Salinity Calculator — Specific Gravity to ppt for Reef Tanks

Your reading

My device shows

Reading SG

Temperature unit

Sample temperature °C Device calibration

Salinity

–

Salinity: –
Specific gravity: –
Density: –
Same water read at 25 °C: –

Log this reading in Reef Lab →

Why the temperature matters

The reason a converter that ignores it is worse than no converter at all.

Specific gravity is not a salt content. It is a ratio of two densities — your sample against pure water — and water expands when it warms. Heat the same water and its density falls, so the number on the scale falls with it, even though not a single gram of salt has left the tank.

Put a number on it. A reading of 1.0264 means 33.9 ppt if the sample was at 22 °C — and 36.3 ppt if it was at 28 °C. Same reading, same device, nearly two and a half ppt apart. That is not a rounding error; it is the difference between mixing new water that matches your tank and mixing water that shifts every animal in it during the change.

So think in ppt, not in SG

Salinity in ppt (or PSU) is a mass fraction: how much salt sits in a kilogram of water. Warming the sample does not change it. That is why Reef Lab converts every reading to ppt on the way in, does all its arithmetic there, and only converts back to specific gravity for display. This page does exactly the same thing.

Where the numbers come from

The conversion uses the International One-Atmosphere Equation of State of Seawater, 1980 — EOS-80, the same standard the printed conversion tables in salt buckets are derived from. It is valid from 0 to 40 °C and 0 to 42 ppt, which covers every case a reef tank can produce.

20/20 and 25/25 on the scale

The most commonly mis-set field in any salinity converter.

The two numbers printed on a hydrometer or refractometer are the calibration temperature: which pure water your sample gets compared against. It is a property of the instrument and does not change when your sample is warm. Almost every reef hobby device is 25/25; older lab glassware is often 20/20.

Getting it wrong shifts every result by about 0.0012 specific gravity — at 35 ppt and 25 °C the same water reads 1.0264 on a 25/25 device and 1.0252 on a 20/20 one. Small, but it stacks on top of the temperature error and always in the same direction.

SG = ρ(S, t_sample) / ρ_pure(t_calibration) Worked example: 35 ppt at 25 °C on a 25/25 device reads 1.0264. The same water at 27 °C reads 1.0257 — a third of the way from optimal to marginal, purely from two degrees.

One warning that outweighs all of the above: a refractometer calibrated with RO water instead of a 35 ppt seawater standard can be off by a full ppt on its own. Calibrate the device first, then worry about the decimals.

Target salinity for a reef tank

The bands Reef Lab rates your readings against — traffic light, no forum search.

Salinity rating, device calibrated 25/25 at 25 °C
Rating	Specific gravity	Salinity
Optimal	1.024 – 1.026	31.9 – 34.5 ppt
Marginal	1.023 – 1.027	30.5 – 35.8 ppt
Critical	< 1.021 / > 1.029	< 27.9 / > 38.5 ppt

Stability beats the exact figure. A tank held at 1.025 year-round does better than one chased between 1.024 and 1.027 — and evaporation moves salinity every single day, which is what an auto top-off is for. All parameters and their ranges →

Questions

What salinity should a reef tank be?

Natural seawater sits at 35 ppt, which reads 1.0264 specific gravity at 25 °C on a device calibrated to 25 °C. For a mixed reef tank, 1.024 to 1.026 is the optimal band, 1.023 to 1.027 still workable, and below 1.021 or above 1.029 is where it gets dangerous. What matters more than the exact number is that it stays put: a stable 1.025 beats a 1.024 that swings to 1.027 and back every week.

Why does temperature change my specific gravity reading?

Specific gravity is a density ratio, and water expands as it warms. The same salt content is therefore less dense — and reads lower — at 27 °C than at 22 °C. A reading of 1.0264 means 33.9 ppt if the sample sat at 22 °C and 36.3 ppt if it sat at 28 °C — the same number on the same device, nearly two and a half ppt apart, which is more than enough to spoil a water change. Salinity in ppt (or PSU) is a mass fraction and does not move with temperature, which is why it is the better number to think in.

What do 20/20 and 25/25 on a hydrometer mean?

That is the calibration temperature: the reference temperature of the pure water your sample's density is compared against. It is a property of the instrument, printed on the scale, and has nothing to do with how warm your sample currently is. Most reef hobby devices are 25/25; some lab glassware is 20/20. Setting it wrong in a converter shifts every result by about 0.0012 specific gravity: at 35 ppt and 25 °C the same water reads 1.0264 on a 25/25 device and 1.0252 on a 20/20 one.

Is ppt the same as PSU?

For aquarium purposes, yes. PSU (practical salinity units) comes from a conductivity measurement and ppt (parts per thousand) is a mass fraction; across the range of seawater the two agree closely enough that the difference is far smaller than any hobby test can resolve. Refractometers marked ppt and controllers reporting PSU can be read as the same scale.

Do I need to calibrate my refractometer with seawater fluid?

Yes, and it is the single biggest source of error in practice. Most cheap refractometers are built for brine, not seawater, and calibrating with RO water alone leaves a bias of up to 1 ppt at reef salinity. Calibrate with a 35 ppt seawater standard fluid at room temperature, and re-check every few months — an uncalibrated device makes every calculation on this page pointless.

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Salinity calculator for reef tanks