Aquarium Substrate Calculator
Work out how much gravel, sand, aragonite or aquasoil to buy for your tank. Get the answer in pounds, kilograms, liters and bags, with sloped beds and rock coverage taken into account.
How to use this calculator
- Measure the floor of the tank. Only length and width matter. Height has nothing to do with how much substrate you need, which is why "pounds per gallon" rules go wrong.
- Choose your depth. For a flat bed, put the same number in both depth boxes. For a slope that rises toward the back, enter the front and back depths and the calculator uses the average.
- Subtract rock if it sits on the glass. If large stones or a rock wall stand directly on the bottom, estimate what share of the floor they cover. Rock placed on top of the substrate does not reduce the amount you need.
- Pick your substrate and bag size. Gravel and sand are usually sold by weight. Aquasoils are usually sold by volume, so set the bag unit to liters for those.
Formulas used
The floor area is length × width for a rectangle, or π × (diameter ÷ 2)² for a round tank. Cubic inches are converted with 1 ft³ = 1,728 in³ and 1 in³ = 16.387 cm³, and 1 ft³ = 28.32 liters.
Worked example: sloped gravel in a 55 gallon tank
A 55 gallon tank has a floor of about 48 × 13 inches. You want gravel 2 inches deep at the front, rising to 3 inches at the back.
- Average depth = (2 + 3) ÷ 2 = 2.5 inches
- Volume = 48 × 13 × 2.5 = 1,560 cubic inches = 0.90 ft³ (25.6 liters)
- Weight = 0.90 × 105 lb/ft³ = 95 lb (43 kg) of gravel
- Bags = 95 ÷ 20 = 4.7, so 5 bags of 20 lb
For comparison, the "1 lb per gallon per inch" rule would say 55 × 2.5 = 137.5 lb, about 45% more than you need for this tank.
Substrate densities
Weight depends on what the substrate is made of. These are the default values the calculator uses. Real products vary with grain size and moisture, so check the bag label if you need to be exact.
| Substrate | lb per ft³ | kg per liter | Basis |
|---|---|---|---|
| Aquarium gravel | 105 | 1.68 | Dry gravel, Engineering ToolBox |
| Sand (silica, pool filter) | 100 | 1.60 | Dry sand, Engineering ToolBox (97–100) |
| Aragonite reef sand | 85 | 1.36 | CaribSea Special Grade listing; aragonites range about 70–100 |
| Eco-Complete planted | 54 | 0.87 | Retailer product specification |
| Aquasoil | about 62 | about 1.0 | Bag label of 9 L / 9 kg for one brand; varies with moisture |
How deep should aquarium substrate be?
There is no single right depth, but common hobby practice looks like this:
- Fish-only tanks: about 1 to 2 inches (2.5 to 5 cm) is enough to cover the glass and is easy to keep clean.
- Planted tanks: about 2 to 3 inches (5 to 8 cm) gives roots room, often deeper at the back.
- Reef tanks: many reefers use a shallow sand bed of about 1 inch. Deeper beds are a separate method with their own pros and cons.
A slope from front to back makes a tank look deeper and puts the thicker substrate where tall plants usually go.
Frequently asked questions
How many pounds of gravel do I need per gallon?
The popular rule of 1 pound per gallon for each inch of depth is only a rough guide, and it usually overestimates for tall tanks and underestimates for wide ones, because substrate depends on the tank's floor area, not its volume. Measure the footprint and use the calculator instead.
Is sand heavier than gravel?
Per cubic foot, dry sand and gravel are similar: Engineering ToolBox lists dry sand at about 97 to 100 lb per cubic foot and dry gravel at about 105. Aragonite reef sand is usually lighter, often 70 to 100 lb per cubic foot, and lightweight planted substrates such as Eco-Complete are around 54.
Why is aquasoil sold in liters instead of pounds?
Aquasoils are porous baked granules whose weight changes a lot with moisture, so brands sell them by volume. Use the liters result from the calculator and pick the liter bag size to get the number of bags.
Should I buy extra substrate?
Yes. Rinsing washes out fines, the bed settles, and you may want to build up a slope later. Buying about 10 percent more than the calculated amount is a sensible margin, and the calculator shows that figure.
Do I measure the inside or outside of the tank?
Use the inside length and width if you can. On a tank with thick glass or a plastic rim, outside measurements add a little extra, which is not a problem if you are already planning to buy a bit more.
Related calculators
- Aquarium volume calculator: gallons and liters for any tank shape
- Aquarium weight calculator: add the substrate to the total tank weight
- Fertilizer dosing calculator: feeding plants in a planted tank
Sources: Dry sand and gravel densities from Engineering ToolBox, dirt and mud densities and ditch filling materials. Aragonite density from the CaribSea Special Grade Reef Sand listing and a Reef Central discussion quoting CaribSea. Eco-Complete density from Reef Supplies. Aquasoil bag weight from the PRODIBIO AquaGrowth Soil 9 L listing.
Last updated: October 2026