There is no single substrate depth that suits every millipede, but a good rule of thumb is to make it deep enough for the animal to burrow fully, turn around comfortably and moult safely. The right depth depends on the species, the adult size and how much it burrows. A thin layer may look neat, but for most millipedes it will not provide enough space, moisture stability or usable habitat.
Think of millipede substrate as the base of the enclosure, not just bedding. It should work like a small woodland floor: deep enough to hold moisture, rich enough to support feeding and decomposition, and loose enough for the animal to move, dig and rest below the surface. If you want the bigger picture on building a successful setup, see the Millipede Enclosure & Substrate Guide and our Millipede Setup & Substrate Guides.
What depth does a millipede need?
Most millipedes spend a lot of time below the surface, especially when resting or moulting. Burrowing helps them feel secure and keeps them in the more stable, humid lower layers of the enclosure. That is why shallow substrate is usually the first limitation in an enclosure. With too little depth, the millipede has less room to retreat, less moisture stability and less space to moult safely.
Smaller millipedes may cope with less depth than larger, more active burrowers, but all millipedes benefit from substrate that gives them genuine usable space below the surface. The useful question is not simply how much substrate you can fit in the enclosure, but whether the species can behave normally in it.
Put the setup together
Build the enclosure around the substrate rather than placing decorations on top of a shallow base. A healthy millipede setup usually includes a deep layer of organic material made up of leaf litter, decomposed leaves and rotting wood, with moisture held through the lower and middle layers rather than sitting on the top.
The exact depth will vary by species and enclosure size, but the substrate should be deep enough for the millipede to burrow fully and turn around comfortably below the surface. Adult size matters here. A larger millipede needs more room than a smaller species, not only because of body length, but because the surrounding substrate has to stay open and functional rather than packed down.
For many keepers, this means aiming for several inches of substrate at minimum, and often more for larger adults or species that spend a lot of time underground. There is no magic number; the better test is whether the animal can disappear into the substrate, move around within it and moult without being exposed or cramped.
When choosing what to use, think in layers of function. Leaf litter gives cover and food. Rotting wood contributes long-term nutrition. The main substrate holds moisture and gives the millipede a place to dig. If you are building a new enclosure, our Best Substrate for Millipedes guide is a useful next step.
Substrate depth and enclosure size go hand in hand. A large enclosure with a shallow base can still be poor husbandry, while a smaller enclosure with enough depth may work well for a single animal or a small group depending on the species. If you are planning a tank, our guide to What Size Enclosure Does a Millipede Need? will help you balance floor space with usable substrate depth.
Check the conditions
Once the enclosure is set up, substrate depth should be assessed alongside moisture and texture. Good millipede substrate should feel damp and workable below the surface, not dry and dusty. The lower layers should remain consistently moist without becoming soggy or waterlogged. That balance matters because burrowing animals rely on the substrate to buffer humidity while still allowing airflow and movement.
A well-functioning substrate usually has an earthy smell and a loose, natural texture. Leaves and wood gradually break down over time, which is a sign that the enclosure is developing into a living woodland floor rather than just sitting as inert bedding. If the substrate is too shallow, too dry or too compacted, the lower layers can lose that buffering effect and become less useful to the animal.
- the millipede burrows readily and remains active below the surface
- the lower substrate stays damp but not wet
- leaf litter and wood slowly decompose
- the substrate stays loose enough for movement
- there is no sour or stagnant smell
By contrast, a setup that is too shallow often shows exposed millipedes, dry lower layers, or animals lingering at the surface because they cannot dig comfortably. Waterlogged substrate, sticky mud or a sour smell usually means the conditions need attention.
What to adjust over time
Substrate depth is not something you set once and forget. As the enclosure matures, the lower layers settle and the organic material breaks down, so the volume may reduce slightly. That is normal. What matters is keeping enough depth available for burrowing and moulting, and topping up the enclosure when the substrate begins to shrink or lose structure.
If the millipede is spending more time on the surface, or if burrowing seems limited, check whether the substrate has become too shallow, too dry or too compacted. Sometimes the fix is simply adding more moist organic material and leaf litter. In other cases, the enclosure may need to be rebuilt with a deeper base from the start.
Juveniles are smaller and may seem comfortable in a less deep setup, but they still benefit from a proper functional substrate. Adults, especially larger species, usually need much more room below the surface. If you keep more than one species, avoid assuming that the same depth will suit them all.
In practice, the safest approach is to build deeper than you think you need, then watch how the millipede uses it. Millipedes are detritivores, so the substrate is not just hiding space; it is part of the habitat and part of the food chain in the enclosure.
