Aquarium Substrate Calculator

Aquarium Substrate Calculator

The 1 lb per gallon rule, sanity-checked

The old rule says 1 pound of gravel per gallon per inch of depth. Run the real numbers and it holds up surprisingly well for small standard tanks. A 20-gallon long has a 30 × 12 inch footprint: 360 square inches × 1 inch of depth is 360 cubic inches, or 0.208 cubic feet. At gravel’s typical bulk density of about 100 lb per cubic foot, that’s 20.8 lb — the rule predicted 20, within 5 percent. The rule falls apart as tanks get taller, because gallons measure total volume while substrate only cares about the floor. A 55-gallon is 48 × 13 inches — just 624 square inches of footprint — and actually needs about 36 lb per inch. The rule tells you to buy 55 lb, a 52 percent overshoot that wastes $20–40 at pet-store gravel prices. A 20-gallon high (24 × 12 inches) runs the other way: about 17 lb per inch, not 20. The calculator above skips the shortcut entirely and works from your actual footprint, which is why its numbers beat the rule on anything other than a standard-proportioned small tank.

How deep should aquarium substrate be?

Two inches (5 cm) is the standard for an unplanted gravel tank — enough to anchor decor and host nitrifying bacteria without trapping excess debris. Planted tanks want 2.5–3 inches so heavy root feeders like Amazon swords can spread, and aquascapes often run deeper still toward the back. Sand should stay shallower, around 1–1.5 inches: fine sand compacts, and below roughly 2 inches it can form oxygen-free pockets where hydrogen sulfide builds up — keep it thin, stir it during water changes, or add burrowers like Malaysian trumpet snails. Crushed coral beds in cichlid and brackish setups typically run 1–2 inches. Sloping the bed from a shallow front to a deep back adds visual depth, and the calculator’s slope option handles the math by averaging the front and back depths. A 20-gallon long sloped from 2 inches at the front glass to 3.5 inches at the rear averages 2.75 inches — about 57 lb of gravel instead of the 42 lb a flat 2-inch bed needs. Budget for the average, not the front measurement, or you’ll come up a bag short on scaping day.

Gravel vs sand vs aqua soil vs crushed coral

The four common substrates differ more in density and pricing unit than most buyers expect. Aquarium gravel runs about 100 lb per cubic foot. Dry sand is similar or slightly denser — reference tables list dry sand at 101.8 lb per cubic foot, with pool filter sand at 100–120. Aqua soil (baked granular clay) is roughly half that at 50–60 lb per cubic foot, which is why planted-tank brands sell it by the liter instead of the pound. Florida crushed coral, with its coarse 2.5–5.5 mm grain, averages about 72 lb per cubic foot. Prices as of July 2026: colored pet-store gravel is the expensive way to cover a floor at about $1.80 per lb ($8.97 for a 5 lb GloFish bag at Chewy); plain natural gravel in 20 lb bags typically lands at $0.75–1.25 per lb. Pool filter sand is the budget option — a 50 lb bag of Quikrete runs $11–25 depending on retailer, or $0.22–0.50 per lb. CaribSea Florida crushed coral is about $1 per lb ($40 for 40 lb). Aqua soil is the premium tier: Fluval Stratum is $29.99 for an 8.8 lb bag and ADA Amazonia Ver.2 is $42.99 for 9 L. Same 2-inch bed in a 55-gallon: 72 lb of gravel, but only about 40 lb of aqua soil.

What substrate costs for common tank sizes

Using 2 inches of depth and July 2026 prices, here’s what the standard tanks cost to floor. A 10-gallon (20 × 10 inch footprint) needs 23 lb of gravel — one 20 lb bag plus one 5 lb bag, about $20–45 depending on natural versus colored — while a single $11–25 bag of pool filter sand covers it twice over. A 20-gallon long (30 × 12) needs 42 lb of gravel, roughly $35–75, or about 44 lb of sand — again one 50 lb pool-sand bag. A 55-gallon (48 × 13) takes 72 lb of gravel at roughly $60–130 in pet-store bags, while a safer 1.5-inch sand bed needs 57 lb: two 50 lb bags, $22–50 total. A 75-gallon (48 × 18) is the clean benchmark, because 2 inches of substrate there is exactly 1.0 cubic foot — about 100 lb of gravel ($80–180), 105 lb of sand ($33–75 across three pool-sand bags), or 28.3 L of aqua soil, which three 9 L ADA bags ($128.97) cover if you go a hair shallower. If the gravel numbers sting, that’s the argument for pool filter sand or bulk natural gravel.

Planted tanks: think in liters, cap in inches

Aqua soil is sold by volume, so switch units: footprint in square centimeters × depth in centimeters ÷ 1,000 = liters. The math lines up neatly with standard rimless tanks — a 60P (60 × 30 cm footprint) at 5 cm of depth needs exactly 9 L, one bag of ADA Aqua Soil Amazonia Ver.2 ($42.99 at Aqua Forest Aquarium, July 2026). The 3 L bag costs $19.99 — that’s $6.66 per liter versus $4.78 for the 9 L, so buy the big bag and keep leftovers sealed and dry for rescapes. A 20-gallon long at 2 inches works out to about 11.8 L: one 9 L bag plus one 3 L bag. For capped setups — a nutrient base under an inert top layer — run the calculator above twice: once for the base layer depth in aqua soil, once for a 1–1.5 inch cap of sand or fine gravel, then add the two bag counts. And remember aqua soil’s low density (50–60 lb per cubic foot) when you bank it high in the back: a dramatic rear slope in a 60P adds liters quickly but surprisingly little weight, which matters for stands rated near their limit.