Aquarium Substrate Calculator
Substrate quantity is footprint times depth — the tricky parts are converting volume to weight (each material has a different density), deciding on depth, and allowing for a slope. This tool handles all three and tells you how many bags to buy.
How much substrate?
Aquasoil is normally sold by the litre; gravel and sand by the kilogram. Enter your bag size in the matching unit.
The math and the densities
Substrate volume is simply footprint × average depth: a front-to-back slope averages out, so a 4 cm front rising to 6 cm back behaves like a flat 5 cm layer. Converting volume to weight uses typical bulk densities — the “as poured” density including the air (later water) between grains:
These are mid-range figures for aquarium products: quartz gravel typically runs 1.5–1.7 kg/L, sand 1.4–1.6 kg/L, and baked-clay aquasoils are much lighter at 0.7–0.9 kg/L (which is why aquasoil is sold by the litre). Crushed coral and aragonite run heavier, around 1.4–1.6 kg/L. If your bag lists its own coverage figures, prefer those.
How deep should substrate be?
- Bare-decorative / fish-only: 2–3 cm is plenty, and easier to keep clean.
- Rooted plants: 5–6 cm at the back, 3–4 cm at the front. Swords, crypts and stem plants want root room.
- Aquascaping slopes: raising the back to 8–10 cm+ creates depth perception; use lava rock or crate fill beneath deep zones to save soil and weight.
- Sand for bottom-dwellers: corydoras and loaches prefer 2–4 cm of fine sand; very deep fine-sand beds can compact and develop anaerobic pockets — stir occasionally or keep them shallow.
Common mistakes
- Buying by the “pounds per gallon” myth. The old “1–2 lb per gallon” shortcut ignores footprint: a tall 100 L tank needs far less substrate than a shallow 100 L tank. Always compute from footprint × depth.
- Not rinsing gravel and sand. Unrinsed substrate clouds a tank for days. Rinse in batches until water runs clear. Exception: never rinse aquasoil — it disintegrates.
- Ignoring chemistry. Crushed coral and many “cichlid sands” raise KH and pH; aquasoils lower them. Match the substrate to your target water, not just the look.
- Under-buying. Bags come in coarse sizes; the calculator rounds up because a thin substrate at the front glass looks worse than a litre of leftover.
Frequently asked questions
Does substrate count toward my tank’s weight on the stand and floor?
Yes — and it’s not trivial. Six centimetres of gravel in a 90×45 tank is ~39 kg, about the weight of another 40 L of water. The tank weight calculator includes a substrate line for exactly this reason.
Can I mix aquasoil with gravel or sand?
Layering works (soil below, fine gravel cap above) but mixed layers migrate: fine particles sink, large grains surface. Keep layers separated with mesh if you need them to last, or commit to a single material.
How much does substrate displace my water volume?
Roughly 30–40% of the substrate’s bulk volume is solid material (the rest is pore space that fills with water). For dosing purposes, subtracting the full substrate volume from tank volume is a safe, conservative simplification.
Do I need to replace aquasoil eventually?
Aquasoil’s nutrient charge depletes over 1–3 years and its granules slowly break down into mud. Heavy root feeders will need root tabs from year one onward; many aquascapers rescape with fresh soil every 2–3 years. Inert gravel and sand last indefinitely.