If there's one piece of millipede care advice that gets repeated more than any other, it's this: provide calcium. It comes up in every care guide, every forum thread, every conversation with experienced keepers. And for good reason. Calcium deficiency is one of the most common causes of health problems and death in captive millipedes.
What calcium does for millipedes
Millipede exoskeletons are hardened with calcium carbonate. Unlike insects, whose exoskeletons are primarily chitin, millipedes deposit significant amounts of calcium carbonate in their cuticle. This makes their exoskeleton harder and more rigid than a comparably-sized insect's, but it also means they have a much higher calcium requirement.
Every time a millipede moults, it sheds its old exoskeleton and needs to harden a new one. That new exoskeleton won't mineralise properly without sufficient dietary calcium. A calcium-deficient millipede will produce a soft, weak exoskeleton that may fail to harden at all. The result is a vulnerable animal that's prone to injury, deformity, and further moulting problems down the line.
Many millipedes eat their shed exoskeletons (exuviae) to reclaim the calcium. This is normal and helpful, but it's recycling rather than adding new calcium to the system. They still need an external source.
Signs of calcium deficiency
The most obvious sign is a failed moult. If a millipede's new exoskeleton is visibly soft, pale, or doesn't harden within a couple of weeks, calcium deficiency is the likely culprit. By the time you see this, the problem is already serious.
Other signs can be subtler. Slower growth in juveniles. A general lack of vigour. Exoskeletons that appear dull or slightly translucent rather than having the glossy, solid appearance of a healthy millipede. These are harder to spot, which is why prevention is better than trying to diagnose a deficiency after the fact.
Calcium sources
There are several options, and most keepers use more than one at a time. Belt and braces.
Cuttlefish bone. This is the most commonly used calcium supplement in the millipede hobby, and for good reason. It's widely available (pet shops, fishing supply stores, or online), inexpensive, and millipedes will rasp at it directly. Leave a piece on the substrate surface permanently and replace it when it's been worn down. You'll often see your millipedes feeding on it, particularly in the days after a moult.
Crushed eggshell. Bake the shells first (180C for about 20 minutes) to sterilise them, then crush them into small pieces or powder. You can mix this into the substrate or sprinkle it on the surface. It breaks down slowly and provides a steady calcium source. Free, too, assuming you eat eggs.
Calcium carbonate powder. Available from reptile supply shops. Dust it over fresh food or mix it into the substrate. It's the same compound the millipede needs, in readily available form.
Crushed oyster shell. Similar to eggshell but coarser. Works well mixed into the substrate as a slow-release source. You can find it sold for poultry keepers.
Limestone chips. Small pieces of limestone mixed into the substrate dissolve very gradually and provide calcium over months. Some keepers use these as a long-term background source alongside more immediately available options like cuttlefish bone.
How much calcium is enough?
There's no precise dosage. Millipedes self-regulate their calcium intake to a large extent: if it's available, they'll eat what they need. The goal is to make sure it's always available rather than trying to measure out a specific amount.
A piece of cuttlefish bone in the enclosure at all times, plus some form of calcium mixed into the substrate, covers most situations. If you're keeping a larger group or a species that moults frequently, you might go through cuttlefish bone faster and need to replace it more often. Watch how quickly it gets consumed and adjust accordingly.
Calcium for juveniles and breeding groups
Young millipedes (pedelings) have proportionally higher calcium demands than adults because they moult more frequently. Each moult builds a new, slightly larger exoskeleton that needs mineralising. A juvenile growing quickly through its first year of life is using calcium at a rate that adults don't match.
This means that if you're breeding millipedes, calcium supplementation is even more important. Make sure it's available throughout the substrate, not just on the surface. Tiny pedelings may not travel to a piece of cuttlefish bone sitting on top. Mixing crushed eggshell or calcium carbonate powder into the substrate itself means it's accessible at all depths.
Calcium and the substrate
In the wild, millipedes get calcium from the soil they live in. Calcium-rich soils produce healthier millipede populations. In captivity, the substrate mix you use may or may not contain much calcium depending on its composition.
Coco coir, which many keepers use for moisture retention, contains very little calcium. Organic topsoil varies. Rotting hardwood has some but not enough on its own. This is why active supplementation matters. The substrate provides food and habitat, but you need to enrich it with calcium to make it nutritionally complete.
Some keepers add a handful of garden lime (calcium carbonate, not the citrus fruit) to their substrate mix. This works, but make sure you're using pure calcium carbonate and not hydrated lime (calcium hydroxide), which is caustic and will harm the millipedes.
The simple version
Keep a cuttlefish bone in the enclosure at all times. Mix some crushed eggshell or calcium powder into the substrate. Replace or top up both as they get consumed. That's it. Calcium supplementation for millipedes is cheap, easy, and probably the single most impactful thing you can do for their long-term health.