The calculator keeps volume, estimated weight, and package count separate. This matters because a sloped layout changes average depth and aquarium substrate can be sold by either litres or kilograms.
Find average depth
For a flat bed, enter the same front and back depth. For a simple straight slope, average depth is the front depth plus back depth divided by two.
Calculate volume and weight
Internal length times width times average depth gives cubic centimetres; dividing by 1,000 gives litres. Density converts the purchase volume to an estimated dry weight.
Match the product label
Choose litres when the bag is sold by volume or kilograms when it is sold by weight. The package count includes the stated 10% setup allowance and rounds up.
How this estimate works
The calculator turns tank footprint and desired depth into volume, converts that volume to weight using substrate density, then rounds bag count up. It is a buying estimate, not a livestock-care decision tool, so substrate suitability should still be checked separately.
Average depth = (front depth + back depth) divided by 2 for a simple straight slope.
Volume in litres = length x width x average depth in cm divided by 1000.
Estimated kg = purchase litres x selected substrate density.
Bag count uses the selected label unit, includes a 10% setup allowance, and rounds up.
Before buying substrate
Check the needs of your livestock and plants, then confirm bag weights, labelled litres, substrate density, rinsing instructions, and setup notes from the brand you plan to use. For sloped aquascapes, compare the flat average estimate with a deeper back-section estimate.
Worked example
A 90 x 45 cm tank with 5 cm of substrate needs about 20.3 litres. At 1.4 kg/L with a small allowance, that is about 31.2 kg, so four 9 kg bags.
Substrate depth examples
Depth depends on layout, livestock, plants, and the substrate type. These examples are planning ranges, not species guidance.
Tank footprint
Depth
Approximate volume
45 x 30 cm
3 cm
4.1 litres
60 x 30 cm
3 cm
5.4 litres
60 x 30 cm
5 cm
9 litres
90 x 45 cm
5 cm
20.3 litres
90 x 45 cm
8 cm
32.4 litres
120 x 45 cm
6 cm
32.4 litres
120 x 50 cm
7 cm
42 litres
Before you add substrate
Decide whether the layout is flat, sloped, or planted before estimating depth.
Check whether the substrate should be rinsed before use.
Confirm the bag weight and density for the exact brand.
Leave enough free space for hardscape, filters, and livestock needs.
Check whether the bag is sold by kg, litres, or both before comparing products.
Keep plant-root areas, decorative foregrounds, and deep back slopes separate when estimating.
Account for displacement from large rocks, wood, caves, and filter hardware.
Keep species-specific substrate requirements separate from the volume estimate.
When this estimate may be wrong
Sloped aquascapes and deep planted areas can use more substrate than a flat average depth.
Rocks, wood, and decor displace substrate and water volume.
Some livestock need specific substrate types or depths that a volume calculator cannot decide.
Bag count changes when a product is sold by litres instead of kg.
Wet, rinsed, or porous substrates can behave differently from dry planning-density estimates.
Deep anaerobic pockets, sharp gravel, or unsuitable grain size can be livestock-specific concerns.
FAQ
How deep should aquarium substrate be?
Many simple aquariums use roughly 2-5 cm, while planted tanks often use deeper areas for roots or slopes. The right depth depends on plants, livestock, layout, and maintenance plan.
Why does density matter?
The same volume of sand, gravel, and aqua soil can weigh different amounts. Density converts litres into kg so the calculator can estimate bag count for products sold by weight.
Should I use litres or kg?
Use litres if the bag is sold by volume. Use kg plus density if the bag is sold by weight. Do not assume kg and litres are interchangeable.
How do I estimate a sloped aquascape?
For a gentle slope, use the average of the front and back depths. For a strong slope, split the tank into foreground and background sections and add the estimates together.
Do rocks and wood reduce substrate needed?
Sometimes. Large hardscape can displace substrate, but it can also create deeper pockets around it. Plan the hardscape first when possible, then estimate the remaining areas.
Does this decide which substrate is safe for my fish?
No. It estimates quantity only. Species-specific substrate type, grain size, sharpness, and depth need separate research.
Should substrate be rinsed before use?
Follow the product instructions. Some sands and gravels need rinsing, while some planted aqua soils should not be rinsed heavily.
Why does the calculator round bags up?
Substrate is sold in whole bags, and running short can interrupt a setup. Rounding up also helps cover small losses, slopes, and settling.
Can I use this for reef sand?
You can use the arithmetic if you enter the footprint, depth, density, and bag size, but reef livestock and substrate suitability must be checked separately.
What if my tank has an internal filter or built-in compartment?
Measure the area that will actually receive substrate. Exclude built-in compartments, large fixed structures, or areas occupied by equipment.
Substrate depth depends on livestock, plants, and tank design. Check species-specific requirements.