# Know the Water You Really Have
A tank may be sold with a headline capacity that does not match the water you can actually fill. This calculator turns inside dimensions into a clearer working estimate, then removes the substrate layer before it tells you how much a chosen water change represents.# Three Tank Shapes in One Quiet Lab
- Rectangular: length multiplied by width multiplied by height.
- Cylindrical: pi multiplied by radius squared multiplied by height.
- Bow front: rectangular footprint plus a two thirds circular segment approximation.
# From Glass to Gravel to Water
The working volume is the geometric volume minus the volume occupied by the substrate layer. The water change result is that working volume multiplied by your chosen percentage. Water mass uses a rounded density of 1 kilogram per litre, so temperature, salinity and dissolved material can shift the real value slightly.Measure inside the tank
Use internal dimensions. If the substrate slopes, measure its depth at a few points and enter the average instead of relying on a product box capacity.
| Output | What it means |
|---|---|
| Gross volume | The geometric volume before substrate displacement |
| Substrate volume | The estimated space occupied by the substrate layer |
| Net water volume | The working water estimate after displacement |
| Water change | The entered percentage of the net water volume |