Home › Calculators › Substrate
Aquarium Substrate Calculator
Enter your tank's length and width and how deep you want the bed. You get the amount in pounds, kilograms and litres, and how many bags to buy. The rule: inside length × inside width × depth ÷ 1,728 × density (in inches) gives pounds. Sand weighs about 100 lb per cubic foot, so a 2-inch sand bed in a 20-gallon long needs about 40 lb.
Substrate calculator
Drawn to scale: the sand-coloured layer is your bed, sloping up to the back if you choose.
Next steps
- Tank volume: how much water your tank really holds once the substrate is in
- Tank weight: water, glass, sand and rock together, for your stand and floor
- Stocking: how many fish, with a tank-mate check
How the calculator works
Substrate is a flat box (or a wedge, if you slope it) sitting on the bottom glass. Work out its volume from the inside footprint and the average depth, then multiply by how much the material weighs per cubic foot.
A slope is just a wedge, so its volume equals a flat bed at the average depth. That is why the calculator only needs the front and back depths.
Example 1: sand in a 20 gallon long
Outside 30 × 12 in, glass about 0.19 in (5 mm), 2 in of sand. Inside: 29.62 × 11.62 = 344.2 in². × 2 = 688.4 in³. ÷ 1,728 = 0.398 ft³. × 100 = 39.8 lb (18.1 kg): 2 bags of 20 lb. See the 20 gallon long dimensions.
Example 2: sloped Eco-Complete in a 55 gallon
Outside 48 × 13 in, glass 0.25 in, 1.5 in at the front rising to 3 in at the back. Inside: 47.5 × 12.5 = 593.75 in². Average depth 2.25 in, so 1,335.9 in³ ÷ 1,728 = 0.773 ft³. × 80 = 61.8 lb: 3.09 bags of 20 lb, so 4 bags. See the 55 gallon dimensions.
Example 3: aquasoil in a 40 breeder (metric)
Outside 91 × 46 cm, glass 0.5 cm, 3 cm at the front rising to 7 cm at the back. Inside: 90 × 45 = 4,050 cm². Average depth 5 cm, so 20,250 cm³ = 20.3 litres: 2.25 bags of 9 L, so 3 bags. Aquasoil is sold by volume, so no weight is needed. See the 40 breeder dimensions.
What each substrate weighs
These are dry, loose weights per cubic foot. Wet or compacted material is heavier, and grain size changes it a little, so treat results as a planning estimate.
| Substrate | lb per ft³ | kg per litre | Source |
|---|---|---|---|
| Sand (e.g. CaribSea Super Naturals, aragonite sand) | 100 | 1.60 | CaribSea |
| Regular aquarium gravel | 80 | 1.28 | Engineering ToolBox (gravel 75–85) |
| Crushed coral, coarse cichlid gravel | 75 | 1.20 | CaribSea |
| Eco-Complete (CaribSea planted) | 80 | 1.28 | CaribSea ("Eco-Planted") |
| Seachem Flourite | ≈ 63 | ≈ 1.0 | Seachem: 7 kg bag ≈ 425 in³ |
| Aquasoils (ADA Amazonia and others) | sold by the litre | Maker's bag volume | |
Sources: CaribSea FAQ (formula and densities) · Engineering ToolBox · Seachem. Checked 24 Sep 2026.
Why "1 to 2 lb per gallon" can be wrong
CaribSea's quick rule is 1–2 lb of substrate per gallon. It's a fair start, but substrate covers the floor, and gallons also count height. A 20 gallon long and a 29 gallon have the same 30 × 12 in footprint, so they need the same sand for the same depth (about 40 lb at 2 in). The rule would tell you 20–40 lb for one and 29–58 lb for the other. Tall tanks get too much from the rule; long, shallow tanks can get too little. Measure the footprint instead.
How deep should the bed be?
- Sand: about 1–2 in (2.5–5 cm) is common for fish-only tanks. Deeper sand beds are a separate, specialist method.
- Gravel: about 2 in (5 cm) is typical and easy to vacuum.
- Planted tanks: rooted plants usually get 2–3 in (5–8 cm), often sloped deeper at the back. Seachem sizes a 7 kg Flourite bag for a 2 in bed in a 10 gallon; Fluval suggests 3–4 cm for Stratum.
These are common starting points, not rules. Your fish, plants and cleaning routine matter more. Sloping the back up adds depth where plants grow and makes the tank look deeper.
FAQ
How much sand do I need for a 20 gallon long?
About 40 lb (18 kg) for a flat 2-inch bed, so two 20 lb bags. A 29 gallon has the same footprint, so it needs the same amount.
How much sand for a 55 or 75 gallon tank?
For a flat 2-inch sand bed: a 55 gallon (48 × 13 in) needs about 69 lb, and a 75 gallon (48½ × 18½ in) about 100 lb. Put the dimensions in the calculator for your exact tank and depth.
Is 1 lb of gravel per gallon enough?
For a standard-height tank, 1 lb per gallon gives roughly a 1-inch bed; most people want 2 in, so plan on closer to 2 lb per gallon. Tall tanks need less than the rule says, because substrate depends on floor area, not height.
Does sand or gravel weigh more?
Sand. It packs tighter, at about 100 lb per cubic foot against about 80 for regular gravel. The same bed of sand weighs about a quarter more, so you need more pounds of it.
Should I buy extra for rocks and slopes?
Rocks sitting on the substrate don't reduce how much you need much, and a slope is already counted. Many people buy one extra bag for settling, tidying and future rescapes. The calculator already rounds up to whole bags.
Put this calculator on your site
Related calculators
- Tank volumeGallons and litres, any shape
- StockingFish count + tank mates
- Heater sizeWatts for your tank
- Tank weightWater, glass and substrate