How the calculation works
A substrate bed is a slab over the floor of the tank. Its volume is the footprint times the depth; a sloped bed uses the average of the front and back depths. Weight is that volume times the bulk density of the material:
cubic feet = length (in) × width (in) × average depth (in) ÷ 1,728 liters = length (cm) × width (cm) × depth (cm) ÷ 1,000 · weight = volume × density Worked example: 2 inches of gravel in a 55 gallon
A 55 gallon tank has a 48 × 13 inch footprint. 48 × 13 × 2 = 1,248 in³ ÷ 1,728 = 0.72 cubic feet (20.5 liters). At about 100 lb per cubic foot that is about 72 lb of gravel, or three 25 lb bags.
Typical substrate densities
| Material | kg per liter | lb per ft³ | Usually sold |
|---|---|---|---|
| Aquarium gravel | ≈ 1.6 | ≈ 100 | by weight |
| Sand | ≈ 1.5 | ≈ 94 | by weight |
| Crushed coral / aragonite | ≈ 1.3 | ≈ 81 | by weight |
| Aquasoil | ≈ 1.0 | ≈ 62 | by volume (liters) |
These are typical values for loose, dry material. Grain size and brand move them by roughly 10%, so round up to the next bag when the result is close.
How deep should the bed be?
| Tank type | Bed depth | Notes |
|---|---|---|
| Fish-only, gravel | 1–2 in (2.5–5 cm) | Enough to anchor decor and hide the glass |
| Fish-only, sand | 1–1.5 in (2.5–4 cm) | Thin beds stay oxygenated |
| Planted | 2–3 in (5–7.5 cm) | Slope to 3–4 in at the back for stem plants |
| Bare-bottom | 0 | Common in breeding and grow-out tanks |
Why "1 lb per gallon" is off
The old rule of 1 lb of gravel per gallon per inch of depth assumes every tank has the same footprint for its volume. It does not. The rule only works for tanks about 13 inches tall. A 55 gallon (21 in tall) needs about 36 lb of gravel per inch of depth, not 55; a 75 gallon about 50 lb, not 75. A 20 gallon long (12 in tall) needs about 21 lb per inch, slightly more than the rule. The calculator shows the rule next to the real figure when you pick a standard size, so you can see the difference.
Frequently asked questions
How much gravel do I need for my aquarium?
Multiply the tank length by its width by the bed depth to get the bed volume, then multiply by the density of the gravel. In inches: length × width × depth ÷ 1,728 gives cubic feet, and aquarium gravel weighs about 100 lb per cubic foot. A 55 gallon tank (48 × 13 in) with a 2 inch bed needs about 72 lb of gravel. The calculator above does this for any size, depth and bag.
How much sand do I need for a 20 gallon tank?
A 20 gallon high has a 24 × 12 inch footprint. A 1.5 inch sand bed is 432 cubic inches, about 7 liters, which weighs roughly 23 lb at a typical 1.5 kg per liter. A 20 gallon long (30 × 12 in) needs about a quarter more for the same depth.
Is the "1 pound per gallon" rule accurate?
Only roughly. The rule says 1 lb of gravel per gallon gives about 1 inch of depth, but the amount of gravel depends on the footprint, not the gallons. Tall tanks have a small footprint for their volume, so the rule overshoots (a 55 gallon needs about 36 lb per inch, not 55); only tanks around 13 inches tall come out close.
How deep should aquarium substrate be?
For a fish-only tank, 1–2 inches (2.5–5 cm) of gravel or sand is enough. Planted tanks with root feeders do best with 2–3 inches (5–7.5 cm), often sloped deeper at the back for depth and taller plants. Sand beds over about 2 inches can trap gas pockets unless they are stirred by snails or fish.
Should I measure inside or outside the glass?
The bed sits inside the glass, so inside dimensions are exact. Outside measurements over-estimate the footprint by a few percent on a typical tank, which is a reasonable safety margin when buying bags.
Why is aquasoil sold in liters instead of pounds?
Aquasoils are light, porous baked-clay granules, and their weight changes a lot with moisture, so manufacturers sell them by volume. Pick "Aquasoil" and a liter bag size in the calculator and it counts bags by bed volume instead of weight.
Related calculators and guides
Substrate takes up water space: enter its share in the aquarium volume calculator to get the real water volume, then size the heater with the heater calculator. The substrate guide covers choosing a material, and the standard tank sizes chart lists every common footprint.