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How the calculation works
Measure the internal footprint and planned depth at the front and back. For a straight slope, average depth is (front + back) ÷ 2. Footprint area × average depth gives bed volume. This covers a rectangular footprint and linear slope; terraces, mounds and large hardscape need a separate adjustment.
Estimated dry mass equals bed liters × dry bulk density in kg/L. The sample density is illustrative, not a sand, gravel or aquasoil specification. Use your exact product’s published bulk density or a measured volume and dry mass. Bag count rounds the resulting mass up to complete bags.
Bulk volume includes air or water-filled pores between particles. It is not the volume of solids that displace aquarium water. Grain size, compaction, moisture and product packing change density, so do not treat the mass estimate as a universal shopping recommendation. Bag counts do not include spillage or a reserve unless you allow for those in your own inputs.
Worked example
A 90 × 45 cm footprint with a slope from 3 to 7 cm averages 5 cm depth. The bed occupies 20.25 liters. At an entered 1.5 kg/L bulk density it needs an estimated 30.375 kg, or seven 5 kg bags.
Common questions
Can I subtract 20 liters from my aquarium water volume?
Not just because the bed is 20 liters. Water fills its pore spaces. Subtract only solid displacement or use a filled-water measurement.
Can I use bag volume instead of bag mass?
This form uses bag mass and dry bulk density. If the maker supplies volume, compare that published volume to the bed-liter result directly; do not invent a mass conversion.