How it works
weight = footprint length × width × bed depth × bulk density
The calculator converts your footprint and bed depth to metres, multiplies length by width by depth to get the loose substrate volume in cubic metres, then multiplies by the bulk density of the substrate you chose to get the mass. Bulk density is mass per unit of loose volume with the pore space between grains included, which is what you actually buy. The defaults are representative dry figures: gravel about 1,400 kilograms per cubic metre and sand about 1,600, both from the FAO aquaculture manual table of material densities; crushed coral about 1,220, from the measured dry bulk density of carbonate sand; and a light, porous planted aggregate about 800, which varies enough by brand that you should switch to Custom and read the density off the bag for a precise order. Depth is the decision that moves the number most: a fish-only tank is happy with about two inches, while rooted plants want three inches or more to anchor, and every extra inch adds the same weight again. As a rule of thumb, roughly one pound of gravel per gallon gives a modest bed, but measuring the footprint is far more accurate because tall tanks and short tanks of the same volume need very different amounts. The result also shows bag count for common 10, 20 and 50 pound bag sizes so you can order without a second calculation.
Sources
- FAO — Simple Methods for Aquaculture, Table 5 Specific mass of construction materials: sand 1,450–2,000 kg/m³, gravel 1,300–1,500 kg/m³ (bulk densities, pore space included).
- Wang et al. 2023, Materials 16(12):4217 (carbonate sand) Measured dry bulk density of coral (carbonate) sand ≈ 1,219 kg/m³ — the basis for the crushed-coral figure.
FAQ
How much substrate do I need for my aquarium?
Multiply the tank base length by its width by the bed depth to get the volume, then multiply by the substrate density. For a common 24 by 12 inch base with a 2 inch gravel bed that is roughly 29 pounds. A quick rule is about one pound of gravel per gallon, but measuring the footprint is more accurate, especially for tall or shallow tanks.
How deep should aquarium substrate be?
For a fish-only tank, one and a half to two inches is enough to look good and hold decorations. For live rooted plants, aim for three inches or more so roots can anchor and spread, and consider a nutrient base layer capped with gravel or sand. Deeper sand beds beyond three inches can trap gas pockets unless you keep burrowing livestock or stir them.
Is sand or gravel better, and do they weigh the same?
Both work; the choice is about livestock and looks. Sand suits corydoras, loaches and a natural look, while gravel anchors plants and decorations well. They do not weigh the same for a given volume: dry aquarium sand is denser than gravel, about 1,600 versus 1,400 kilograms per cubic metre, so a sand bed of the same depth weighs a little more. The calculator accounts for that difference automatically.
How much crushed coral do I need for a marine tank?
Crushed coral and aragonite are used in marine and African cichlid tanks to buffer pH upward. Size the amount exactly like any other substrate: footprint times depth times density, using the crushed-coral option, which is around 1,220 kilograms per cubic metre. A one to two inch bed is typical; deeper beds in reef tanks act as biological filtration but need more careful maintenance.
Why does the calculator ask for a footprint, not gallons?
Substrate covers the floor of the tank, so the amount depends on the base area and the depth, not the total volume. Two tanks can hold the same gallons but have very different footprints; a tall 29-gallon and a long 40-gallon breeder need quite different amounts of substrate. Measuring the base length and width gives an accurate figure that a gallon-based rule of thumb cannot.
How many bags of substrate should I buy?
Divide the total weight by your bag size and round up. The calculator does this for common 10, 20 and 50 pound bags. Buy one extra bag if you plan to slope the substrate higher at the back, build up around rockwork, or want spare for future aquascaping. Rinse new gravel or sand thoroughly before adding it to avoid clouding the water.
Weights are estimates from representative dry bulk densities, which vary with grain size, moisture and brand — check the bag and use the Custom option for an exact spec. Rinse substrate before use. General aquarium guidance, not professional advice.