How Much Substrate (and Drainage Material) a Terrarium Actually Needs

"How much substrate do I need?" is really two separate questions stacked on top of each other: how big is the container in the first place, and how does that size split across drainage, charcoal, substrate, and top dressing? Guessing either one tends to end in either a half-empty jar or a leftover bag of soil with nowhere to go, and buying drainage material by eye is an easy way to come home with several times more LECA than one jar will ever use. Running both questions through this site's own calculators, on one real container, makes the whole process concrete.

Step one: find the vessel's actual volume

Take a straight-sided cylinder jar, 20cm across and 30cm tall — a genuinely common mid-sized terrarium jar. Its total volume works out to about 9.4 litres (roughly 2.5 US gallons). Not all of that is usable for layers and plants, though: leaving room for airflow, growth, and simply not packing a jar to the absolute rim, a realistic 60% fill factor puts the usable volume at around 5.65 litres. That's the number worth keeping in mind when you're shopping, not the raw 9.4L figure — buying substrate for the full total volume routinely leads to more soil than the build will ever actually use.

Step two: split that height across the four layers

Volume alone doesn't tell you how deep to build any individual layer, because a terrarium's layers are worked out from height, not volume directly. The layers should also fill only part of the jar: this site's substrate layer calculator fills a third of the height by default (a rule of thumb you can change), leaving the rest for the plants and the air a closed jar's water cycles through. Until October 2026 it split the jar's full height to the rim, which is how an earlier version of this guide came to plan a jar packed with soil.

Built closed, a third of that 30cm (10cm of fill) splits into a 2.5cm drainage reservoir, 0.3cm of charcoal, 7.2cm of substrate and 1cm of top dressing: 11cm of layers and 19cm of room above them. Built open, the same fill gives the same 2.5cm reservoir, 0.3cm of charcoal, 7.7cm of substrate and 1cm of top dressing. The reservoir comes from the drainage layer calculator (18% of the substrate depth above it, never less than 2.5cm without a hole), so both calculators now give the same depth for the same jar.

Substrate is the layer that actually needs measuring out to buy, since it's the largest by far and the one sold by volume. At a 20cm-diameter floor (about 314cm²), a 7.2cm-deep closed-terrarium substrate layer works out to about 2.26 litres; the whole 11cm stack is about 3.46 litres, comfortably inside the 5.65 litres the vessel calculator calls usable for layers and plants together. The old full-height split asked for 6.8 litres of substrate alone, more than that whole usable volume, which was the sign something was off.

Step three: convert the drainage depth into a material volume to buy

The layer calculator gives you a drainage depth, but LECA, gravel, and lava rock are sold by volume, not depth, so there's one more conversion worth making. The drainage layer calculator turns the closed build's 7.2cm of substrate into a 2.5cm reservoir, which across the same 314cm² floor is about 0.79 litres, a little over 3 cups. The open build's 7.7cm of substrate also sits on the 2.5cm minimum, so it needs the same amount.

That's a small but real detail worth internalising: the drainage calculator sizes the reservoir from the substrate depth above it, 18% of that depth with a 2.5cm floor for a vessel with no hole. Most jars filled a third of the way sit on that floor; a deep substrate layer in a tall tank is where the percentage takes over. If your vessel is a globe or narrows toward the bottom, choose that shape in the drainage calculator: the bottom few centimetres of a globe hold far less than its widest circle suggests.

Why the substrate figure isn't just "a bag of potting soil"

The 2.26L substrate figure above is a volume, not a shopping instruction on its own — what fills that volume matters as much as how much of it there is. Standard outdoor or houseplant potting soil compacts more densely than a terrarium wants and often holds too much water for a sealed jar with no drainage hole. A terrarium-specific mix, or a well-draining potting soil cut with extra perlite or orchid bark, takes up a bit more apparent volume for the same weight of actual soil, since the added perlite and bark chunks introduce air space the calculator's raw litre figure doesn't distinguish from solid substrate. In practice that means the 2.26L figure is a reasonable target to buy toward, not a number to hit with surgical precision — a bag that gets you close, with a little left over for topping up as the substrate settles over the first few weeks, works better than trying to buy the exact volume and running short.

Converting to units you'll actually shop in

Retail substrate and drainage material aren't always sold in litres. In the US, potting mix is commonly sold by the dry quart or the dry gallon, and drainage material like LECA sometimes by the cup or by weight rather than volume. A US dry quart is about 1.1 litres (a liquid quart is a little less than a litre), and a litre is roughly 4.2 cups — both useful mental conversions when a bag's labelled size doesn't match the unit a calculator gave you. The drainage layer calculator on this site already reports its result in both litres and cups for exactly this reason, so the 0.79L drainage figure for the closed jar above is the same as roughly 3.3 cups of LECA or gravel, which the smallest bags sold for aquarium or terrarium use will cover.

Putting the whole jar's shopping list together

For the 20cm x 30cm jar built closed, that leaves a shopping list of roughly: 0.79L of drainage material (LECA, gravel, or lava rock), a thin dusting of activated charcoal, about 2.26L of terrarium substrate, and a modest top-dressing amount — moss or fine gravel — for the visible surface layer. Substrate is usually the only one worth buying with real headroom, since it's cheap relative to specialty drainage material and easy to store the leftover of; drainage material is worth measuring more precisely, since overbuying LECA for one small jar is a more expensive mistake than overbuying a bag of potting mix.

A second size, to see how the numbers scale

Numbers don't scale in a straight line with size, which is worth seeing once. A larger rectangular tank, 45 x 30 x 30cm, holds a full 40.5 litres total (about 24.3L usable) — more than four times the smaller jar's total volume, even though both containers are the same 30cm tall. The difference is entirely floor area: the tank's 1,350cm² footprint is over four times the jar's roughly 314cm² circular floor, and since volume for a fixed height is just floor area multiplied by that height, the total volume scales with the footprint almost exactly as much as it would if the tank were four times taller instead. This is exactly why "how much substrate to buy" is worth calculating from your actual container's dimensions rather than assuming height alone tells the story — two vessels of identical height can need very different amounts of substrate if their footprints differ.

A note on substrate ingredients and sustainability

Many terrarium and houseplant substrate mixes lean heavily on peat moss for its excellent water retention, and it's worth knowing what that ingredient actually is before buying it by the bag. Peat forms in waterlogged bogs over centuries, and harvesting it drains and disturbs a slow-forming wetland habitat while releasing carbon that had been locked away in the peat for a very long time — a genuine environmental cost that's led many growers to look for alternatives. Coco coir, made from coconut husk fibre, is the most common substitute: it holds water in a broadly similar way to peat, comes from a renewable byproduct of the coconut industry rather than an extracted wetland, and performs well in most terrarium substrate blends when cut with the same perlite or orchid bark a peat-based mix would use.

This isn't a case where one option is unsafe and the other is fine — a peat-based mix works perfectly well for the plants themselves, and it isn't a fabricated hazard the way a genuinely toxic plant is. It's a sourcing consideration similar in spirit to the moss-sourcing question covered elsewhere on this site: a coco-coir or other peat-free mix gives a comparable growing result while avoiding the specific environmental cost that peat extraction carries, and it's an easy swap to look for on a bag's ingredient list without changing anything else about the substrate volume calculations above.

Over-buying and under-buying: the common mistakes

The most common overbuying mistake is filling the whole jar on paper: calculating layers against a vessel's full height or total volume instead of the share the layers should take — buying toward that jar's full 9.4 litres when an 11cm stack takes about 3.5 litres and leaves the rest for the plants. The most common underbuying mistake runs the other way: working out a layer's depth and forgetting it still needs to be converted to an actual volume against the real floor area, which is easy to skip if you're eyeballing rather than running the numbers through a calculator.

Drainage material is where a third, more specific mistake shows up: sizing a round-bottomed vessel by its widest circle. A globe's floor is a point and a tapered pot narrows toward the bottom, so the bottom 2.5cm of either holds far less than the widest circle times 2.5cm. In a 25cm globe the 2.5cm reservoir is about 0.23 litres; the widest circle times that depth would be over a litre. Choose the globe or tapered shape in the drainage layer calculator and it sizes the reservoir over the vessel's own shape.

Running your own numbers

The three calculators used here — jar & vessel volume, substrate layer ratio, and drainage layer — are meant to be chained exactly the way this example did: volume first for a sense of scale, then the layer calculator for your container's height, the share to fill and open/closed type, then the drainage calculator using that substrate depth and your vessel's shape. Each one takes a couple of measurements and a shape or type selection, so the whole chain for a single container takes a few minutes, and it's worth redoing whenever you switch to a genuinely different vessel rather than reusing an old jar's numbers for a new one. If you'd rather see a range of common sizes side by side before measuring your own container, the vessel size reference runs this same three-step chain across eight typical vessel shapes and sizes in one table.