Saltwater Mix Calculator

How much salt mix a batch of RO/DI needs to reach reef salinity — for a new tank, or for the fraction of the tank you are replacing this week.

Last reviewed August 2026 · Free · Nothing you enter is stored

Weigh the salt, then check the water

Mixing saltwater is one of the few tasks in the hobby with a genuinely correct answer, because salinity is measurable and the target is agreed on. Reef systems are run at a specific gravity of 1.025, which is a salinity of 35 parts per thousand and a conductivity of roughly 53 mS/cm at 25 °C. Those are three descriptions of the same water, so hitting any one of them means you have hit the others.

What varies is how much salt it takes to get there, because mixes are not identical in composition or in how densely they pack. Weight is the reliable unit. Volume is convenient but unreliable, since a cup of salt that has settled in the bottom of the bucket contains appreciably more than a cup scooped from a freshly opened one. If you have a kitchen scale, use it.

Red Sea is unusually specific about this and publishes a ratio table for Coral Pro, which is worth reproducing because it shows how small the increments are.

Red Sea Coral Pro ratios

Two tenths of a pound across a five gallon batch is the whole distance between 1.021 and 1.025. That is why guessing by eye does not work and why the last step of mixing is always a measurement rather than a calculation.

Instant Ocean is the other salt most people start with, and its package instructions target a specific gravity of 1.022 — below reef salinity. This is why keepers routinely mix it heavier than the label says. The heavier ratio circulated in the hobby is hobbyist-derived and is not an official Instant Ocean specification, so treat it as a starting point and confirm it with your own reading rather than trusting either number blindly.

Red Sea publishes an explicit table for Coral Pro salt: 1.6 lb per 5 gallons reaches a specific gravity of 1.025 at 35 ppt. Instant Ocean's package instructions target 1.022 rather than reef salinity, which is why keepers routinely mix it heavier than the label says. Flow of twenty to forty times tank volume per hour is Bulk Reef Supply's guidance, with soft corals at the bottom of that range and SPS at the top. The one to 1.5 pounds of rock per gallon rule dates from the live rock era; modern porous dry rock does the same biological work with less mass, and open aquascapes are now generally preferred to packed walls of rock.

Measuring salinity without fooling yourself

A refractometer with automatic temperature compensation is the practical standard. It is quick, it needs only a few drops, and calibrating it against pure water or a purchased calibration fluid takes a minute. Everything else in common use has a caveat attached.

Swing-arm hydrometers are the cheapest option and the least accurate instrument most people own. Bubbles clinging to the arm, salt residue from the previous reading and temperature all move the result, and the error does not repeat consistently enough to be corrected. Glass hydrometers are better, but they are calibrated at 77 °F, so a reading taken on water at any other temperature has to be corrected before it means anything at all.

Whichever tool you use, the discipline is the same: measure the finished batch before it goes anywhere, and measure the display tank on a regular schedule so you notice drift while it is still small.

Mix in a bucket, never in the display

Salt does not dissolve instantly. On the way to fully dissolved the water passes through a period of elevated pH and unsettled chemistry, and undissolved grains sitting on a coral do real damage. So the method is fixed. Mix in a separate container with a powerhead running for several hours, bring the batch up to tank temperature with a heater, confirm the salinity, and only then move it into the system. Never mix salt in a tank that has anything living in it.

Water changes on a reef are usually smaller and more frequent than on a freshwater tank. Ten percent weekly or thirty-five percent monthly is the common default, and either one is defensible. The point of a reef water change is as much about replacing consumed trace elements as it is about exporting nitrate, which is why the schedule tends to be steady rather than reactive.

Top-off is the opposite problem

Evaporation removes pure water and leaves every gram of salt behind, so the salinity of what remains goes up. The fix is to replace what evaporated with fresh RO/DI and nothing else. Topping off with saltwater adds salt that was never lost, and doing it daily drives salinity up relentlessly until corals close and fish start behaving oddly for reasons the owner traces back to everything except the top-off jug.

A working figure for evaporation is around 1.25 percent of system volume per day, which is what this calculator shows. Treat it as illustrative rather than predictive: evaporation scales with surface area and airflow, not with volume, so an open-top tank under a strong light in a dry room loses far more than a covered tank of the same size in a humid one. Measure your own over a few days and you will have a number worth automating around.

Rock, and the rule that outlived its era

The traditional figure is one to one and a half pounds of rock per gallon, and it is still quoted everywhere. It is worth saying plainly that it dates from the live rock era, when rock was the filter and its mass was a reasonable proxy for how much bacteria the system could host. Modern porous dry rock does the same biological work with less mass, and open aquascapes with room for flow and for fish to swim are now generally preferred to the packed walls of rock that the old rule produced. Use it as an upper bound if you use it at all.

Reef parameter targets

Salinity is the parameter you set when mixing. These are the others worth watching once the tank is running, along with what you need to read them.

Alkalinity of 8 to 9 dKH, calcium around 420 ppm and magnesium around 1350 ppm are the numbers most reef keepers dose towards. The more useful lesson, though, is that stability matters more than hitting any exact figure. A tank held steadily at 7.5 dKH does better than one swinging between 7 and 10 in pursuit of 8.5, and most of the visible trouble people attribute to a wrong number is actually the result of a fast change in it.

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