Quick Answer
To work out aquarium substrate, multiply the base area of the tank by the bed depth you want, then multiply that volume by the material's bulk density. Tank height is irrelevant — the bed sits on the floor. A 40 breeder footprint of 36 x 18 inches with a 2.5 inch bed holds 26.55 litres of substrate, but that is 42.52 kg of gravel at 1.60 kg/L against 34.02 kg of planted aquasoil at 1.28 kg/L. Because bags only come whole, the same tank at the same depth needs five 9 kg bags of gravel but only four of aquasoil.
The formula
Substrate is sold by weight and measured by volume, so every substrate calculation is really two steps: work out the volume of the bed, then convert that volume to weight using the material’s bulk density.
weight = volume × bulk density
bags = weight ÷ bag size, rounded up
Tank height never enters the calculation. A 30 cm tall tank and a 60 cm tall tank with the same base need exactly the same substrate, because the bed sits on the floor of the tank and does not care what happens above it. That is why this calculator asks for fewer dimensions than a volume calculator does — if you also need the water volume, use the aquarium volume calculator, which does take height.
The base area depends on the shape. For a rectangular tank it is length × width. For a cylinder it is πr². For a hexagonal tank it is (3√3 ÷ 2) × side². For a corner tank it is a quarter circle, πr² ÷ 4. A bowfront adds the bowed segment to the rectangle: length × width + (2 ÷ 3) × length × bow depth.
The same maths appears in imperial form on CaribSea’s own FAQ, which tells you to multiply length × width × bed depth in inches, divide by 1,728 to get cubic feet, then multiply by the substrate density in pounds per cubic foot. That is the identical calculation in different units, and this calculator reproduces its answers exactly.
How to use it
- Pick your tank shape. Six shapes are supported. The dimension fields change to match — a cylinder asks only for its diameter, a cube only for one side.
- Enter the footprint. Measure the inside of the glass, not the outside, and not the cabinet. Each field has its own unit picker, so you can enter 48 in for the length and 33 cm for the width without converting anything by hand.
- Set the bed depth you want. This is the finished depth of the layer, not the depth of the bag.
- Choose the substrate type. This sets the bulk density, and it changes the answer more than most people expect.
- Enter the bag size you are buying. The calculator rounds up to whole bags and tells you how much of the last bag is left over.
If you have a standard US glass tank, the quick-fill buttons load the footprint for you. Note that they only fill length and width — the gallon rating in the name refers to the whole tank, and two tanks with the same rating can have different footprints.
Bulk density by substrate type
Bulk density is the weight of a loosely poured litre of the material, and it is the number that turns a volume into a shopping list. The figures below are CaribSea’s own published densities, given in their source unit and converted here.
| Substrate | Density (lb/ft³) | Density (kg/L) | Litres per 9 kg bag |
|---|---|---|---|
| Gravel or sand | 100 | 1.60 | 5.62 L |
| Aragonite / reef sand (dry) | 90 | 1.44 | 6.24 L |
| Special grade reef sand | 85 | 1.36 | 6.61 L |
| Planted aquasoil | 80 | 1.28 | 7.02 L |
| Crushed coral / cichlid gravel | 75 | 1.20 | 7.49 L |
The spread from top to bottom is 33%. A 9 kg bag of crushed coral fills 7.49 litres of tank floor; the same 9 kg of gravel fills only 5.62 litres. If your bag states its own density, enter it directly using the custom option — manufacturers vary within a category, and a stated figure always beats a category average.
Why the material changes the bag count
This is the part most substrate calculators get wrong, because they assume one density for everything. Take a 40 breeder footprint, 36 × 18 inches, with a 2.5 inch bed. The volume is fixed at 26.55 litres no matter what you fill it with. The weight is not.
| Material | Volume | Weight | 9 kg bags | Left over |
|---|---|---|---|---|
| Gravel or sand | 26.55 L | 42.52 kg | 5 | 2.48 kg |
| Planted aquasoil | 26.55 L | 34.02 kg | 4 | 1.98 kg |
| Crushed coral | 26.55 L | 31.89 kg | 4 | 4.11 kg |
Gravel is exactly 25% heavier than aquasoil for the same bed, and 33% heavier than crushed coral. Because bags only come whole, that difference is not academic: the same tank at the same depth needs five bags of gravel but four of aquasoil. Ordering by weight without checking the material is how people end up a bag short on setup day, or with an unopened bag they cannot return once it is in the car.
How deep should the bed be
Depth is a straight multiplier — double the depth and you double the weight, the volume and roughly the cost. The table below is a 20 long footprint, 30 × 12 inches, filled with gravel at 1.60 kg/L and bought in 5 kg bags.
| Bed depth | Volume | Weight | 5 kg bags |
|---|---|---|---|
| 2.0 cm (0.8 in) | 4.65 L | 7.44 kg | 2 |
| 2.5 cm (1.0 in) | 5.81 L | 9.30 kg | 2 |
| 3.0 cm (1.2 in) | 6.97 L | 11.16 kg | 3 |
| 5.0 cm (2.0 in) | 11.61 L | 18.60 kg | 4 |
| 7.5 cm (3.0 in) | 17.42 L | 27.90 kg | 6 |
| 10.0 cm (3.9 in) | 23.23 L | 37.20 kg | 8 |
A bed under about 2 cm struggles to cover the base evenly and will not hold planted roots. Beds beyond 10 cm are unusual outside aquascaping, where the substrate is deliberately sloped to build depth toward the back. If you are sloping the bed, enter the average depth, not the deepest point — a bed running 3 cm at the front and 7 cm at the back averages 5 cm.
CaribSea offers a general rule of thumb of 1 to 2 pounds of substrate per gallon of tank. That is a useful sanity check on the result rather than a substitute for it, because it takes no account of footprint shape: a tall narrow tank and a shallow wide tank of the same gallonage need very different amounts.
Substrate for standard tank sizes
Every row below is a 5 cm bed of gravel at 1.60 kg/L, in 5 kg bags, using the standard US footprint for each tank class.
| Tank | Footprint | Volume | Weight | 5 kg bags |
|---|---|---|---|---|
| 5 gallon | 16 × 8 in | 4.13 L | 6.61 kg | 2 |
| 10 gallon | 20 × 10 in | 6.45 L | 10.33 kg | 3 |
| 20 long | 30 × 12 in | 11.61 L | 18.60 kg | 4 |
| 40 breeder | 36 × 18 in | 20.90 L | 33.48 kg | 7 |
| 55 gallon | 48 × 13 in | 20.13 L | 32.24 kg | 7 |
| 75 gallon | 48 × 18 in | 27.87 L | 44.64 kg | 9 |
The 40 breeder and the 55 gallon are worth comparing. The 55 holds far more water, but its footprint is 4,026 cm² against the breeder’s 4,181 cm² — so the 40 breeder actually needs slightly more substrate. The 55 gets its capacity from height, and height is exactly the dimension substrate ignores.
Tank shape and footprint
Round and angled tanks catch people out because the width across the tank is not the width of the floor. Each row below is a 5 cm gravel bed.
| Shape | Base area | Volume | Weight |
|---|---|---|---|
| Rectangular, 60 × 30 cm | 1,800 cm² | 9.00 L | 14.42 kg |
| Cylinder, 45 cm across | 1,590 cm² | 7.95 L | 12.74 kg |
| Hexagonal, 25 cm sides | 1,624 cm² | 8.12 L | 13.01 kg |
| Corner, 60 cm radius | 2,827 cm² | 14.14 L | 22.65 kg |
Mistakes that cost you a bag
- Measuring the outside of the tank. Glass and trim can take a centimetre or more off each internal dimension. On a 20 long that is a few hundred grams; on a 75 it can be a kilo.
- Rounding the depth up “to be safe”. On a 20 long, a 4 cm bed needs 14.88 kg and fits in 3 bags of 5 kg. Going to 5 cm needs 18.60 kg — a fourth bag, for one extra centimetre.
- Using a weight quoted for a different material. See the 25% and 33% gaps above.
- Assuming the bag is the same everywhere. Bag sizes are not standard: Seachem’s Flourite comes in 3.5 kg and 7 kg, while CaribSea’s range runs from 3.5 lb to 40 lb. Enter the size you are actually buying.
- Forgetting hardscape. Large rocks and thick wood sitting on the glass displace substrate. On a heavily scaped tank you may need noticeably less than the flat-bed figure.
Weight on the tank and the stand
Substrate is dense, and it all sits on the base pane. A 75 gallon with a 5 cm gravel bed carries 44.64 kg of substrate before a drop of water goes in. That load matters when you are choosing a stand, and it matters when the tank is not sitting perfectly flat — if you are checking whether the base glass is up to the job, the aquarium glass thickness calculator covers that side of the problem.
It also matters for water volume. Substrate displaces water, so a tank with a deep bed holds less than its rating suggests. That 20.90 litres of substrate in a 40 breeder is 20.90 litres of water the tank no longer holds, which changes every dose you measure by tank volume.
Frequently asked questions
Because substrate depth is set by you, not by the tank. The bed sits on the base and its volume is base area × bed depth. Height only affects how much water sits above it, which is a different calculation.
For a standard 48 × 13 inch footprint with a 5 cm bed, 32.24 kg — that is 20.13 litres, or 7 bags of 5 kg with some left over. Halve the depth and you halve the weight.
Yes, by about 20% per litre — 1.28 kg/L against 1.60 kg/L. Put the other way round, gravel is 25% heavier than aquasoil for the same bed. On a 40 breeder at 2.5 inches that is the difference between 4 bags and 5.
The finished depth of the bed in the tank. Bag volume is handled separately, through the bag size field, and the calculator converts between the two using the bulk density.
The average. A bed running 3 cm at the front and 7 cm at the back averages 5 cm, and that average is what determines the total volume.
Yes. Switch the result units to imperial and weights appear in pounds, volumes in US gallons and density in pounds per cubic foot. The dimension fields each have their own unit picker, so you can mix inches and centimetres in the same calculation.
Grain size, shape and moisture all shift bulk density, so products within a category differ. If your bag states a figure, choose the custom option and enter it — a stated density is always better than a category average.
Usually yes, because rooted plants need depth to anchor in, and a bed under about 2 cm will not hold them. Deeper beds scale linearly: going from 3 cm to 6 cm doubles the amount you buy.
