Aquascaping equipment
Calculating Aquarium Substrate Quantities
Calculate substrate litres, compare flat and sloped beds, see whole-bag boundaries and separate-material quantities, and download reproducible calculations.
Measure the bed you intend to build
Measure the aquarium's internal length and front-to-back width in centimetres. Then choose appropriate bed depths for the actual plants, livestock and substrate system. The calculation estimates material for that plan; it does not decide the right depth or certify that a steep slope will stay stable.
Use internal base dimensions rather than external cabinet dimensions or the aquarium's headline litres. A tall and a shallow tank can hold similar water volumes while needing different substrate quantities because their footprints differ. Height above the bed is not part of the substrate-volume formula.
Check the selected material's instructions first. A complete substrate, a concentrated base with a cap and a local planting pocket need different calculations. Keep the substrate-choice guide alongside the measurements so the arithmetic describes the system you will actually use.
Calculate flat-bed or simple-slope litres
For a flat bed, enter the same depth at front and rear. For a straight even slope, enter the two end depths and the calculator uses their average. The result is gross loose-bed volume before accounting for buried hardscape, settlement or a deliberately irregular layout.
Plan the bed volume
Aquarium substrate calculator
Use internal centimetres for a rectangular footprint. Enter the same depth twice for a flat bed. For a simple even slope, use the front and rear depths. Bag size must be the product's stated volume in litres, not its weight in kilograms.
This calculates loose bed volume and rounds up to whole bags. It does not prescribe a suitable planting depth, subtract buried hardscape, convert kilograms or calculate the aquarium's usable water volume.
Enter a bag volume only if the product gives a reliable volume in litres. The selected Soil Powder pack later on this page is 3 litres. A bag marked 7 kg is a weight pack and must not be entered as seven litres. The tool deliberately avoids a universal weight conversion.
Flat beds: multiply three centimetre measurements
The flat rectangular formula is length × width × depth ÷ 1,000 = bulk litres. The division converts cubic centimetres into litres. A bed measuring 60 × 30 cm at 4 cm depth is 7,200 cm³, or 7.2 litres.
Tropica publishes this geometric method for its Soil and Soil Powder products. The same geometry describes a rectangular envelope for another loose material, but the envelope does not establish its weight, preparation requirements or suitability for a particular fish. Those questions remain product-specific.
| Internal base | Flat 4 cm | Flat 5 cm | Slope 3–7 cm |
|---|---|---|---|
| 45 × 30 cm | 5.4 L | 6.75 L | 6.75 L |
| 60 × 30 cm | 7.2 L | 9 L | 9 L |
| 90 × 45 cm | 16.2 L | 20.25 L | 20.25 L |
| 120 × 45 cm | 21.6 L | 27 L | 27 L |
A straight 3–7 cm slope has the same 5 cm average depth as a flat 5 cm bed, so their envelope volumes match for the same footprint. Compared with a flat 4 cm bed, both need more material because their average depth is greater. A slope does not automatically require more material than every flat bed.
These depths are examples for arithmetic. A product requiring a particular cap or plants needing a different root zone should determine the actual plan. Do not deepen every bed to match a convenient table row.
Slopes: average only when the shape supports it
For a straight even front-to-back slope, average depth is (front depth + rear depth) ÷ 2. A 60 × 30 cm footprint sloping from 3 to 7 cm averages 5 cm, giving 9 litres. That works because the depth changes evenly across the rectangular footprint.
A high mound behind one rock is not an even slope across the whole width. Divide irregular layouts into sensible rectangles or planting zones and add their estimated volumes, avoiding overlap. Terraces with retaining structures also need their own plan for stability and access.
If a footprint has a sand path and a different planting zone, calculate the area of each material separately. Do not calculate each as though it covers the full base. For a concentrated base plus cap, estimate each layer separately using the exact maker's depth requirements and the area actually covered.
Round up only after calculating the required material
Divide the required bulk litres by the bag's stated litres, then round up to whole bags. A 7.2 litre flat-bed estimate needs three 3 litre bags, providing 9 litres of labelled material. A 9 litre sloped estimate needs three of the same bags. A 90 × 45 cm bed averaging 4.5 cm needs 18.225 litres and therefore seven 3 litre bags.
Rounding provides unused capacity in some examples, but it is not a universal allowance for settlement or future additions. Decide any additional reserve explicitly and check whether the spare can be stored appropriately. Avoid applying several hidden percentage allowances and calling the result exact.
Why can a small depth change add a whole bag?
We held a 60 × 30 cm footprint and 3-litre pack size constant, then changed the average depth around a whole-bag boundary. These are planned-volume examples, not recommended planting depths. For a flat bed, the average is its depth; for an even slope, it is the mean of the front and rear depths.
| Mean depth | Bed volume | Whole bags | Unused volume |
|---|---|---|---|
| 4.9 cm | 8.82 L | 3 | 0.18 L |
| 5 cm | 9 L | 3 | 0 L |
| 5.1 cm | 9.18 L | 4 | 2.82 L |
At 5 cm, the bed is exactly 9 litres: three 3-litre packs. At 5.1 cm, the estimate is 9.18 litres, so the purchasing calculation becomes four packs. That extra 0.18 litres of bed crosses a packaging boundary and leaves 2.82 litres of labelled capacity unused. It does not mean the aquarium needs the entire fourth bag poured in.
Uncertainty matters at a boundary. Choose the appropriate layout first, then use the pack calculation to understand the purchase. Keep any deliberate reserve separate from the mathematical round-up; neither figure predicts settlement or guarantees a bag's installed coverage in every layout.
Round different materials separately
Consider a hypothetical 60 × 30 cm base split into two side-by-side 30 × 30 cm zones. Zone A uses one material at a flat 3 cm depth; zone B uses a different material at a flat 7 cm depth. Assume each is sold in 3-litre packs solely for this example.
| Zone | Bed volume | Whole bags | Unused volume |
|---|---|---|---|
| A: 3 cm depth | 2.7 L | 1 | 0.3 L |
| B: 7 cm depth | 6.3 L | 3 | 2.7 L |
The actual bed totals 2.7 + 6.3 = 9 litres, but buying the two materials separately needs one pack plus three packs. Dividing the combined 9 litres by 3 would suggest three packs in total and overlook the material-specific shortfall. If both zones genuinely used the same interchangeable material, their combined 9 litres would instead fit three packs.
The example isolates purchasing arithmetic. It does not select suitable materials, depths or a retaining system. A visually attractive boundary may still need an appropriate barrier and stable construction. Keep the actual material instructions, livestock needs and active-soil water effects in the layout decision.
Check the inputs and reuse the calculations
Calculate bulk litres first, divide by the stated pack litres, and round up to a whole pack. Purchased litres equal whole packs multiplied by pack litres; unused labelled volume is purchased litres minus the planned envelope. For separate materials, perform that sequence for each material before adding the bag counts.
Download all 29 substrate scenarios as a CSV. It includes four footprints with flat and even-slope plans, illustrative 3 L and 9 L pack sizes, the boundary examples and the two separate-material zones. Pack sizes are calculation inputs, not claims about the available sizes of every product. Values are recorded to six decimal places where needed; whole-bag counts remain integers.
The geometry follows Tropica's length × width × depth method. The controlled comparisons and bag-boundary analysis are our own calculations, using the same volume and rounding method as the substrate calculator. These are bulk material litres, with no universal conversion to kilograms or solid water displacement.
Tropica Soil Powder below is a genuine volume-labelled example. It is an active substrate that affects pH and requires its own establishment routine; the convenience of its litre pack does not make it the right substrate for every aquarium.
A finer active soil for a planned new planted setup
Tropica Aquarium Soil Powder, 3 litres
A complete active substrate with the maker’s 1–2 mm grain specification. Soil Powder affects water chemistry and lowers pH, so select it with the intended water and livestock in mind.
- 3 litre selected bag
- 1–2 mm Soil Powder variant
- Complete substrate; affects pH
A new planted aquarium where the active chemistry, establishment routine and fine grain are a deliberate choice.
- Tropica recommends 25–50% changes at least twice weekly for the first four weeks; monitor the actual setup.
- Not the same product as regular Aquarium Soil or the nutrient Substrate that needs a cap; do not assume it cycles a filter.
For kilogram packs, use exact-product coverage information
Bulk density is the mass of a given loose volume, including the way grains pack with spaces between them. Sand, quartz gravel and porous clay can differ, so one litre does not universally equal one kilogram. A weight label alone cannot tell you how many litres occupy the bag.
Use the exact product's calculator, stated coverage or reliable bulk-density information. If a maker provides kilograms per litre, multiply required litres by that value, then round to the selected bag weight. Keep the source and conditions attached; a density for one grain or material should not be transferred to another.
Seachem provides a project calculator for Flourite Black, including 7 kg and 3.5 kg bags. The selected pack below is 7 kg. Use that product-specific tool or obtain reliable coverage information rather than entering the pack weight into the litre calculator above.
A stable porous clay-gravel planting base
Seachem Flourite Black planted-aquarium gravel, 7 kg
A mineral-rich porous clay gravel intended as an integral planted-aquarium bed. Seachem describes it as uncoated and not altering pH; it is different from a pH-lowering active soil.
- 7 kg selected pack
- Porous clay gravel
- Maker states no pH alteration
A compatible freshwater planted layout with an appropriate grain texture, separate nutrition plan and gentle preparation.
- Transit dust can require rinsing and careful filling; follow the product’s preparation guidance.
- Its mineral content does not establish a complete nitrogen/phosphorus feeding plan for every plant.
If the supplier cannot establish coverage for a weight pack, retain the calculated bed litres and ask for the corresponding quantity. Guessing a universal conversion may leave a large shortfall or an unnecessary extra load on the aquarium plan.
Keep hardscape displacement and usable water separate
A rock extending into the planned bed occupies some of that envelope. Only the buried portion displaces substrate, not the entire visible rock. Estimate its effect cautiously or calculate the remaining zones; the simple calculator does not pretend to measure that irregular solid automatically.
Loose-bed litres are also not the exact litres of water lost when substrate is added. Water fills spaces between grains, so subtracting the whole 9 litre bed envelope from the aquarium's water envelope overstates solid displacement. Use the usable-water calculator with an appropriate solid-displacement estimate, and confirm measured fill volume where possible.
That distinction matters when calculating conditioner or fertiliser doses. Bed planning, actual water litres and product dose are separate calculations. Do not dose a bottle against an external tank capacity simply because the substrate estimate was precise to two decimal places.
Check the physical plan before opening the bags
Confirm the material, selected pack, preparation and layer arrangement. Leave intake clearance, stable hardscape support and access for a controlled siphon. A calculated rear slope that buries a heater or makes the filter impossible to service is a layout problem the formula cannot solve.
Keep any leftover material identified and stored according to its instructions. Review the depth after gentle filling and settlement, without repeatedly stirring an occupied bed. The calculation is a purchasing aid for an appropriate design, with assumptions visible enough to change when the design changes.
Before installing the calculated quantity, check active soil effects on water and stable rock placement. Bag volume does not establish chemistry or the support required for heavy hardscape.
Common questions
How many litres of substrate do I need for a 60 × 30 cm base?
At an illustrative flat depth of 4 cm, the calculation is 60 × 30 × 4 ÷ 1,000 = 7.2 litres. Choose the actual depth for the selected substrate system and plants rather than treating 4 cm as a universal requirement.
How do I calculate a sloping substrate bed?
For a straight even slope, average the front and rear depths and multiply by the internal footprint. Irregular mounds, terraces and separate materials need zone estimates instead; an average of two endpoints cannot describe every layout.
Can I enter a 7 kg bag as seven litres?
No. Kilograms measure mass, while the calculator uses bulk volume. Different materials pack at different densities. Use the exact product’s volume, coverage calculator or reliable bulk-density information before converting the calculated litres to weight packs.
Why does the calculator round up to another bag?
The loose-bed requirement may be larger than a whole number of the selected bags. It rounds up because a fraction of a sealed pack cannot supply the missing material. This is separate from choosing an explicit reserve for settlement or future use.
Should I subtract every litre of substrate from the tank water volume?
Do not subtract the whole loose-bed envelope as solid displacement. Water enters the gaps between grains. Keep substrate purchasing volume and usable aquarium water separate, using an appropriate solid estimate or measured fill volume for dosing.
Does the calculator tell me a safe planting depth?
No. It estimates material for the dimensions entered. Substrate-system requirements, plant roots, fish behaviour, equipment clearance and hardscape stability determine the appropriate design; correct arithmetic cannot make an unsuitable depth or unstable slope suitable.