Aquarium water change calculator
Volume of water to prepare for a change, corrected for substrate and decor.
Fill in the fields and the result will appear here automatically.
Preparing replacement water starts with the actual water volume, not just the tank label. Substrate, decor and the fill line change that volume. The entered displacement is subtracted first, then your selected change percentage is applied. This helps size preparation containers. The appropriate schedule and percentage depend on the inhabitants and water quality and are not prescribed here. Conditioner dosing follows the product directions and how it is added, so replacement volume is not automatically the basis for every dose.
How it works
Formula and logic
For V litres, displaced share d% and selected change p%, net water N=V×(1−d/100), replacement W=N×p/100 and remaining water N−W. V>0, 0≤d<100, 0<p≤100. Include filter volume and the fill line in your own estimate; neither is calculated separately. This volume model prescribes no change schedule or product dose.
Example
A 240-litre tank with 12% taken by substrate holds 211.2 litres of water; a 25% change is 52.8 litres. With 100 L, no displacement and 100% change, replacement is 100 L and remainder 0; this boundary is arithmetic, not a full-change recommendation.
Fields and units
- Tank volume, l — L
- Share of water changed, % — unitless
- Share taken by substrate and decor, % — unitless
How to use
- — Enter the labelled volume of the tank in litres.
- — Enter your selected change percentage; the formula does not establish its suitability.
- — Estimate displacement and the fill line from actual water; account separately for filter water.
- — Prepare the calculated volume; dose conditioner according to product directions for your addition method.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- Seachem Prime: instructions distinguish new-water and direct-tank dosing; no universal dose MSD Veterinary Manual: change volume depends on stocking and water testing; no prescribed fixed schedule
- Limitation
- Include filter volume and the fill line in your own estimate; neither is calculated separately. This volume model prescribes no change schedule or product dose.
FAQ
What share does substrate usually take?
Measuring the water actually added is more reliable than a general percentage. Displacement depends on layer height, footprint, porosity and decor;5%,12% or 20% can be scenario assumptions, not a rule for every tank.
Why not dose conditioner by the tank label?
Follow the particular product directions. Prime, for example, distinguishes pretreating new water from adding directly to the aquarium, with different volume bases. This change-volume calculation checks neither chlorine concentration, water chemistry, product nor dosage.
How large should a routine change be?
No percentage suits every tank: inhabitants, stocking density, filtration and water tests matter. The default 25% is an example. This calculation converts a selected share into litres without assessing whether the change is suitable for your system.
Does the filter volume count?
External filter and hose water is not added separately. If V covers only the tank, those litres remain outside the model. For water treatment use the volume basis required by the directions; omitted filter water is not a guaranteed safety margin.