Buy too little substrate and you are short mid-scape; buy too much and bags sit unused. This calculator turns your tank footprint and desired depth into a precise volume and weight for gravel, sand, or aquasoil, so you can order the right number of bags first time.
How it works
Substrate volume is the base footprint times the depth, then weight is volume times the material’s bulk density:
volume(L) = length(cm) × width(cm) × depth(cm) / 1000
weight(kg) = volume(L) × density(kg/L)Typical bulk densities are about 1.6 kg/L for gravel, 1.5 kg/L for sand, and 0.8 kg/L for lightweight aquasoil. Dividing the litre figure by your bag size gives the number of bags, rounded up.
Worked example
A 60 × 30 cm tank base with 5 cm of gravel: 60 × 30 × 5 / 1000 = 9 litres, about 14.4 kg of gravel. A 6-kg bag covers 4.3 bags — buy 5. Add roughly 10 percent extra for settling and dust loss, particularly with sandy substrates that lose a lot to rinsing. For a sloped aquascape, use the depth at the midpoint of the slope, not the back or the front, for a reasonable average.
Choosing the right substrate for your setup
Different substrate materials serve very different purposes, and the choice affects how deep you need to go.
Inert gravel is the workhorse for community freshwater tanks. It requires 3–5 cm for a fish-only tank and does not affect water chemistry. It is the heaviest per litre, which matters for stands and shelving.
Fine sand suits bottom-dwelling species — corydoras, kuhli loaches, and sand-sifting cichlids — that need to forage and burrow without abrading their barbels. At 1–2 mm grain size, 3–5 cm depth is adequate. Finer sand at more than 6 cm can develop anaerobic dead zones unless disturbed regularly.
Aquasoil (baked clay / volcanic substrate) is essential for serious planted tanks. It releases nutrients slowly into the root zone and lowers pH, which suits soft-water plant species. The trade-off is cost and a limited nutrient lifespan (typically two to three years before it exhausts). Because it is much lighter than gravel or sand, the same volume weighs roughly half as much — relevant when ordering by the kilogram online.
Depth guidance by setup type
| Setup | Recommended depth | Notes |
|---|---|---|
| Fish-only (gravel) | 3–5 cm | Easier to vacuum; less biological load in substrate |
| Community planted | 5–7 cm | Allows root development; slope up to 8 cm at rear |
| Deep sand bed (marine) | 8–15 cm | Needs burrowers or stirring to prevent pockets |
| Iwagumi / Dutch | 6–10 cm | Often sloped sharply; measure the average depth |
What the weight result tells you
Beyond bags, the weight figure helps you assess stand load. A typical 90 × 45 cm aquascape with 7 cm of aquasoil still comes in well under 10 kg, but a reef tank with 10 cm of aragonite sand across a large footprint can add 30–40 kg to the total tank weight — a real consideration for furniture and floor ratings.