Calculation method
Read the geometry, exact unit conversions, worked examples and practical limits behind every Aquakind calculation.
Tank geometry
For a rectangular tank, area = internal length × internal width. For a vertical cylinder, area = π × (internal diameter ÷ 2)². Volume in litres = base area in cm² × vertical depth in cm ÷ 1,000. The full internal capacity uses the tank height; gross water uses the actual water depth.
Actual water and displacement
Estimated actual water = gross volume at the waterline − entered displacement. Enter displacement for submerged substrate, rock and other contents, once. The app rejects displacement greater than the gross water volume. At an unchanged waterline, a porous substrate’s outer bulk volume is not its exact water displacement, because water can occupy pores and gaps. Measure displacement where precision matters.
A worked volume example
An internally measured 60 × 30 × 36 cm rectangular tank holds 64.8 L to the top. At 32 cm water depth, gross volume is 57.6 L. If submerged contents displace 6 L, estimated actual water is 51.6 L, or approximately 13.6313 US gal / 11.3504 imperial gal.
Substrate and slopes
For a rectangular tank with a straight, uniform front-to-back slope, average depth = (front depth + back depth) ÷ 2. Volume = footprint area × average depth. A 60 × 30 cm footprint with 3 cm front and 5 cm back depth needs 7.2 L of substrate bulk volume. Cylindrical tanks use one uniform depth. This does not account for mounds, rocks, compaction or a buying allowance.
Optional substrate mass
Mass in kg = bulk volume in L × the bulk density you enter in kg/L. A 7.2 L substrate layer at a user-provided density of 1.4 kg/L gives 10.08 kg. This density is a worked example, not a general recommendation. Moisture, grading, settling and packaging affect measured bulk density.
Water replacement and balance
Target replacement = starting actual water × your chosen percentage ÷ 100. Actual removed percentage = removed ÷ starting actual water × 100. Final water = starting water − removed + added. With 51.6 L and a chosen 25%, the target is 12.9 L. Removing and adding 12.9 L restores 51.6 L. This models one removal followed by one refill, not continuous exchange or repeated cycles.
Water-level changes
Volume change = base area × vertical level difference ÷ 1,000. A 60 × 30 cm rectangular footprint loses 1.8 L for every 1 cm drop. A 0.5 cm difference represents 0.9 L or 900 mL. The result assumes straight vertical walls and an unchanged submerged footprint. Evaporation leaves dissolved material behind; volume alone does not specify suitable refill water.
Units and numeric limits
One inch is exactly 2.54 cm. One litre is 1,000 cm³. US liquid gallons use exactly 3.785411784 L; imperial gallons use exactly 4.54609 L. Length inputs accept 0–100,000 cm (or the equivalent in inches), with positive footprints and tank heights. Amount inputs accept 0–1,000,000,000,000; a starting water volume must be positive. Percentages are 0–100. Optional bulk density is greater than zero and at most 1,000 kg/L.
Scope and precision
Calculations keep floating-point precision; the screen rounds to four decimal places, while CSV retains the underlying values. Small values show extra significant digits rather than becoming zero. These are geometric estimates, not certified capacity measurements, medicine-dose tools, fish-stock recommendations or structural load assessments. External filters, sump volumes, pipework, curved glass and horizontal cylinders are excluded.