Saltwater tool
Aquarium Salt Mix Calculator
Enter the water volume and your starting and target salinity to see how much sea salt mix to add, in grams, kilograms and US cups. If the batch is already too salty, it tells you how much water to replace with fresh water instead.
How to use the salt mix calculator
- Enter the volume of water you are mixing or correcting, in US gallons or litres.
- Set the starting salinity. For a fresh batch of RO/DI or dechlorinated water, use 1.000 SG or 0 ppt.
- Set the target salinity. Full-strength seawater is about 1.026 SG or 35 ppt. A fish-only or quarantine system may run slightly lower.
- Calculate, then add the mix gradually. Dissolve it fully in a separate container and recheck salinity before it goes into a stocked tank.
How the salt dose is calculated
The calculator assumes about 1 gram of dry reef mix per litre of water for each ppt of salinity you need to raise. At 35 ppt that works out to roughly 35 g/L, or about 132 g per US gallon. That matches the usual bucket instruction and is deliberately a starting estimate: salt brands range from about 130 to 150 g per US gallon, and humidity changes the weight of the mix in the bucket.
The result is shown in grams, kilograms and US cups. Cups assume about 272 g of dry mix per US cup. Scoop-and-level is never as repeatable as a scale, so weigh the first batch and note what your brand actually needs.
Specific gravity is converted with an assumed relationship of about 1,346 ppt per specific gravity unit, so 1.026 SG is roughly 35 ppt at 25 °C (77 °F). That is a hobby approximation, not a laboratory measurement.
Worked example
Mixing 50 US gallons (189.3 litres) of fresh water to 1.026 SG takes about 6,624 g of mix — roughly 6.62 kg, or 24.35 US cups. Start slightly under that, dissolve fully, and top up in small increments because the last thousandth of specific gravity is easy to overshoot.
When the batch overshoots
If the starting salinity is above the target, adding more mix only makes it worse. The calculator has two choices to fix it: remove a fraction of the water and replace it with fresh water at 0 ppt, or mix a weaker batch and blend the two. The water replacement shown is the direct exchange: discard that much salt water and top the container back up to the same level with RO/DI or dechlorinated fresh water.
For example, 100 litres at 40 ppt has to lose 12.5 litres (12.5% of the batch) and gain 12.5 litres of fresh water to land at 35 ppt. For a stocked tank, do that in two or three steps, let the water circulate, and recheck between them. Large sudden salinity drops stress fish and invertebrates.
What this calculator does not do
- It does not know your brand's exact dose. Use the bucket label as the final word and adjust after weighing.
- It ignores the small volume added by dissolved salt. Mix to the marked water line, not to a guessed final volume.
- It does not correct for a hydrometer or refractometer calibrated at a different temperature. Calibrate the instrument first.
- It is not a treatment for hyposalinity, disease or a tank emergency. Those need a measured protocol and a calibrated instrument.
Salinity is one number. If you are also setting up a new tank, our aquarium tank volume calculator gives you the net water volume this dose should be based on. And if the mix is right but alkalinity keeps drifting, see our guide to raising alkalinity in a reef tank.
Frequently asked questions
How much salt mix do I need per gallon of water?
At full-strength seawater the common dose is about 35 grams per litre, which is roughly 132 grams per US gallon. This calculator uses 1 gram of mix per litre per ppt of salinity at the target you enter, then converts the result to grams, kilograms and US cups. Check your bucket: brands range roughly 130 to 150 grams per US gallon, so treat the number as a starting point.
Is 1.026 specific gravity the same as 35 ppt?
On a 25 °C (77 °F) scale referenced to pure water, yes, about 35 ppt. This calculator converts specific gravity with an assumed 1.026 SG = 35 ppt relationship (about 1,346 ppt per specific gravity unit). That is an approximation for a hobby reading, not a lab measurement.
Why did my mix come out saltier than the target?
Usually one of four things: water filled above the mark, salt measured by volume instead of weight, mix added before the water was fully dissolved, or a hydrometer that has drifted out of calibration. Mix, aerate and recheck at the same temperature before adding more salt or water.
Can I add dry salt mix straight into the aquarium?
No. Premix it in a separate container with water at the same temperature, let it fully dissolve, and check salinity before it goes in. Dry mix landing on corals, fish or equipment can burn tissue and leaves undissolved residue.
How do I lower salinity when a batch overshoots?
Remove the fraction of the water shown by this calculator and replace it with fresh water at 0 ppt, such as RO/DI or dechlorinated tap water. Do the swap in steps for a stocked tank and recheck salinity after the water has circulated. Do not try to remove salt from water directly.
Does water temperature change the reading?
Yes. Hydrometers and refractometers are calibrated at a reference temperature, commonly 25 °C (77 °F). A reading taken far from that temperature can be off by a few thousandths of a specific gravity point, so let the sample reach room temperature and calibrate the instrument before trusting the number.