Aquarium substrate bags: litres vs kg

Litres and kg answer different questions

Tank footprint and depth produce a volume in litres. Bag labels may then describe either volume, weight, or both. If a bag is sold by litres, compare it directly with the volume result. If it is sold by kg, use density to estimate how many kg are needed.

• Use litres when the product label lists litres per bag.
• Use kg and density when the product label lists weight only.
• Do not assume a 9 kg bag is the same as a 9 litre bag.

Use supplier information first

Generic density assumptions are useful for comparison, but the product's own coverage notes should take priority. Porous aqua soils, wet packed substrates, and dense sands can differ noticeably.

• Check brand data for kg, litres, and suggested coverage.
• Round up if running short would interrupt the setup.
• Keep a note of the density assumption used.

Calculate directly from a litre-labelled bag

When the product states a usable volume per bag, the comparison is straightforward. Use the purchase-volume result, which includes the setup allowance, and divide it by the litres printed on the bag. The calculator does this when Bag label unit is set to litres. A 9 litre bag is therefore treated as 9 litres of product; it is not converted through the density field for package count.

• Use the number of litres on the exact package, not a similarly named product.
• Keep the 10% allowance if running short would interrupt the layout.
• Reduce the allowance only when you have measured exclusions or already own spare substrate.

Use density for weight-labelled bags

A kilogram-labelled bag needs one extra step because the tank geometry produces litres. Density estimates how many kilograms occupy the calculated purchase volume. Presets are planning values rather than product specifications: dry sand, gravel, porous soil, and wet-packed material can have noticeably different mass per litre. Enter a supplier figure whenever one is available, then divide the estimated kilograms by the package weight.

• Do not compare kilogram and litre package numbers as if they were equivalent.
• Coverage claims are useful only when their stated bed depth matches your layout.
• Retain the volume result even when buying by weight; it is the direct result of the tank measurements.

Label comparison