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. For that same 30cm-tall jar, this site's substrate-layer ratios give two different splits depending on whether you're building it open or closed.

Built closed, 30cm of height splits into roughly 4.5cm of drainage material, under 1cm of charcoal, 21.6cm of substrate, and 3cm of top dressing. Built open, the same 30cm height gives about 3cm of drainage, under 1cm of charcoal, 23.1cm of substrate, and 3cm of top dressing — a shallower drainage reservoir, and correspondingly a little more room for substrate, since an open vessel doesn't need as deep a buffer for excess moisture as a sealed one does.

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 21.6cm-deep closed-terrarium substrate layer works out to roughly 6.8 litres of substrate on its own — more than the jar's total usable volume figure from step one might suggest, which is exactly why working through the layer-by-layer breakdown matters more than eyeballing a single "usable volume" number.

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

The substrate-layer ratio 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. Feeding that 21.6cm substrate depth (for the closed build) into the drainage layer calculator, for the same 314cm² floor, gives an actual drainage material height of about 3.9cm and a volume of roughly 1.2 litres — a little over 5 cups. The open build's shallower 3cm drainage ratio, sitting under its slightly deeper 23.1cm substrate layer, works out to about 4.2cm of actual material and roughly 1.3 litres, a touch more than the closed version despite its smaller ratio, because it's supporting a deeper substrate layer above it.

That's a small but real detail worth internalising: the drainage material calculator takes the substrate depth as an input, not the drainage-layer ratio itself, so a deeper substrate layer calls for a bit more drainage material underneath it even when the ratio suggests a "thinner" drainage layer on paper.

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

The 6.8L 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 6.8L 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 quart or the dry gallon, and drainage material like LECA sometimes by the cup or by weight rather than volume. A litre converts to a little over 1 US quart, and to 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 1.2L drainage figure for the closed jar above is the same as roughly 5 cups of LECA or gravel, a size most bags sold for aquarium or terrarium use will get you to without much waste either way.

Putting the whole jar's shopping list together

For the 20cm x 30cm jar built closed, that leaves a shopping list of roughly: 1.2L of drainage material (LECA, gravel, or lava rock), a thin dusting of activated charcoal, about 6.8L 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 calculating substrate against a vessel's total volume instead of its usable volume — buying for that jar's full 9.4 litres instead of the roughly 5.65L that's actually going to hold layers and plants, once headspace for airflow and growth is factored in. The most common underbuying mistake runs the other way: measuring only the substrate layer's ratio-based percentage of the container's height and forgetting that percentage 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: using the substrate-layer ratio's drainage percentage on its own, rather than feeding the resulting substrate depth into the dedicated drainage layer calculator. The two numbers look similar but answer different questions — one is a rough share of the container's height, the other is an actual buyable volume for your floor area and substrate depth — and treating the ratio percentage as a shopping quantity on its own tends to come out low, since it skips the floor-area conversion the drainage calculator does explicitly.

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 ratio for your container's height and open/closed type, then the drainage calculator using that substrate depth against your floor area. 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.