Aquarium Water Change Calculator
Enter your tank volume and the share you want to change to get the exact amount of water to replace.
How much to change
Water to replace
50 L
≈ 13.2 US gal
A weekly change of 20–30% suits most stocked freshwater tanks; adjust to your own test readings. Reference information, not a substitute for testing your water.
How we calculate
We take the water volume you enter and multiply it by the percentage you choose.
For the tank's real (net) water volume — less substrate, rocks and the gap below the rim — use the Volume calculator first.
Percentages are capped at 0–100%. Results are shown in liters and approximate US gallons.
Frequently asked questions
- How much water should I change?
- For most stocked freshwater aquariums a 20–30% weekly change is a common starting point. Heavily stocked tanks or high nitrate readings may need more. Let your own water tests guide you.
- Should I use gross or net volume?
- Use the actual water volume, which is less than the tank's nominal size once substrate, rocks and the gap below the rim are subtracted. Our Volume calculator estimates it.
- How do I prepare the new water?
- Match temperature and treat tap water with a conditioner before adding it. Dosing follows the product label — this tool only sizes the volume.
- Can I share my settings?
- Yes — press Share to copy a link that reopens the calculator with the same volume and percentage.