ScapeMath

Salinity & specific gravity converter

A hydrometer reading only means something together with the water temperature and the temperature the instrument was calibrated at. This converter handles both, using the international seawater equation of state.

°C
Salinity
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SG at 25/25 °C
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SG at 60/60 °F
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Your instrument should read
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Density
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How it works

Specific gravity (SG) compares the density of your water with pure water. Warm water expands, so the same salt mix reads lower when warm. A hydrometer is therefore marked for a reference temperature, usually 60 °F (15.6 °C) for traditional instruments or 25 °C for many reef products. We use the UNESCO one-atmosphere equation of state of seawater (EOS-80, Millero & Poisson 1981):

ρ(S, t) = ρ_w(t) + A(t)·S + B(t)·S^1.5 + C·S² (valid S 0–42, t −2 to 40 °C) ρ_w(t) = 999.842594 + 6.793952e−2·t − 9.095290e−3·t² + 1.001685e−4·t³ − 1.120083e−6·t⁴ + 6.536332e−9·t⁵ A = 0.824493 − 4.0899e−3·t + 7.6438e−5·t² − 8.2467e−7·t³ + 5.3875e−9·t⁴ B = −5.72466e−3 + 1.0227e−4·t − 1.6546e−6·t², C = 4.8314e−4 SG (reading) = ρ(S, t_water) ÷ ρ_w(t_reference)

To go from a reading back to salinity, the calculator solves this equation for S numerically. Salinity here is practical salinity, which is within 0.5% of grams of salt per kilogram (ppt) for natural seawater and well-made salt mixes.

Worked examples

Natural seawater, 35 ppt

At 25 °C, ρ = 1,023.343 kg/m³ and pure water at 25 °C is 997.048 kg/m³, so SG 25/25 = 1.0264. This is the reef-keeping target you often see quoted.

A 60 °F hydrometer in a 26 °C (78.8 °F) tank

It reads 1.0230. Solving EOS-80 for 26 °C water against 15.6 °C pure water gives 33.6 ppt, which is SG 1.0253 in 25/25 terms. That's about 1.4 ppt low for a reef. For 35 ppt this hydrometer should read about 1.0240 at 26 °C.

Limits

References

  1. Millero, F.J. & Poisson, A. (1981). International one-atmosphere equation of state of seawater. Deep-Sea Research 28A: 625–629
  2. UNESCO (1981). Background papers and supporting data on the International Equation of State of Seawater 1980. Tech. Papers in Marine Science 38
  3. IOC, SCOR & IAPSO (2010). TEOS-10: The international thermodynamic equation of seawater

Formulas on this page are checked by automated unit tests against the reference values above. Last reviewed October 29, 2026. Found an error? Tell us.

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