To use this calculator simply fill in the yellow blanks and hit the Tab key to move to the next line.
Your results are in blue

Beginning Information
Total Gallons of water in tank, sumps, Refugium, etc. (estimate)  
Cubic feet of LR and sand (Estimate)  
Current Salinity in PPT (exact) - Use chart at bottom to convert if needed PPT
Total Estimated Volume of Water in Gallons  
Calculation Information
How many gallons are you adding? (exact)  
After adding water and waiting for equlibrium what is your new Salinity? PPT
Final Information
Percent Change:  
Actual Volume in Gallons including the addition of your test water:  

Credit for calculations goes to ReefSparky from http://www.3reef.com
For additional information please see original post:
Determing The Water Volume Of Your Whole Reef System
 
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