Saltwater Aquarium Salinity Calculator
Type a salinity reading in any unit and instantly see it in all others. Then switch to the adjustment tab to find out exactly how many grams of salt to add, or how much freshwater to put in, to hit your target salinity. Nothing uploaded.
Convert between salinity units
Edit any field and the others update instantly.
Temperature correction ?
Adjustment calculator
Enter your current tank salinity and your target to see how much salt to add - or how much freshwater to put in.
Water change salt calculator
How much salt do you need to mix into your replacement water batch? Enter the volume you are changing and the target salinity for the fresh batch.
Learn more: Salinity measurement and aquarium chemistry
Why salinity measurement is confusing for reef keepers
Three different units are in common use in the hobby. SG (specific gravity) is read by swing-arm hydrometers. PSU or ppt (parts per thousand) is the scientific unit and what most optical refractometers display. Some digital refractometers display both. New hobbyists regularly confuse a reading of "35" (ppt) with "1.035" (SG), which is dangerously high seawater concentration.
How to use the calculator
Type your reading in SG, PSU, or refractometer ppt and the other two update instantly. Use the preset buttons for standard fish-only or reef targets. Switch to the adjustment tab to enter your tank volume and current/target SG to find out how many grams of salt to add or how much freshwater to dilute. The water change tab calculates mixing for replacement water.
FAQ
What SG should a reef tank be?
Most reef aquarists target 1.025-1.026 SG (approximately 34-35 PSU), which matches natural tropical seawater. Stability is more important than the exact value - swings of more than 0.001 SG per day can stress corals and fish.
How do I raise salinity in a saltwater tank?
Calculate the grams of salt needed: multiply your tank volume in litres by the difference in PSU between current and target. For example, to raise a 100 L tank from 30 PSU to 35 PSU, add (35-30) x 100 = 500 g of salt. Dissolve it in a bucket of tank water before adding it to avoid localised high salinity spots near corals.
What is the difference between SG and ppt in saltwater?
SG (specific gravity) measures how dense the seawater is relative to pure water. PSU/ppt measures the mass of dissolved salt per kilogram of water. They are closely related: SG 1.025 = approximately 33 PSU at 25°C. Most refractometers read in ppt, while hydrometers often read in SG.