About the Aquarium Substrate Calculator
This aquarium substrate calculator works out how much gravel, sand, aragonite or planted aqua soil to buy for your fish tank. Enter the inside length and width of the tank, the depth you want at the front and back (a slope toward the back is common in planted and aquascaped tanks) and the type of substrate. You get the weight in pounds and kilograms, the volume in liters and the number of bags.
It is useful when setting up a new tank, switching from gravel to sand, or building up a deep bed for rooted plants. Pet stores sell gravel and sand by weight but aqua soils by volume, so the calculator gives both. The “one pound per gallon” rule of thumb only gives about an inch of gravel in a standard tank and is far off for long, shallow or tall tanks.
Densities are typical dry bulk values; real products vary by grain size and moisture, so buy one extra bag rather than coming up short. As a guide, 1–2 inches (2.5–5 cm) suits most fish-only tanks, 2–3 inches (5–7.5 cm) suits planted tanks, and sand beds deeper than about 3 inches need care to avoid trapped gas pockets.
With the default inputs, the substrate needed is 41.6 lb. Change any value above to recalculate instantly.
How to use the aquarium substrate calculator
- 1Choose inches or centimetres and measure the inside length and width of the tank.
- 2Enter the substrate depth you want at the front and at the back.
- 3Pick the substrate type, or enter a custom density from the product label.
- 4Enter the bag size you plan to buy.
- 5Read the weight, volume and number of bags — round up if in doubt.
Formula and method
The substrate forms a slab across the tank floor, so its volume is length × width × depth. For a sloped bed the calculator uses the average of the front and back depths, which gives the exact volume of a straight slope. Inches are converted to centimetres (2.54 cm per inch) and cubic centimetres to liters (1,000 cm³ per liter).
Weight is volume multiplied by the dry bulk density of the material: about 1.6 kg/L for gravel, 1.5 kg/L for sand, 1.4 kg/L for aragonite or crushed coral and about 1.0 kg/L for lightweight planted aqua soils. Kilograms are converted at 2.2046 lb per kg, and bags are rounded up to whole bags.
- length, width
- Inside floor dimensions of the tank
- average depth
- (front depth + back depth) ÷ 2
- bulk density
- Weight of one liter of dry substrate, in kg
Worked examples
20-gallon long (30 × 12 in), 2 inches of gravel
30 × 12 × 2 = 720 cubic inches, or 11.8 liters. At 1.6 kg per liter that is about 18.9 kg (41.6 lb) of gravel, so three 20 lb bags.
Metric 90 × 45 cm planted tank, 3 cm front sloping to 8 cm back, aqua soil
The average depth is (3 + 8) ÷ 2 = 5.5 cm, so the volume is 90 × 45 × 5.5 ÷ 1,000 = 22.3 liters. Aqua soil weighs about 1 kg per liter, so roughly three 9–10 L bags.
48 × 18 in tank with 1.5 inches of sand
48 × 18 × 1.5 = 1,296 cubic inches, which is 21.2 liters. At 1.5 kg per liter that is 31.9 kg or about 70 lb of sand — two 50 lb bags.
Frequently asked questions
How much gravel do I need for my aquarium?+
Multiply the tank’s inside length × width × desired depth in inches, then multiply by about 0.058 to get pounds of gravel. A 30 × 12 inch tank needs about 21 lb per inch of depth, so roughly 40–45 lb for a 2-inch bed.
Is it really 1 pound of gravel per gallon?+
Only roughly. One pound per gallon gives about an inch of gravel in a standard-height tank. Long, shallow tanks have more floor area per gallon and need more; tall tanks need less. Calculating from floor area is more accurate.
How deep should aquarium substrate be?+
About 1–2 inches (2.5–5 cm) is enough for most fish-only tanks. Planted tanks usually need 2–3 inches (5–7.5 cm) for roots, often sloped deeper toward the back to create depth.
How much sand do I need for a fish tank?+
Sand is slightly lighter than gravel by volume — roughly 0.054 lb per cubic inch. For a 48 × 18 inch tank, 1.5 inches of sand is about 70 lb. Rinse it well before adding water to avoid cloudiness.
Why does aqua soil come in liters instead of pounds?+
Planted aqua soils are baked, porous clay granules that weigh only about 1 kg per liter and vary in moisture, so manufacturers sell them by volume. Use the liters result to choose bags.