieves very little. If it's filling up, a bigger or additional formicarium is easy to justify.
When to choose a large formicarium
Large nests suit established colonies that can really use the extra room, and what that means depends on the species. A large Camponotus colony and a large Lasius colony hold very different numbers of ants, which is why labelling a nest "for 500 ants" can mislead. Physical occupancy is what counts. A large nest answers a colony that needs a lot of space. A young colony shouldn't be expected to grow into one from the far side of an empty box.
Move the colony or add another nest?
An upgrade doesn't have to mean forcing the colony to relocate. In a modular setup you can connect extra nesting space to the existing system and let the colony investigate and use it as its needs change. That gives you a progression like this:
Founding setup → small nest → small nest plus an additional nest → larger modular system
instead of removing the ants and starting over at each size. Exactly how you manage expansion depends on the species and setup, but designing for it makes growth much easier to handle.
Outworld size
Nest size is only half of it. As the colony grows, foraging grows too. An outworld that was easy with five workers gets very busy with hundreds, and a bigger colony means more food, more ants foraging at once, more waste and more frequent feeding. When you're deciding whether a colony has outgrown its setup, check both nesting space and foraging space. Sometimes the nest is fine and the outworld is what needs upgrading.
Species make a difference
Small species can pack large populations into compact nesting areas, and escape prevention matters more because tiny workers exploit tiny gaps. Large species need bigger chambers and passages, so a modest population can fill a lot of space. A fast-growing colony benefits from an expansion route prepared before its nest is completely full, while a slow-growing one gains nothing from years of space in advance. These differences are why worker count alone can't settle the size question.
What if the ants won't use the new nest?
Connecting a new formicarium doesn't guarantee the colony will move in. Ants respond to conditions and don't share your enthusiasm for new housing. They may prefer the old nest because of its humidity, temperature, darkness, familiarity, available space or the state of the brood. If the old nest still suits them, they have no reason to leave. That doesn't mean the new nest is wrong, and unless there's an urgent welfare reason, forcing a move just because you've bought something new is usually unnecessary.
Blocking off unused chambers
Some designs let you seal or open sections as the colony grows, so the physical nest stays put while only part of its chamber area is available at first. Modularity does the same thing differently, adding separate nesting modules as required. Either way, the colony gets space gradually and not all at once.
How much spare space is right?
A nest doesn't have to be packed before you can expand. You want some capacity for more workers and brood. But room to grow is different from giving a small colony many times the space it occupies. Think of sizing as a progression rather than a threshold: a nest that's comfortable now, with some headroom for near-term growth, and not the final home the colony might need several years from now.
A simple size progression
For a typical colony, the general pattern looks like this:
- Founding queen: a test tube or an appropriate species-specific founding setup. No formicarium needed.
- Queen with first workers: test tube plus a small feeding area or outworld. The tube stays the nest.
- Young growing colony: a small formicarium plus a suitable outworld, once the colony really needs more nesting space.
- Established colony: a medium formicarium or extra modular nesting space, based on occupancy, brood and species.
- Large established colony: a large nest or several connected nests plus an appropriately sized outworld. Modularity is especially handy here as demands keep rising.
This tracks colony development and isn't a worker-count chart.
Choosing an Antastic Tom size
The same logic applies across the range:
- Founding Nest: early colony development, when only a little dedicated nesting space is appropriate. Here
- Small Nest: young colonies ready for a dedicated formicarium. Here
- Medium Nest: established colonies starting to need more chamber space. Here
- Large Nest: larger established colonies that can use much more nesting area. Here
The right choice still depends on your species and colony. If you're torn between two sizes, choose for where your ants are now and not the size they might reach, since the modular system gives you somewhere to go next.
The right size is the one your colony needs now
No magic worker number tells every keeper when to upgrade. Look at the species, worker size, brood, how much of the nest is occupied, the condition of the current nest, foraging activity, expected growth and your expansion options. If your ants are doing well and still have suitable space, there's no need to invent an upgrade problem. When they do need more room, move up gradually.
Browse our Ant Formicariums & Nests UK collection to compare sizes, or read What Is a Formicarium? if you're still getting to grips with how nests and outworlds fit together.
Start small. Expand when the colony gives you a reason to.