Enter your system's water volume, its salinity before evaporation, and how much water has evaporated. The calculator shows the new salinity in ppt and specific gravity, and how much fresh RO/DI water brings it back.
The formula
Evaporation removes pure water and leaves the salt, so the salt mass is conserved:
New salinity = starting salinity × V ÷ (V − water evaporated)
Example: 50 US gal at 35 ppt loses 2 gal → 35 × 50 ÷ 48 = 36.5 ppt (SG 1.0275).
Specific gravity is converted with the same seawater density equation (EOS-80, at 25 °C) used by our salinity converter. Most reef tanks are kept at about 33.5–35 ppt (1.025–1.026 SG). To stop the creep, top off with RO/DI as the water leaves; our evaporation and auto top-off guide covers measuring evaporation and ATO failsafes.
FAQ
How much does evaporation raise salinity? Only fresh water evaporates, so the salt stays behind. New salinity = starting salinity × system volume ÷ (system volume − water evaporated). A 50-gallon system at 35 ppt that loses 2 gallons reaches about 36.5 ppt (specific gravity 1.0275).
How much RO/DI should I add back? Exactly the volume that evaporated. Adding that much fresh RO/DI water returns the system to its starting salinity. Do not top off with saltwater, which adds more salt every time.
What volume should I enter? The total water in the system: display tank plus sump at running level, minus what rock and sand displace. Not the tank's label size.