Saltwater Specific Gravity Calculator: Hit Your Target Salinity Exactly

What Is a Specific Gravity Calculator?

A specific gravity calculator is a dosing and dilution tool built for saltwater aquarium keepers, translating a tank’s current and target specific gravity readings into a concrete action: either how much dry synthetic sea salt mix to dissolve into the system, or how much fresh RO/DI water to add to dilute it. This saltwater specific gravity calculator uses your actual water volume alongside both readings to give a realistic estimate rather than a rough guess, which matters more in marine tanks than almost any other aquarium parameter, since corals and many invertebrates are notably sensitive to salinity swings.

Specific gravity confuses a lot of newer reef keepers because it isn’t the same thing as salinity, even though the two terms get used interchangeably in casual conversation. Specific gravity is a measurement of water density relative to pure freshwater, while salinity is a direct measurement of dissolved salt content, usually expressed in parts per thousand. Hobbyist hydrometers and refractometers almost always display specific gravity, sometimes alongside an estimated salinity conversion, which is exactly the relationship this calculator and the reference chart below are built to clarify.

This tool is built for reef tank owners dialing in a new coral system, fish-only saltwater keepers doing routine top-offs, and anyone managing a hyposalinity quarantine protocol for a sick fish. It applies to any marine system using synthetic sea salt mix and RO/DI water, and is not intended for freshwater tanks, which use an entirely different dosing approach covered by a standard aquarium salt calculator instead.

How to Use This Calculator

Getting an accurate result from this specific gravity calculator takes under a minute:

1. Enter your tank’s water volume. Use your actual water volume, not the tank’s rated capacity, since rock, sand, and equipment displace a meaningful percentage of total volume.
2. Enter your current specific gravity. Take this reading with a calibrated hydrometer or refractometer, ideally at or near 77°F (25°C) for the most accurate result.
3. Enter your target specific gravity, or tap one of the quick presets for Reef Standard, Fish-Only, or Hyposalinity to auto-fill a commonly used target.
4. Click “Calculate Adjustment Needed.” If your target is higher than your current reading, the calculator estimates the salt mix needed. If your target is lower, it estimates the RO/DI water needed to dilute the tank.
5. Make the adjustment gradually. Especially for reef tanks, spread larger corrections across several top-offs or water changes rather than a single large dose.

No sign-up or download is required — everything calculates instantly in your browser.

Specific Gravity vs. Salinity, Explained

Specific gravity (SG) is the ratio of your saltwater’s density to the density of pure freshwater at a reference temperature, which is why it’s always expressed as a decimal close to 1.000, such as 1.025. Salinity, on the other hand, measures the actual mass of dissolved salts per unit of water, typically in parts per thousand (ppt). Natural seawater sits at roughly 35 ppt salinity, which corresponds to a specific gravity of about 1.025–1.026 at 77°F. The two measurements move together predictably within the narrow range aquarium keepers actually use, which is why most hobbyist references, including the chart below, provide a direct conversion between them.

The practical reason this distinction matters is that most affordable hobbyist tools, whether a swing-arm hydrometer or a handheld refractometer, are calibrated to display specific gravity, not salinity directly. Understanding the relationship helps you cross-check a manufacturer’s salt-mix instructions, which are sometimes given in ppt, against your own SG-based readings.

Target Specific Gravity Ranges

Most experienced reef keepers pick one consistent target within the reef-standard range and hold it steady rather than chasing exact natural seawater numbers, since stability matters more to coral and invertebrate health than hitting a precise figure.

Specific Gravity to Salinity Conversion Chart

These approximate conversions, referenced at 77°F, are commonly used across the reefing hobby to translate a hydrometer or refractometer reading into a parts-per-thousand salinity figure:

For a deeper technical background on how salinity is defined and measured in natural bodies of water, see the overview of seawater composition and salinity.

Hydrometer vs. Refractometer

Two instruments dominate the hobby for measuring specific gravity. Swing-arm and floating box hydrometers are inexpensive and easy to use, but they’re prone to calibration drift, trapped air bubbles, and parallax reading error, which can throw a reading off by a meaningful margin. Optical and digital refractometers, calibrated periodically with a reference fluid, are generally more precise and more repeatable, which is why most experienced reef keepers eventually switch to one as their primary tool. Whichever instrument you use, calibrating it regularly against a known reference solution and rinsing it with RO/DI water between uses meaningfully improves reading accuracy.

How Temperature Affects Readings

Specific gravity is temperature-dependent because water density itself changes with temperature. Most hobbyist hydrometers and refractometers are factory-calibrated to read accurately at 77°F (25°C), which is also the standard reference temperature used throughout this guide. Measuring noticeably above or below that calibration point, without a temperature-compensating instrument, can shift your reading enough to matter for sensitive reef systems. The most reliable approach is simple: let your sample reach close to 77°F before testing, or use a refractometer with built-in automatic temperature compensation (ATC) if your tank commonly runs warmer or cooler than that benchmark.

Worked Adjustment Examples

Applying the math to real tank scenarios makes the concept easier to use with confidence.

Enter your own tank’s numbers into the calculator above to get amounts tailored to your exact volume and readings.

Hyposalinity: What It Is and When to Use It

Hyposalinity is a well-established treatment method that lowers specific gravity to roughly 1.009–1.012 to disrupt the reproductive cycle of marine parasites such as Cryptocaryon irritans, commonly called marine ich. Because most invertebrates, including corals, clams, and many crustaceans, cannot tolerate salinity that low, hyposalinity is always carried out in a separate, bare-bottom quarantine or hospital tank rather than the main display. The transition into and back out of hyposalinity should happen gradually, typically over 24–48 hours in each direction, and salinity should be held at the low target for a minimum of several weeks to fully break the parasite’s life cycle before slowly restoring normal salinity.

Evaporation, Top-Offs, and Salinity Stability

Evaporation is the single biggest day-to-day driver of specific gravity drift in a marine tank. When water evaporates, only the H2O leaves the system — the dissolved salt stays behind, which means specific gravity rises steadily between top-offs unless it’s corrected with plain freshwater. This is exactly why the golden rule of reef keeping is to always top off evaporated water with RO/DI freshwater, never with pre-mixed saltwater; adding saltwater to replace evaporation only concentrates the salt further and pushes specific gravity higher over time, sometimes without the hobbyist noticing until livestock starts showing stress.

Many experienced reef keepers install an automatic top-off (ATO) system paired with a small reservoir of RO/DI water, which keeps evaporation losses replaced continuously rather than in periodic manual doses. This approach smooths out day-to-day specific gravity swings considerably, since even a conscientious manual top-off routine still lets salinity drift up between refills, then drop back down each time freshwater is added. For tanks without an ATO, checking specific gravity two to three times a week and topping off consistently at the same time of day is the next best habit to build.

Specific Gravity When Adding New Fish or Corals

New livestock is one of the more common reasons a tank’s specific gravity gets accidentally shifted. Bagged fish and corals from a local fish store or online shipment often arrive at a specific gravity different from your display tank, sometimes by a meaningful margin if the source system runs its salinity differently than yours. Drip acclimation, which slowly mixes display tank water into the shipping bag over 30 to 60 minutes, is the standard method for closing that gap gradually rather than dumping a bag of mismatched-salinity water directly into the tank. This matters especially for invertebrates and corals, which tend to be considerably less tolerant of a sudden specific gravity change than most hardy fish.

If you’re unsure what specific gravity your supplier keeps their systems at, a quick check with a hydrometer or refractometer on the bag water before acclimating gives you a clear starting point, and lets you judge how long a drip acclimation session should run based on how large the gap is.

Common Specific Gravity Mistakes

• 1Topping off with saltwater instead of freshwater. Only water evaporates, not salt, so top-offs should always use plain RO/DI freshwater to keep salinity stable.
• 2Making large, sudden adjustments. Big swings in either direction stress fish and corals; spread bigger corrections across multiple top-offs or water changes.
• 3Never calibrating a hydrometer or refractometer. Instruments drift over time; check yours periodically against a reference calibration fluid.
• 4Testing at inconsistent temperatures. Let samples reach close to 77°F before reading, or use a temperature-compensated instrument.
• 5Running hyposalinity in a display tank. Always isolate hyposalinity treatment to a dedicated quarantine tank away from corals and invertebrates.

Specific gravity is one of the more forgiving reef parameters once you understand it, but it punishes rushed corrections more than almost any other water quality metric. A slow, deliberate approach to raising, lowering, or holding salinity steady will consistently protect fish, corals, and invertebrates better than trying to hit a perfect number in a single step.

Related Fish & Pet Calculators

If this saltwater specific gravity calculator was useful, a few companion tools are worth bookmarking alongside it. Before adding new fish to a marine system, checking stocking capacity with the Fish Inch Per Gallon Calculator helps avoid overcrowding a tank you’ve just carefully dialed in for salinity. For households balancing a saltwater tank alongside other pets, the Pet Alliance Calorie Calculator keeps a dog or cat’s diet on track, and the Dog Age Calculator is a quick way to translate your dog’s age into human years. Explore the full library of free tools on the Pet Calculator Hub.

Frequently Asked Questions