Refractometer or hydrometer?

Refractometer, every time. A $25 ATC refractometer beats every glass swing-needle hydrometer made. Calibrate with 35 ppt standard solution monthly — RO/DI water calibration is the most common mistake and reads 0.001–0.002 SG low across the board.

What's the right reef salinity?

35 ppt / 1.0264 SG / 53 mS/cm @ 25 °C. Anywhere from 33–36 ppt is fine for most reefs as long as it's stable. Drift down to 30 stresses livestock; climb past 37 starts impacting metabolism.

Why does conductivity scale linearly here but not at fresh-water levels?

The salinity-to-conductivity relationship is linear only in the narrow range hobbyists care about (25–45 ppt). The PSS-78 standard equation is nonlinear and accounts for ion ratios, but in reef tanks the linear approximation is accurate to within 0.2 ppt — close enough for tank maintenance.

Calculator

Salinity Converter (SG ↔ ppt ↔ mS/cm)

Three instruments, three units, one quantity. Refractometers read SG (or salinity in ppt if multi-scale). Old swing-needle hydrometers read SG. Aquarium controllers and probes read conductivity in mS/cm. Natural reef seawater sits at 35 ppt ≈ 1.0264 SG @ 25 °C ≈ 53 mS/cm. This converter handles all four with a temperature-correction option for hydrometers calibrated at temperatures different from your tank.

Salinity converter

I have a reading in

Value

Instrument calibration temp

Sample water temperature (°F)

Only relevant when correcting an SG reading.

Correct SG for sample temperature (glass hydrometers only — refractometers correct internally if ATC-equipped)

Salinity

34.47ppt

Specific gravity

1.0260SG

Conductivity

52.20mS/cm

Reef target: 35 ppt = 1.0264 SG @ 25 °C = ~53 mS/cm. Refractometers should be calibrated with 35 ppt standard, not RO/DI water.

How this is calculated

Anchor (Randy):
  S = 35  →  SG = 1.0278 @ 4 °C
             SG = 1.0269 @ 60 °F (15.6 °C)
             SG = 1.0266 @ 20 °C
             SG = 1.0264 @ 25 °C  // modern refractometer standard
             conductivity = 53 mS/cm @ 25 °C

Linear approximation, S = 25–40:
  S       = 35 × (SG − 1) / (SG@35 − 1)
  cond    = 53 × S / 35

Temperature correction for SG (hydrometers):
  trueSG  = reportedSG × (ρ_water @ calib_T / ρ_water @ sample_T)

The temperature correction matters more than people realize. Randy's canonical example: a 1.023 reading on a 60 °F-calibrated hydrometer at a tank temperature of 86 °F corrects to about 1.027. That's the difference between "I need to top off salt" and "I'm above NSW already". Refractometers with ATC (automatic temperature compensation) handle this internally; cheap eBay refractometers usually don't.

FAQ

Refractometer or hydrometer?: Refractometer, every time. A $25 ATC refractometer beats every glass swing-needle hydrometer made. Calibrate with 35 ppt standard solution monthly — RO/DI water calibration is the most common mistake and reads 0.001–0.002 SG low across the board.
What's the right reef salinity?: 35 ppt / 1.0264 SG / 53 mS/cm @ 25 °C. Anywhere from 33–36 ppt is fine for most reefs as long as it's stable. Drift down to 30 stresses livestock; climb past 37 starts impacting metabolism.
Why does conductivity scale linearly here but not at fresh-water levels?: The salinity-to-conductivity relationship is linear only in the narrow range hobbyists care about (25–45 ppt). The PSS-78 standard equation is nonlinear and accounts for ion ratios, but in reef tanks the linear approximation is accurate to within 0.2 ppt — close enough for tank maintenance.

Share / embed this calculator

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<iframe
  src="https://reefcalcs.com/embed/salinity/"
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Sources & references

01
Randy Holmes-Farley — Specific Gravity: Oh How Complicated
https://reefs.com/magazine/chemistry-and-the-aquarium-specific-gravity-oh-how-complicated/
02
Randy Holmes-Farley — Optimal Parameters
https://www.reef2reef.com/threads/optimal-parameters-for-a-coral-reef-aquarium-by-randy-holmes-farley.173563/
03
Salinometry — Practical Salinity Scale 1978 (PSS-78)
https://salinometry.com/pss-78/
04
Global Seafood Alliance — Typical chemical characteristics of seawater
https://www.globalseafood.org/advocate/typical-chemical-characteristics-of-full-strength-seawater/

Last reviewed 2026-05-25