Saltwater Mix Calculator
Enter your water volume and target salinity to see how much marine salt mix you need, in pounds, cups, and kilograms. Perfect for new saltwater setups and routine water changes on FOWLR and reef tanks.
Use the actual water you are mixing, which is less than the tank label once rock and sand are in.
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pounds of salt
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cups (approx)
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Always mix to a meter, not to the math. Add salt to RODI water (never pour saltwater or dry salt into a stocked tank), run a powerhead and airstone for 12 to 24 hours, then confirm the reading with a refractometer. Exact salt per gallon varies by brand, so adjust the final amount to hit .
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How Much Salt You Need to Make Saltwater
Mixing your own saltwater is one of the core skills of the marine hobby, and the math is simpler than it looks. As a planning rule, most synthetic marine salt mixes need about half a pound of salt per US gallon, which is roughly half a cup, to reach a reef salinity near 35 parts per thousand (specific gravity 1.026). To hit a lower target, you simply use proportionally less salt. The calculator above scales that half-pound-per-gallon baseline to your exact volume and target, then converts it to cups and kilograms so you can measure with whatever you have on hand. A 50-gallon batch at full reef strength works out to about 25 pounds of salt.
Salinity Targets for Different Tanks
Not every saltwater tank wants the same salinity. Fish-only and fish-only-with-live-rock (FOWLR) systems do well a little lower, around specific gravity 1.020 to 1.021, which is about 28 ppt. Reef tanks with corals, clams, and sensitive invertebrates should sit at natural seawater levels, between 1.025 and 1.026, or 33.5 to 35 ppt. The reason corals are pickier is that they actively pull calcium, magnesium, and carbonates from the water to build skeletons, and those processes run best at full ocean strength. Whatever your target, stability beats perfection: a steady reading that is slightly off matters far less than a number that bounces around from week to week.
Always Mix to a Refractometer, Not to the Math
Treat the numbers from this tool as a starting estimate, not gospel. Every salt brand is formulated a little differently, so the exact grams per gallon that produce 35 ppt will vary from one bucket to the next, and even from batch to batch as a bucket settles. The only way to know your true salinity is to measure it with a calibrated refractometer. Swing-arm and floating hydrometers are notoriously unreliable, drifting several points out of true, which is more than enough to stress corals. Calibrate your refractometer with reference solution or zero-TDS RODI water, then dial the final salt amount up or down until the meter reads your target.
The Right Way to Mix a Fresh Batch
Order matters. Always add salt to RODI water, never the other way around, and never pour dry salt or concentrated saltwater directly into a stocked tank, because the local salinity and chemistry spike can burn livestock. Start with clean, zero-TDS RODI water at roughly the same temperature as your display tank. Add the salt gradually while a powerhead circulates the container, then drop in an airstone and let the batch mix and aerate for 12 to 24 hours. This dissolves everything fully, off-gasses excess carbon dioxide, and lets pH and temperature settle. Confirm the salinity and temperature one last time, then use the water for a change or a new fill.
Starting with RODI water is not optional for a healthy reef. Tap water carries chlorine, chloramine, phosphates, nitrates, copper, and silicates that feed nuisance algae and can poison invertebrates, so a good RODI filter is one of the best investments a new saltwater keeper can make. Pair it with a quality salt mix, a refractometer, and a little patience, and clean stable water becomes routine.
Keep going: dial in the rest of your saltwater system.
Frequently Asked Questions
What is the ideal salinity for a reef tank?
Most reef keepers target a specific gravity of 1.025 to 1.026, which is about 33.5 to 35 parts per thousand (ppt). That matches natural ocean water and keeps corals, clams, and invertebrates happy. Fish-only and FOWLR tanks can run a little lower, around 1.020 to 1.021 (about 28 ppt), because fish tolerate a wider range. Whatever your target, the key is keeping salinity stable, since sudden swings stress livestock far more than a steady reading slightly off the ideal.
Refractometer or hydrometer, which should I use?
Use a refractometer. Swing-arm and floating hydrometers are cheap but drift out of calibration, get fooled by bubbles, and are easily off by several points, which is enough to harm corals. A refractometer reads salinity by light refraction and, once calibrated with reference solution or RODI water, is far more accurate and repeatable. Calibrate it before each major mixing session. Treat the number this calculator gives you as a starting estimate, then always confirm the real reading with the meter.
Can I use table salt or water softener salt to make saltwater?
No. Never use table salt, kosher salt, water softener salt, or aquarium "tonic" salt to make a marine mix. Real seawater contains dozens of elements, including calcium, magnesium, carbonates, and trace minerals that corals and invertebrates need, and table salt has almost none of them. Many table salts also include anti-caking agents and iodide that can harm livestock. Always buy a proper synthetic marine salt mix made for reef or saltwater aquariums.
How long should I mix and age new saltwater before using it?
Add the salt to RODI water (never the reverse), then run a powerhead and an airstone to circulate and oxygenate it. Most mixes dissolve clear within an hour, but plan on aerating and mixing for 12 to 24 hours before use. This lets the chemistry stabilize, off-gasses excess CO2, and brings pH and temperature in line with your display tank. Always confirm salinity with a refractometer and match the temperature before adding the new water.
Why do I need RODI water instead of tap water?
Tap water carries chlorine, chloramine, phosphates, nitrates, copper, and silicates that fuel algae and can poison invertebrates. A reverse osmosis deionization (RODI) filter strips these out and gives you a clean zero-TDS base, so the only minerals in your tank are the ones from your salt mix. Starting with RODI water is one of the biggest factors in avoiding nuisance algae and keeping a reef stable. Aim for a TDS reading of 0 ppm from your RODI unit.
How much salt does it take to make saltwater?
As a planning rule, most synthetic marine salt mixes need about half a pound (roughly half a cup) per US gallon to reach a reef salinity near 35 ppt or specific gravity 1.026. A 50-gallon batch is therefore around 25 pounds of salt. Exact amounts vary by brand and by your target salinity, so this calculator scales the estimate to your numbers. Always add salt gradually, mix, then confirm and fine-tune to your refractometer.