TANKBLOOM CALCULATOR
Aquarium Substrate Calculator
Calculate how much gravel, sand, or aquasoil your aquarium needs—from substrate volume and estimated weight to the number of bags to buy.
Flat or sloped beds
Gravel, sand & aquasoil
US & metric units
Gravel, sand & aquasoil
US & metric units
PLAN YOUR SUBSTRATE
Calculate your substrate
Start with your tank’s inside dimensions. Your bag size comes directly from the product label.
THE QUICK ANSWER
How much substrate do you need for an aquarium?
Multiply the aquarium’s inside floor area by your chosen substrate depth to find the bed volume. Use that volume to plan gravel, sand, or aquasoil purchases. If your bag is labeled in pounds or kilograms, convert volume to estimated weight using the bulk density of your actual product when available. A tank’s gallon rating alone is not enough: a 20-gallon long has a larger footprint than many 20-gallon high tanks.
Three things to know before buying
- Measure the bottom, not the gallon label. Different footprints require different amounts.
- Calculate liters first. Weight changes with the type and packing of substrate.
- Buy by the bag’s printed unit. Liters, pounds, and kilograms are not interchangeable.
TANK-SIZE EXAMPLES
Substrate amounts for common fish tank sizes
These are bed volumes, not bag weights. The example footprints are approximate common US tank dimensions. Actual internal measurements may be smaller; use the calculator above for your tank.
| Tank type | Example footprint | 1-inch deep | 2-inch deep |
|---|---|---|---|
| 5-gallon | 16 × 8 in | 2.1 L | 4.2 L |
| 10-gallon | 20 × 10 in | 3.3 L | 6.6 L |
| 20-gallon high | 24 × 12 in | 4.7 L | 9.4 L |
| 20-gallon long | 30 × 12 in | 5.9 L | 11.8 L |
| 40-gallon breeder | 36 × 18 in | 10.6 L | 21.2 L |
| 55-gallon | 48 × 13 in | 10.2 L | 20.5 L |
| 75-gallon | 48 × 18 in | 14.2 L | 28.3 L |
Why this matters: at 2 inches deep, the example 20-gallon long needs about 11.8 L, while the 20-gallon high needs about 9.4 L. Both tanks are called “20 gallons,” but their base areas differ.
FORMULAS, MADE PRACTICAL
How to calculate aquarium gravel, sand, and soil
The same volume formula works for most materials. It’s the weight estimate that changes by substrate.
Flat substrate bed
Liters = length (cm) × width (cm) × depth (cm) ÷ 1,000
A 60 × 30 cm aquarium with a 5 cm bed uses 9 L of substrate by volume, before any purchase allowance.
Straight, even slope
Average depth = (front depth + back depth) ÷ 2
A 3 cm front and 7 cm back average 5 cm. The same 60 × 30 cm rectangular footprint still needs 9 L. This is not a formula for irregular hills or terraces.
Estimated weight
Weight (kg) = volume (L) × bulk density (kg/L)
Using an illustrative gravel density of 1.6 kg/L, 9 L weighs about 14.4 kg (31.7 lb). Actual product density may differ.
For a soil base capped with sand or gravel, calculate each layer separately. Mixing materials in one weight calculation can give the wrong estimate, because their bulk densities and package units differ.
BUY THE RIGHT AMOUNT
How many bags of aquarium substrate should you buy?
After calculating bed volume, check the bag: is it sold in liters, kilograms, or pounds? That label determines how the calculator counts packages. Always round up to a whole bag.
Allowance is optional. It increases the amount you shop for, not the amount that geometrically fits into the bed. The calculator defaults to zero extra, and you can deliberately add a buffer for adjustments.
DEPTH & MATERIAL CHOICE
How deep should aquarium substrate be?
There is no single ideal depth for every freshwater, planted, or reef tank. Choose the depth for your livestock, rooted plants, substrate product, and maintenance plan—not from a universal inches-per-gallon rule.
Decorative gravel
A thin decorative layer generally needs less material than a planted bed. If you keep rooted plants, give them adequate anchoring space and follow the plant and substrate product’s instructions.
Aquarium sand
Fine sand can form a compact surface. Suitable depth depends on the animals, grain size, and upkeep. A reef sand bed may have different requirements from a freshwater display; do not automatically copy planted-tank depths.
Planted aquasoil
Check the manufacturer’s instructions for the product you bought. Some active aquasoils influence water chemistry and contain nutrients; some planted systems need only one complete soil rather than a separately capped base.
Layered and sloped beds
A gradual rise toward the back can create perspective in planted aquascapes. For example, Tropica describes a planted layout with approximately 3–4 cm at the front and 6–8 cm at the back; that is a layout example, not a prescription for every tank.
Manufacturer guidance: Tropica’s hardscape and substrate guide explains planted-bed slopes and substrate layering. ADA’s Aqua Soil product information documents product-specific substrate features and water-chemistry effects. Always follow the directions for your own material.
COMMON QUESTIONS
Frequently asked questions
How much gravel do I need for a 10-gallon aquarium?
Using a typical 20 × 10 inch footprint, a flat bed needs approximately 3.3 L at 1 inch or 6.6 L at 2 inches. To estimate pounds, you must also know the gravel’s bulk density. Measure your tank’s inside base for a closer result.
How much substrate do I need for a 20-gallon tank?
The footprint matters more than the capacity label. At 2 inches, a nominal 30 × 12 inch 20-long needs about 11.8 L, but a 24 × 12 inch 20-high needs about 9.4 L. Actual inside dimensions may differ.
How many bags of aquarium sand do I need?
First calculate substrate volume, then enter the size printed on your sand bag. A 9 L bed with a 10% allowance needs 9.9 L; that is 2 bags of 5 L. If your sand is sold in pounds, use an appropriate bulk density for the weight conversion instead.
How many pounds of gravel should I buy?
There is no reliable universal pounds-per-gallon rule. For example, at an illustrative density of 1.6 kg/L, 9 L weighs about 31.7 lb before any buying allowance. Your particular gravel may be lighter or heavier.
Can I use the calculator for a sloped or layered substrate?
Yes. A straight front-to-back slope is supported for rectangular tanks. You can calculate up to three separate layers, each with its own material and bag size. Irregular terrain and raised terraces need more detailed measurements.
Does the substrate bed volume equal the water it displaces?
No. Spaces between sand or gravel grains hold water. A 9 L substrate bed does not necessarily displace 9 L of aquarium water. This calculator estimates substrate to purchase, not exact remaining water capacity.
TANKBLOOM METHODOLOGY
What the result measures—and what it cannot know
The calculator uses horizontal footprint geometry and substrate depth to calculate a bulk bed volume. Material weight is calculated only when a density value is available; the built-in generic estimates are provisional and may not match your product. Bag counts use the package size and unit you enter, then round up. Intermediate calculations use unrounded values.
For a bow-front tank, the footprint is a geometric approximation. Actual packaged substrate volumes, settling, and hardscape may alter purchasing needs. Verify product directions and actual measurements before buying.
Need to calculate your tank’s water capacity as well? Use TankBloom’s Aquarium Volume Calculator. Water capacity and substrate bed volume are different measurements.
Read the labels correctly
Calculated: volume from dimensions and depth.
Estimated: weight based on bulk density; modeled geometry where applicable.
Shopping estimate: whole bags after the optional allowance and package-unit conversion.
Important: bed volume is not exact water displacement.