Why does specific gravity depend on temperature?

Water density falls as it warms, so the same salt content reads as a lower specific gravity in warmer water. A reading must be tied to a reference temperature to be meaningful. This converter standardises to specific gravity at 25C referenced to pure water at 25C, the convention most reef hydrometers use.

What salinity should a reef tank be?

Natural seawater is about 35 ppt, which is roughly 1.0264 specific gravity at 25C or about 53 mS/cm conductivity. Most reef keepers target 34 to 36 ppt. Fish-only marine tanks tolerate a slightly wider range, around 30 to 35 ppt.

How accurate are swing-arm hydrometers?

Plastic swing-arm hydrometers are convenient but drift and trap bubbles, often reading several points off. A calibrated refractometer or a conductivity probe is far more reliable. Use this converter to cross-check whichever instrument you own.

What is the difference between ppt and PSU?

Parts per thousand (ppt) is a mass fraction of dissolved salts, while the practical salinity unit (PSU) is defined from conductivity ratios. Near typical seawater values the two are numerically almost identical, so this tool treats them as interchangeable for aquarium use.

Does conductivity change with temperature too?

Yes, conductivity rises with temperature, but most aquarium conductivity meters report a temperature-compensated value referenced to 25C. The conductivity figure here is that 25C-referenced value, so read your meter in its compensated mode.

What is the Aquarium Salinity / Specific Gravity Converter?

Convert aquarium salinity between parts per thousand (ppt/PSU), specific gravity at a chosen temperature, and conductivity. Uses seawater density relations with hydrometer temperature correction for marine and reef aquarists. It runs free in your browser on Gera Tools, with nothing uploaded.

Aquarium Salinity / Specific Gravity Converter

Name: Aquarium Salinity / Specific Gravity Converter
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Marine and reef aquariums live or die by stable salinity, yet hobbyists measure it three different ways: refractometers in ppt, hydrometers in specific gravity, and probes in conductivity. This converter ties all three together and corrects specific gravity for the temperature at which you took the reading.

How it works

Salinity in ppt is the master value. Specific gravity for seawater is estimated from a density relation, and the reading is corrected from your measurement temperature back to the 25 °C reference using the thermal expansion of water:

density(S, T)  ≈ rho_pure(T) + S × (0.667 + 0.0008 × T) ... g/L scale
SG(25/25)      = density(S, 25C) / density(0, 25C)
conductivity   ≈ S × 1.5 mS/cm  (near seawater, 25C compensated)

The pure-water density curve and the salt-density slope reproduce standard seawater values closely in the aquarium range: 35 ppt gives about 1.0264 specific gravity at 25 °C and roughly 53 mS/cm.

The temperature correction problem in practice

Most plastic swing-arm hydrometers are calibrated at a specific temperature, commonly 25 °C or 60 °F. Reading one in water that is 28 °C without correction will give you a number that is off by a few points of specific gravity — enough to make a 35 ppt reef look like 32 or 33 ppt if you take the reading at face value.

The correction matters most in summer when aquarium water warms, or if you mix salt and test at room temperature before the tank has reached its target. This converter lets you enter the actual water temperature at the time of the reading so the displayed specific gravity is corrected back to the 25 °C/25 °C reference.

Instrument comparison and when to trust each

Instrument	Typical resolution	Temperature sensitivity	Reliability
Swing-arm hydrometer	~0.001 SG	High — must correct	Moderate; bubbles and salt creep cause drift
Refractometer	~1 ppt	Moderate — calibrate with correct ATC	Good if calibrated with standard solution
Conductivity probe	~0.1 mS/cm	Low — most report 25 °C-compensated	Best; consistent and easy to calibrate

If you rely on a refractometer, calibrate it with a known standard (not tap water, which does not have the same refractive index as seawater) every few weeks. A fresh bottle of calibration fluid made for reef-aquarium refractometers removes most of the reading error.

Target ranges

Reef tank (corals): 34–36 ppt, ideally 35 ppt (≈ 1.0264 SG at 25 °C, ≈ 52–54 mS/cm). Corals are more sensitive to salinity swings than most fish.
Fish-only marine (FOWLR): 30–35 ppt is generally tolerated. Slightly lower salinity marginally reduces the reproductive cycle of some external parasites.
Brackish tank: varies by species. Archerfish and figure-eight puffers do well around 10–15 ppt; full-grown monos and scats prefer higher salinities.

Checking stability over time

Single-point readings miss slow evaporation-driven salinity creep. Keep a log of readings and the date; a gradual upward trend over several weeks in a stable tank means evaporation is outpacing top-off. Use this converter to track whether your daily evaporation top-off with fresh RO water is keeping the salinity flat.