Aquarium Substrate Calculator

How many pounds of gravel, sand or aquasoil your tank's footprint needs at the depth you are aiming for, and how many bags that comes to.

Last reviewed August 2026 · Free · Nothing you enter is stored

Why the answer is a weight, not a volume

Substrate is sold by the pound, so the useful number is pounds rather than gallons or litres. Getting there takes two steps. The first is geometry: length times width times depth gives the volume the substrate has to fill, and that volume has nothing to do with the height of the tank. The second is density, which is where the material matters. Gravel runs around twelve pounds per gallon of volume, sand fourteen to fifteen because the grains pack tighter, and aquasoil only about ten because the granules are porous and light.

That difference is large enough to change what you buy. The same 48 by 18 inch footprint at two inches deep takes roughly 90 pounds of gravel, 105 pounds of fine sand, or 75 pounds of aquasoil. Sizing by gravel and then switching to sand at the shop leaves you a bag short.

Be aware that real products vary by roughly thirty percent in density, and that variation exists even inside a single manufacturer's own range — a coarse grade and a fine grade of the same branded substrate will not weigh the same per gallon. This is the reason to buy over rather than under. A spare 20-pound bag costs a few dollars; a half-finished aquascape waiting three days for a delivery costs your patience.

Densities are the figures used across substrate calculators, cross-checked against CaribSea's own published pounds-per-cubic-foot data. Real products vary by roughly thirty percent between grain types even within one manufacturer's range, so buy a little over rather than a little under. A plain gravel display wants one to two inches; a planted tank wants two to three so roots have somewhere to go, and more at the back if you are sloping the scape.

How deep to go

One to two inches is right for a plain display tank. It covers the glass, gives the fish a floor that looks like a floor, and stays shallow enough that a gravel siphon reaches the bottom of it easily. There is nothing to gain from more if you are not growing anything.

Two to three inches is the planted range. Roots need somewhere to go, and a stem plant in an inch of gravel spends its life being uprooted every time a fish disturbs it. Carpeting plants want the top of that range or a little more. If you are sloping the substrate, run it deeper at the back and shallower at the front, which gives the tank depth visually and also means the front glass stays clean.

Above about four inches the lower layer starts to be a liability in freshwater. Water movement through a deep bed is very slow, oxygen does not reach the bottom of it, and the layer goes anaerobic. In a heavily planted tank this matters much less because roots carry oxygen down with them, which is why deep beds work in planted tanks and cause trouble in bare ones. If you want depth for the look, plant it or keep something that burrows.

Rinsing, and the one substrate you must not rinse

Gravel and sand both arrive coated in dust, and play sand or pool-filter sand arrives coated in a great deal of it. Rinsing means running water through a bucket of it, stirring, pouring the cloudy water off, and repeating until it runs clear. It takes longer than anyone expects and skipping it means a tank that stays milky for several days while the filter slowly clears what you could have removed in twenty minutes.

Aquasoil is the exception, and it is important. Aquasoil is a manufactured clay granule with nutrients baked into it, and washing it breaks the granules down into mud. The result is a permanently cloudy tank and a substrate that has lost the porous structure you paid for. Pour it in dry, fill the tank slowly onto a plate or a bag to avoid disturbing it, and expect some initial cloudiness and an ammonia release for the first couple of weeks while it settles.

Sand and gravel behave differently for the fish

This is the one substrate argument with a clear answer rather than a matter of taste. Corydoras, kuhli loaches, geophagus cichlids and other barbelled bottom feeders sift substrate through their gills to find food. Sharp-edged gravel wears their barbels down over months until they are stubs, which affects how the fish feeds for the rest of its life. Sand is genuinely better for these species, and smooth rounded gravel is an acceptable compromise where sand is not practical.

Working the other way, fine sand compacts. A deep bed of it with nothing moving through it develops gas pockets, and disturbing one releases the lot at once. The risk is much overstated for shallow beds and real for deep ones. Keeping fine sand under about three inches, planting it, or keeping malaysian trumpet snails to turn it over continuously all solve the problem.

Crushed coral deserves a specific warning because it is not a neutral choice. It dissolves slowly, raising hardness and buffering pH upward. That is exactly what an African cichlid tank wants and exactly what a soft-water tank of tetras and shrimp does not. Use it deliberately or not at all.

Substrate types compared

Mixing types works better than people assume, provided the heavier material goes on top. Aquasoil capped with a thin layer of sand is a standard planted-tank approach that keeps the nutrients where the roots are and the finer grain where the fish are.

Substrate takes water out of the tank

Every gallon of substrate you add is a gallon of water the tank no longer holds. Two inches of gravel across a 75-gallon footprint displaces around eight gallons, and once you add the inch or so of freeboard below the rim the tank holds closer to 62 gallons than 75.

That gap matters at dosing time. Medications, dechlorinator and fertilisers are all measured per gallon of water, not per gallon of tank label, so treating a 75 that actually holds 62 runs every dose about twenty percent strong. For dechlorinator this is irrelevant. For copper-based medication or anything dosed to the millilitre it is the sort of error that kills invertebrates for reasons nobody traces back. Our aquarium volume calculator subtracts your actual substrate depth and fill line and gives you the number to dose against.

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