How much substrate do I need?
Footprint times depth gives you a volume; the material's density turns it into weight. That's the whole calculation, and it's why the answer depends on the tank's floor, not its gallons — a 40 breeder and a 40 high hold the same water and need very different amounts of gravel.
pounds = L × W × depth (in) ÷ 1,728 × lb/ft³ · kg = L × W × depth (cm) ÷ 1,000 × kg/L
The "divide by 22" shortcut you'll see elsewhere is that formula with a single density baked in (1,728 ÷ 22 ≈ 79 lb/ft³, a light gravel). It's fine for gravel and useless for anything else: the same shortcut for aquasoil would be "divide by 33". Here is a 2 in bed on a 75 (48 × 18 in) in each material, computed by this page's engine:
| Material | Density | 2 in on a 75 | Bags |
|---|---|---|---|
| Gravel (quartz / pea) | 100 lb/ft³ · 1.6 kg/L | 100 lb · 45 kg | 5 × 20 lb |
| Sand (pool filter, play, black diamond) | 97 lb/ft³ · 1.55 kg/L | 97 lb · 44 kg | 5 × 20 lb |
| Aragonite reef sand | 94 lb/ft³ · 1.5 kg/L | 94 lb · 42 kg | 5 × 20 lb |
| Crushed coral | 87 lb/ft³ · 1.4 kg/L | 87 lb · 40 kg | 5 × 20 lb |
| Aquasoil (ADA, Fluval Stratum, UNS…) | 53 lb/ft³ · 0.85 kg/L | 53 lb · 24 kg · 28 L | 4 × 9 L |
Poured (bulk) densities, voids included — what a bag weighs per liter of the pile it makes. Sources in the engine's constants file.
Substrate chart for standard tanks
Gravel by depth, plus aquasoil in liters at 2 in, for the common sizes. Sand runs about 3% lighter than gravel for the same depth; aragonite reef sand about 6%.
| Tank | Footprint | 1 in gravel | 2 in gravel | 3 in gravel | 2 in aquasoil | 2 in displaces |
|---|---|---|---|---|---|---|
| 5.5 gallon | 16 × 8 in | 7 lb | 15 lb | 22 lb | 4 L | 0.7 gal |
| 10 gallon | 20 × 10 in | 12 lb | 23 lb | 35 lb | 7 L | 1 gal |
| 15 gallon | 24 × 12 in | 17 lb | 33 lb | 50 lb | 9 L | 1.5 gal |
| 20 gallon long | 30 × 12 in | 21 lb | 42 lb | 62 lb | 12 L | 1.9 gal |
| 20 gallon high | 24 × 12 in | 17 lb | 33 lb | 50 lb | 9 L | 1.5 gal |
| 29 gallon | 30 × 12 in | 21 lb | 42 lb | 62 lb | 12 L | 1.9 gal |
| 37 gallon | 30 × 12 in | 21 lb | 42 lb | 62 lb | 12 L | 1.9 gal |
| 40 gallon breeder | 36 × 18 in | 37 lb | 75 lb | 112 lb | 21 L | 3.4 gal |
| 55 gallon | 48 × 13 in | 36 lb | 72 lb | 108 lb | 20 L | 3.2 gal |
| 65 gallon | 36 × 18 in | 37 lb | 75 lb | 112 lb | 21 L | 3.4 gal |
| 75 gallon | 48 × 18 in | 50 lb | 100 lb | 150 lb | 28 L | 4.5 gal |
| 90 gallon | 48 × 18 in | 50 lb | 100 lb | 150 lb | 28 L | 4.5 gal |
| 120 gallon | 48 × 24 in | 67 lb | 133 lb | 200 lb | 38 L | 6 gal |
| 125 gallon | 72 × 18 in | 75 lb | 150 lb | 225 lb | 42 L | 6.7 gal |
| 150 gallon | 72 × 18 in | 75 lb | 150 lb | 225 lb | 42 L | 6.7 gal |
| 180 gallon | 72 × 24 in | 100 lb | 200 lb | 300 lb | 57 L | 9 gal |
| 210 gallon | 72 × 24 in | 100 lb | 200 lb | 300 lb | 57 L | 9 gal |
Computed by this page's engine from standard outside dimensions (inside footprint is a hair smaller; the difference is less than a handful). Displacement at 60% solid.
How deep should aquarium substrate be?
Deep enough for what lives in it and no deeper. Every extra inch is more weight, more trapped waste, less water — and in a planted tank the only inches that matter are the ones with roots in them.
| Setup | Depth | Why |
|---|---|---|
| Fish-only, gravel or sand | 1–2 in (2.5–5 cm) | Enough to anchor decor and hide the bottom pane; more just traps waste. |
| Planted, aquasoil | 2 in front → 3–4 in back (5 → 8–10 cm) | Roots need depth; the slope adds perspective and puts the deepest soil where the big plants go. |
| Shrimp / nano | 1–1.5 in (2.5–4 cm) | Shallow beds stay clean and are easy to siphon. |
| Reef, shallow sand bed | 1–2 in (2.5–5 cm) | Looks natural, easy to keep clean; or go bare-bottom. |
| Reef, deep sand bed | 4–6 in (10–15 cm) | Only if you want the anaerobic denitrifying zone — and its maintenance. |
| Cichlids, sand-sifters, corydoras | 1–2 in (2.5–5 cm) fine sand | Soft on barbels; deeper only invites digging to the glass. |
Ranges reflect common hobby practice; a specific fish's needs override the table.
A bed that runs 1 in at the front to 3 in at the back is, on average, 2 in deep, and that's what the calculator uses when you switch to "sloped" — a straight-line slope averages to the middle. On a 75 in aquasoil that's 28 L instead of the 42 L a flat 3 in bed would take: the slope buys the look of a deep bed for 33% less soil. Soil doesn't hold a slope on its own forever, though; plan on terracing with rock or a substrate support, and on the front creeping up over the first months.
Gravel vs sand vs aquasoil: what changes
Weight. Two inches of gravel on a 75 is 100 lb; the same bed in aquasoil is 53 lb. On a big planted tank the choice of substrate is worth more to your floor than the choice between glass and acrylic. Volume vs weight. Gravel and sand are sold by the pound; aquasoil is sold by the liter, because the bags are light and lumpy and nobody wants to explain why 9 liters weighs 8 kg. That's why the calculator switches its bag units when you pick aquasoil. Behavior. Sand packs and needs less depth than gravel to look full; a 2 in sand bed is heavier than it looks and a 4 in one is a maintenance decision (see deep sand beds above). Aquasoil is porous and rich; it buffers pH down and breaks down into mud over years, and it costs several times what gravel does per bag — a reason to slope it rather than fill flat.
A bag of aquasoil labeled 9 L makes a pile about 9 L in size — but roughly 40% of that pile is the gaps between grains, which fill with water. Poured into your tank it displaces about 5.4 L of water, not 9. The same goes for gravel: 100 lb of it on a 75 makes a 28 L bed but pushes out only 4.5 gal. The calculator reports both, and the displaced number is the one your true water volume — and every dose on this site — uses.
How many bags of substrate should I buy?
The calculator rounds up to whole bags of the size you pick and tells you what's left over. Two things worth knowing before you round: bags of the same nominal weight vary by a pound or two, and a poured bed settles a little once it's wet and you've planted in it — so if the math says you'll have a quarter bag spare, that's about right, not wasteful. If it says you'll have a quarter bag short, buy the extra one; a thin patch at the front glass is the thing you'll notice every day. Sloping, capping sand over soil, or building up the back for hardscape all need more than the flat number — use the sloped mode and enter the depths you actually want.