Specific Gravity & Salinity Converter
Created by: Liam Turner
Last updated:
Convert aquarium specific gravity, refractometer readings, and ppt salinity into one another with temperature correction and reef-safe interpretation.
Specific Gravity & Salinity Converter
AquariumConvert aquarium specific gravity, ppt salinity, and refractometer readings with temperature correction and reef-safe interpretation.
What is a Specific Gravity & Salinity Converter?
A Specific Gravity & Salinity Converter changes one saltwater reading into another. You can enter specific gravity, ppt salinity, or a refractometer reading and see the matching values in the other common formats.
This is useful because hydrometers, refractometers, and salinity targets do not all show the same kind of number. Converting between them makes it easier to mix saltwater, check your instruments, and understand whether a reading is freshwater, brackish, marine, or reef strength.
The calculator also applies a simple temperature correction so the result is easier to compare with standard reef references.
How Salinity Conversion Works
The calculator first normalizes the input into specific gravity, then applies a small temperature correction so the reading can be compared at the standard reference temperature. After that, it converts specific gravity into ppt salinity using a practical reef-hobby approximation. Refractometer reading is treated as roughly equivalent to ppt salinity when the device is properly calibrated for seawater.
SG corrected = SG measured + 0.0002 × (T°C − 25)
ppt salinity = (SG corrected − 1) × 1000 × 1.34
SG from ppt = 1 + (ppt ÷ 1340)
Refractometer reading ≈ ppt salinity at 25°C when calibrated correctly
These formulas are intended for practical aquarium interpretation rather than laboratory salinity certification, which is exactly what most saltwater hobbyists need when mixing water or checking tank stability.
Example Conversions
Example 1: Reef check. A measured specific gravity of 1.025 at 77°F converts to about 33.5 to 34.0 ppt depending on the exact conversion model. That lands very close to the usual reef target range and would generally be interpreted as acceptable for a mature reef system if the reading is stable and the instrument is calibrated correctly.
Example 2: Brackish planning. A system reading 1.010 specific gravity is nowhere near reef conditions even though it is clearly not freshwater. The converter classifies that zone as brackish and helps prevent the common mistake of treating a low marine-style density reading as adequate for corals or full-strength seawater livestock.
Example 3: Temperature correction. If a hydrometer reading is taken at a higher temperature than the standard reference, the corrected specific gravity can shift slightly upward. That small correction matters when you are near a reef threshold and want to know whether the number is truly inside the 1.025 to 1.026 target range or only appears close because of temperature effects.
Common Applications
- Converting refractometer salinity into specific gravity when following reef-care guides written in SG instead of ppt.
- Checking whether a hydrometer reading is actually reef-safe after accounting for water temperature.
- Comparing freshwater, brackish, marine, and reef salinity ranges when setting up species-specific systems.
- Cross-checking two different salinity instruments that report in different units.
- Planning mixed saltwater to a target reef value before a water change or new system fill.
- Explaining whether a reading is merely marine or truly in the reef-ideal zone for coral systems.
Tips for More Reliable Salinity Readings
Calibrate refractometers with the correct calibration fluid rather than plain water whenever possible. Rinse tools between readings, avoid dried salt residue, and compare readings at a consistent temperature. Stability is usually more important than obsessing over tiny decimal differences, but if corals are involved, being consistently near the reef target range matters enough that temperature correction and calibration should not be treated casually.
Frequently Asked Questions
How do I convert aquarium specific gravity to salinity?
A practical hobby conversion starts by correcting the measured specific gravity for temperature, then translating that corrected value into parts per thousand salinity. This calculator uses a reef-keeping approximation that is accurate enough for everyday aquarium use, especially when you want to compare hydrometer, refractometer, and ppt targets without manually switching between density tables and temperature references.
What specific gravity should a saltwater aquarium be?
Most reef aquariums run best near specific gravity 1.025 to 1.026 at the standard reference temperature of 77°F or 25°C, which corresponds to roughly 34 to 35 ppt salinity. Fish-only saltwater systems are often kept a little lower, while brackish systems run much lower still. The important part is not chasing decimals constantly but holding a stable, correctly measured salinity range.
Why does water temperature matter when reading specific gravity?
Specific gravity is temperature-sensitive because water density changes as temperature changes. A reading taken above or below the standard reference temperature needs a correction if you want to compare it fairly against reef-target values. Automatic temperature compensation helps, but it does not eliminate the need to understand that raw readings from different temperatures can look slightly different even when the real salinity is unchanged.
Is refractometer salinity the same as specific gravity?
No. A refractometer usually reports salinity in ppt or a related scale, while specific gravity is a density ratio compared with pure water. They are closely related and can be converted, but they are not the same unit. This calculator helps bridge those formats so an aquarist can move between refractometer readings, salinity targets, and specific-gravity references without relying on inconsistent forum charts.
What range counts as freshwater, brackish, marine, or reef salinity?
Freshwater is essentially near zero salinity, brackish systems often sit around 1.005 to 1.015 specific gravity, marine fish-only systems usually run around 1.020 to 1.025, and reef aquariums typically aim around 1.025 to 1.026. These are planning ranges rather than absolute laws, but they are useful for quickly classifying a reading and seeing whether a system is close to its intended habitat.
Why do hydrometers and refractometers sometimes disagree?
Hydrometers can read low if bubbles cling to the arm or if they are not calibrated well, while refractometers can drift if they are not zeroed correctly or are calibrated with the wrong standard solution. Temperature, residue, and instrument quality all matter. When the two disagree, calibration and temperature correction are usually the first things to check before assuming the water chemistry itself is wrong.
Sources and References
- NOAA seawater density and salinity reference data used as the background for practical salinity conversion.
- Milwaukee and other aquarium refractometer calibration guidance for standard 25°C reef measurement practice.
- Common reef husbandry references supporting 34 to 35 ppt and 1.025 to 1.026 as typical reef targets.
- Practical marine and brackish aquarium salinity ranges used for classification and interpretation.