Drainage Layer Calculator
Enter your container’s shape and size, planned substrate depth, and whether it has a drainage hole, and get the reservoir height and how much material to buy for that shape.
Drainage material needed
Buying the right amount
The volume figure is the loose, poured volume of drainage material you'll need to reach the calculated height across the whole floor area — round up slightly when buying, since material settles once it's poured and levelled. A mesh or landscape-fabric barrier is commonly laid on top of the drainage layer before the substrate goes in, to stop soil from washing down and clogging it.
If you're building a very large planted display rather than a small jar, it's worth measuring an actual test pour into a similar-shaped container to confirm the volume-to-height relationship before committing to a full purchase.
Frequently Asked Questions
Why does having a drainage hole change the answer so much?
A pot with a hole lets truly excess water leave the container, so its drainage layer only needs to be a thin buffer that keeps the substrate from sitting directly in any water that briefly collects. A sealed jar or bottle has nowhere for water to go, so it needs a genuine reservoir sized to hold what the substrate can't absorb.
Why is the height tied to the substrate depth above it?
A deeper substrate stack holds and later releases more water, so the reservoir beneath it needs to scale up too. The calculator uses a percentage of the substrate depth, with a sensible minimum height enforced for very shallow builds where the percentage alone would be too thin to do anything.
Why does a globe need so much less material than its width suggests?
The reservoir sits at the very bottom, and the bottom of a globe is a narrow cap. A 25 cm globe's bottom 3.2 cm holds about 0.37 L; its widest circle times 3.2 cm would be 1.57 L. Until October 2026 this calculator took only a floor area, and for a round vessel the area to hand is its widest circle, so it sized globes about four times too large. Choose the globe or tapered-pot shape and it now sizes the reservoir over the vessel's own shape.
What material should I use for the drainage layer?
Expanded clay pebbles (LECA), aquarium gravel, and lava rock are all common choices — the calculator gives you a volume and height regardless of which one you pick. Lighter materials like LECA are easier to lift and rearrange; gravel and lava rock sit more firmly in place.
Can the drainage layer be too deep?
Yes — a reservoir that is unnecessarily deep just eats into the space you could otherwise use for substrate and root depth, without adding any benefit once it comfortably holds the water your setup produces. The calculator's numbers aim for a functional minimum rather than an oversized buffer.
Planning estimate only — actual material needed can vary slightly with grain size and how tightly it settles. Round up when buying.