Salinity & specific gravity calculator

How it works

Seawater gets denser as it gets saltier and lighter as it warms. The international (UNESCO 1983) equation of state gives density for any salinity and temperature. Specific gravity compares that density with pure water at a reference temperature.

density ρ(S, T) = ρ_water(T) + A(T)·S + B(T)·S^1.5 + C·S²
specific gravity = ρ(S, sample temperature) ÷ ρ_water(reference temperature)

• S is salinity in ppt (parts per thousand, grams of salt per kilogram of water) and T is temperature in °C. A, B and C are fixed coefficients from the UNESCO equation.
• Hydrometers are read against pure water at their calibration temperature, usually 60 °F (15.56 °C), even if the tank is warmer. We solve the equation backwards to find the salinity that would give your reading at your water temperature.
• Refractometers read salinity directly and compensate for temperature, so use the ppt option. The second scale on the chart shows what each salinity is as a specific gravity at 77 °F.
• Two reference points are shown so you can compare with other charts: 77 °F (25 °C), the usual reef reference, and 60 °F (15.6 °C), the usual hydrometer calibration.

Worked example

A reef tank at 82 °F, read with a 60 °F swing-arm hydrometer showing 1.021:

• The raw reading looks like a low salinity, but the water is warm, so the hydrometer floats lower than it would at 60 °F.
• Solving the equation gives a salinity of about 31.7 ppt, a specific gravity of about 1.0239 at 77 °F.
• That is 3.3 ppt below a 35 ppt target, so the tank needs salt, not less water.

Tips from the fish room

• Read at the same temperature every time. Take the reading at the tank’s normal temperature so you can compare it with last week.
• Check against a known sample. A bottle of 35 ppt calibration fluid, or a fresh batch of salt mixed to the maker’s instructions, tells you if your instrument has drifted.
• Top off, don’t re-mix. Evaporation leaves salt behind, so replace it with fresh water. Change the salt level only with new saltwater.
• Adjust slowly. Move salinity about 1 ppt a day at most, and re-test before doing it again.

FAQ

Sources

1. Algorithms for computation of fundamental properties of seawater (Technical Papers in Marine Science 44), UNESCO, Fofonoff & Millard, 1983. The international equation of state for seawater density, with published check values. Natural seawater is 35 ppt.
2. Measuring Salinity and Specific Gravity in your Aquarium, D-D The Aquarium Solution. 35 ppt reads SG 1.02637 against 25 °C water, 1.02519 against 20 °C water and 1.02435 against 15.56 °C water.

Formula and sources last checked September 30, 2026. How we test formulas.

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