Aquarium Substrate Calculator

Why substrate amount matters

Buying too little substrate means a second trip to the store (and often a mismatched bag or dye lot); buying too much wastes money and leaves you storing a half-used bag of gravel or aqua soil. Substrate is also sold by weight, not by "how much my tank needs," so translating a tank's footprint and desired depth into pounds/kilograms and a bag count up front avoids both problems.

The formula used

1. Area: rectangular tanks use length × width; cylindrical tanks use π × (diameter / 2)².
2. Volume: area × substrate depth, converted to cubic feet and liters.
3. Weight: volume (ft³) × the substrate's density in pounds per cubic foot.
4. Bag count: weight ÷ 5 and weight ÷ 10, each rounded up — so you always buy enough, never one bag short.

Density varies a lot by substrate type, which is why picking the right one matters:

Substrate	Density (lb/ft³)	Notes
Gravel	100	Most common all-purpose substrate
Sand	100	Rinse thoroughly before adding — packs finer than gravel
Aquatic plant soil	45	Lighter and more porous than gravel/sand
Crushed coral / coral sand	95	Raises and buffers pH — for cichlid/brackish tanks, not planted setups
Fluorite	93	Fired clay substrate marketed for planted tanks
Custom	you enter it	For any substrate not listed, check the bag or manufacturer spec sheet

Depth guidance by substrate type

The right depth depends on what's living (or growing) in the tank, not just the footprint:

• Gravel: 1–2 in for fish-only tanks; 2–3 in if capping with root-feeding plants.
• Sand: 1–2 in — deeper beds risk anaerobic pockets.
• Aquatic plant soil: 2–3 in flat, sloping to 3–4 in at the back for a planted layout.
• Crushed coral: 1–2 in — it's there for pH buffering, not root anchorage.
• Fluorite: 2–3 in, often capped with a thin layer of sand for looks.

How to use it

1. Pick your tank shape (rectangular or cylindrical) and enter its dimensions in inches or centimeters.
2. Enter your target substrate depth.
3. Pick a substrate type — or "Custom" to enter your own density if your product isn't listed.
4. Read off the area, volume, weight, and how many 5-lb or 10-lb bags to buy.

FAQ

Why do gravel and sand need different depths? Sand packs much finer than gravel, so a deep sand bed can trap waste and go anaerobic (starved of oxygen) in the lower layers, producing pockets of hydrogen sulfide. Gravel's larger grain size lets water and burrowing critters move through it more freely, so it tolerates more depth.

My bag says coverage in square feet at a certain depth — can I still use this? Yes — the weight and bag-count results here are shape/depth-independent once computed, so they'll match a bag's stated coverage as long as the density you use (or the one we assume) matches the actual product.

Does this calculator store my data? No. Everything runs in your browser — your tank dimensions and results are never sent to or stored on our servers.

Is the density table exact for every brand? No — density varies by grain size, moisture, and manufacturer. Treat the built-in values as a solid planning estimate, and use "Custom" with your bag's stated weight-per-volume if you need an exact figure.