A 48 by 13 inch tank bottom covered 2 inches deep with natural gravel needs 72.8 lb, and that is 4 bags of 20 lb. The figure includes a 5% allowance, and the four bags leave about 7 lb spare. Slope the same bed from 1.5 inches at the front to 3 inches at the back and the average depth is 2.25 inches, so you need 81.9 lb and a fifth bag.
How the amount of substrate is calculated
Substrate is a layer: bottom area times depth gives the volume, and volume times the bulk density of the material gives the weight. Bulk density is what a cubic foot of loose product weighs, and makers publish it. CaribSea gives the same method in its FAQ: length times width times depth in inches, divided by 1,728 cubic inches per cubic foot, times the density.
Pounds = length (in) × width (in) × depth (in) ÷ 1,728 × density (lb per cu ft) × (1 + waste)
For the example: 48 × 13 × 2 = 1,248 cubic inches, ÷ 1,728 = 0.722 cubic feet, × 96 lb = 69.3 lb, and 5% more is 72.8 lb. Divide by the 20 lb bag and round up: 3.64 becomes 4 bags. A sloping bed uses the average of the front and back depth, which is exact for a straight slope: (1.5 + 3) ÷ 2 = 2.25 inches.
Density by material
| Material | Density | Bag sizes | Source |
|---|---|---|---|
| Aragonite reef sand | 85 lb per cu ft | 10, 20 lb | CaribSea Arag-Alive Special Grade Reef |
| Oolite sand | 96 lb per cu ft | 10, 20 lb | CaribSea Arag-Alive Bahamas Oolite |
| Crushed coral | 72 lb per cu ft | 10, 15, 40 lb | CaribSea Florida Crushed Coral |
| White sand for cichlids | 73 lb per cu ft | 20 lb | CaribSea African Cichlid Mix |
| Natural gravel and pebbles | 96 lb per cu ft | 5, 20, 40 lb | CaribSea Super Naturals Gemstone Creek |
| Planted tank substrate | 54 lb per cu ft | 20 lb | CaribSea Eco-Planted |
| Aqua soil | about 62 lb per cu ft | 9 L (9.0 kg) | ADA Amazonia, 9.0 kg per 9 L |
Densities differ within a maker's range: the Super Naturals colours run from 92 to 100 lb per cubic foot. If your bag prints a different density, choose "Other" and type it in. Always use the density for your product, not a generic one: the table spans 54 to 96 lb per cubic foot, almost a factor of two.
Sand versus gravel and how deep
Depth changes the bag count faster than density does. A 75 gallon (48 × 18 in) under 2 inches of reef sand at 85 lb per cubic foot needs 89.3 lb with 5% waste, or 5 bags of 20 lb. Aqua soils and planted substrates are lighter but you usually lay them deeper. The right depth depends on the plants and animals you keep, so the depth field is yours to set and the calculator does the sums.
The pounds-per-gallon rule
CaribSea's FAQ says the general rule is 1 to 2 pounds per gallon. Enter your tank volume and the page compares your answer with that rule. The example bed needs 66 lb without waste, 1.2 lb per gallon for a 55 gallon tank, inside the 55 to 110 lb the rule gives. The rule ignores the footprint, so a tall, narrow tank and a long, shallow one get the same answer from it, while the calculator uses the actual bottom area.
Aquarium gravel, bags and how many to buy
Pick the bag size that leaves the least waste: the calculator rounds up and shows the leftover so that you can compare a 20 lb bag with a 40 lb one. Substrate weighs on the floor too, but it rarely changes the load much: 66 lb is about 12% of the filled weight of the 55 gallon example on the aquarium volume calculator. Substrate also displaces water, which the same page subtracts to give net water.
Next steps
Size the water first with the fish tank calculator, then check how many fish it holds with the aquarium stocking calculator. A marine tank needs its salt mix from the salinity calculator. Everything for tank setup is on the aquarium hub.