Calculator
Salinity Calculator
A hydrometer reads specific gravity, a refractometer reads ppt, a controller probe reads conductivity — three numbers for one thing, and reef advice mixes them constantly. Enter whichever your instrument shows and get the other two, plus a straight verdict on whether you're where a reef wants to be (35 ppt / SG 1.0264).
Glass hydrometer temperature correction
Swing-arm and glass hydrometers read true only at their calibration temperature (25°C / 77°F). If your tank runs warmer, the reading comes in low. Enter your SG reading and water temperature for the corrected value. Refractometers with ATC don't need this.
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What the reef target actually is
Natural seawater is 35 ppt — the same as SG 1.0264 and ~53 mS/cm at 25°C. That's the number to aim for with corals and inverts; a fish-only tank tolerates a little lower. The band worth staying inside is roughly 33–36 ppt. What matters more than the exact figure is stability — a steady 34 beats a 35 that swings, because salinity drifts up as water evaporates and drops when you top off.
Why your two instruments disagree
Swing-arm hydrometers are cheap and notoriously off — bubbles cling to the arm, and they're calibrated for one temperature. A refractometer is the reef standard, but only if you calibrate it with RO/DI or 35 ppt calibration fluid, not "close enough." If your hydrometer and refractometer disagree, trust the calibrated refractometer. This tool converts cleanly between all three, but it can't fix an uncalibrated instrument — garbage in, garbage out.
The assumptions (flagged, as always)
Conversions are linear fits anchored to natural seawater (35 ppt = SG 1.0264 = 53.0 mS/cm at 25°C) and are accurate across the reef band (~28–40 ppt); outside it, readings are flagged as extrapolated. Specific gravity everywhere here is referenced to 25°C / 77°F, the aquarium standard — a different reference temp (some lab hydrometers use 60°F) shifts SG by a few points. The temperature correction uses water's density change near 25°C (~0.00025 SG per °C) and applies to glass/swing-arm hydrometers only.
Mixing a fresh batch?
Match your saltwater to your display before it goes in — same salinity, same temperature. The salt cost calculator ranks mixes by true cost per mixed gallon, and the water change calculator works out how much to swap.
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