⚠️ Live animals — read first
- Salinity changes must be gradual: never shift livestock more than ~2 ppt per day. Sudden swings kill faster than a slightly-off value.
- The hyposalinity tab is general treatment information, not veterinary advice. It is for fish-only hospital tanks — it kills invertebrates and corals.
- Salt-mix amounts are estimates (brands vary 230–300 g per cup). Always verify with a calibrated refractometer before adding livestock.
4-Way Salinity Converter
Enter a value in any unit at your water temperature — get all four back, computed with the UNESCO equation of state (Millero & Poisson 1981) and PSS-78 conductivity scale.
Results
Hydrometer / Refractometer Temperature Correction
A hydrometer calibrated at 25°C that reads 1.023 in 28°C water is not measuring 1.023 worth of salt — warm water is less dense, so the float sits lower. Enter your reading and water temperature to get the true salinity.
True salinity
Salt-Mix Dosing Calculator
How much dry salt mix to add — for mixing fresh saltwater or adjusting an existing tank. Brand-agnostic math: 35 g of mix per liter of water ≈ 35 ppt (≈ ½ cup per US gallon).
Dosing result
Mix with a pump/powerhead for 24 h, match temperature, then verify with a refractometer before use. Cup weight varies by brand (230–300 g); grams are the reliable number.
Target Salinity by Livestock
Common target ranges at 25°C (77°F). Ranges, not commandments — stability beats chasing a perfect number.
| Livestock | ppt | Specific gravity | Conductivity (mS/cm) | Notes |
|---|
Hyposalinity Treatment Planner
Plan: lower to SG 1.009 (≈ 12 ppt at 25°C) over 48 hours, hold 4–6 weeks, then raise slowly (≤ 2 ppt/day).
Treatment plan
| Step | After | Target ppt | Target SG @25°C |
|---|
How the math works
This converter implements two published oceanographic standards, client-side:
- Density & specific gravity — UNESCO Equation of State 1980 (Millero & Poisson 1981):
ρ(S,t,0) = ρ_w(t) + A(t)·S + B(t)·S3/2 + C·S². Aquarium SG = ρ(sample at t) / ρ(pure water at 25°C), matching reef reference tables: 35 ppt @25°C → SG 1.0264. - Conductivity ↔ salinity — Practical Salinity Scale 1978 (PSS-78): defining points 42.914 mS/cm @35 ppt/15°C and 53.088 mS/cm @35 ppt/25°C (the polynomial fit carries a documented ~0.04% residual at 25°C — far below hobby probe accuracy).
- Hydrometer correction — a hydrometer calibrated at Tcal reads ρ(sample at T)/ρ(pure water at Tcal); the true salinity is found by inverting the UNESCO equation at your water temperature.
Frequently asked questions
How do I convert salinity from ppt to specific gravity?
Divide-and-conquer is unnecessary: at 25°C (77°F), 35 ppt of seawater has a specific gravity of 1.0264, and the salinity to specific gravity relationship is nearly linear at about 0.00075 SG per ppt — so 30 ppt ≈ 1.0226 and 27 ppt ≈ 1.0203. But temperature shifts the reading, so use the converter above: it applies the UNESCO equation of state at your actual water temperature instead of a rough rule of thumb.
How do I convert specific gravity to ppt?
Specific gravity alone is ambiguous without temperature, because warm water is less dense. The aquarium convention is SG = density of your sample at its temperature divided by the density of pure water at 25°C. At 25°C, the SG to ppt shortcut is roughly (SG − 1) / 0.000754 = ppt — so SG 1.023 ≈ 30.5 ppt and SG 1.020 ≈ 26.5 ppt. The converter above inverts the UNESCO equation numerically at your exact temperature, which is more accurate than the linear shortcut.
How do I convert conductivity (mS/cm) to salinity in ppt?
Conductivity probes measure in mS/cm, and the Practical Salinity Scale 1978 (PSS-78) defines the mS/cm to ppt conversion: 42.914 mS/cm at 15°C and 53.088 mS/cm at 25°C both equal exactly 35 ppt. Because conductivity rises about 2% per °C, you must know the water temperature — a reading of 50 mS/cm means ~33 ppt at 25°C but ~37 ppt at 20°C. Enter your probe reading and temperature in the converter for the PSS-78 computation.
What is the ideal salinity for a reef tank?
The ideal salinity for a reef tank is 35 ppt, which reads as specific gravity 1.0264 (about 53.1 mS/cm) at 25°C / 77°F — that is natural seawater, and corals and invertebrates evolved in it. Most successful reef keepers hold reef tank salinity at 34–36 ppt (1.025–1.027). Stability matters more than hitting exactly 35.0: a rock-steady 34 ppt beats a 35 ppt that swings with every top-off.
My hydrometer reads 1.023 but my tank is 28°C — what is my real salinity?
About 31.8 ppt (SG 1.0239 at 25°C) — your water is saltier than the hydrometer suggests. Warm water is less dense, so a hydrometer calibrated at 25°C floats lower in 28°C water and under-reads. The error is roughly 0.001 SG per 3°C. Use the Hydrometer tab above: enter the reading, your water temperature, and the device's calibration temperature for the corrected value.
Do I need to calibrate my refractometer, and how?
Yes — refractometer calibration is simple but essential, and don't do it with RO water. Zeroing a refractometer with RO/DI only checks the 0 point; the scale slope can still be off. Calibrate with 35 ppt (53 mS/cm) calibration solution, which pins the reading exactly where reef salinity lives. Recheck monthly and after any knock. ATC (automatic temperature compensation) models handle temperature for you; without ATC, let the sample sit to room temperature first.
How much salt mix do I need for my reef tank?
The universal mixing rule is 35 grams of salt mix per liter of water for 35 ppt — about half a US cup per gallon. So a 100-liter batch needs ~3.5 kg (≈13 cups), and a 20-gallon batch needs ~10 cups. Brands vary (one cup weighs 230–300 g depending on the mix), so weigh in grams when you can, mix with a pump for 24 hours, match temperature, and confirm with a calibrated refractometer before use. The Salt Mix tab above does this math for any volume and target.
What is hyposalinity treatment and how does it work?
Hyposalinity is a treatment for marine ich (Cryptocaryon irritans): lowering salinity to SG 1.009 (≈12 ppt) for 4–6 weeks disrupts the parasite's life cycle while most marine fish tolerate it. It must be done in a bare fish-only hospital tank — the same salinity kills invertebrates, corals, and live-rock fauna. Lower gradually over 48 hours, hold 4–6 weeks, then raise no faster than 2 ppt per day. Monitor pH and alkalinity closely, since both drop at low salinity. This is general hobby information, not veterinary advice — when in doubt, consult an aquatic veterinarian.
What is the difference between ppt, specific gravity, and PSU?
They all describe how salty the water is. ppt (parts per thousand) is grams of dissolved salt per kilogram of seawater — the most unambiguous unit. Specific gravity is the density ratio versus pure water (aquarium convention: versus pure water at 25°C), so 35 ppt = SG 1.0264. PSU (practical salinity units) is the dimensionless number from the conductivity-based Practical Salinity Scale — for hobby purposes, 35 PSU = 35 ppt. Use ppt when precision matters; SG when reading a hydrometer.
Why does temperature change my salinity reading?
Temperature doesn't change how much salt is in the water — it changes the water's density and conductivity, which is what your instruments actually measure. Warm water is less dense (hydrometer floats lower → reads low) and conducts better (probe reads high). A swing-arm hydrometer calibrated at 25°C under-reads by about 0.001 SG for every 3°C above calibration. True salinity in ppt is temperature-independent; the reading is not. That's why every conversion on this page asks for temperature.
Are swing-arm hydrometers accurate?
Approximately — typically ±0.001 to ±0.002 SG even after temperature correction, and they drift as bubbles or deposits collect on the arm. That's acceptable for fish-only systems but marginal for reef tanks, where 0.002 SG is ~2.6 ppt. Tap out bubbles, rinse in freshwater after use, and replace yearly. For reef, a refractometer calibrated with 35 ppt solution is the hobby gold standard.
What salinity should a FOWLR or fish-only tank be?
Fish-only with live rock (FOWLR) is commonly kept at 30–33 ppt (SG 1.0226–1.0249), and fish-only systems without invertebrates often run 27–30 ppt (SG 1.0203–1.0226). Slightly lower salinity eases the fish's osmoregulation workload and gives more margin against salinity creep from evaporation. Stay above ~27 ppt — below that you lose the buffering chemistry of full seawater.
What salinity do brackish aquariums need?
Brackish means between freshwater and seawater: roughly 5–15 ppt (SG 1.0037–1.0113 at 25°C), but the right number is species-dependent — figure-8 puffers, monos, scats, and archerfish all prefer different points in that range. Research your species rather than aiming mid-range blindly, and acclimate slowly since many brackish fish move between salinities in the wild.
How do I lower aquarium salinity safely?
Add fresh RO/DI water — slowly. For small corrections, top off with freshwater instead of saltwater and re-test. For large drops (e.g., 35 to 12 ppt for hyposalinity), do it over 48 hours in steps, because a sudden drop shocks fish osmotically. Never change livestock salinity faster than about 2 ppt per day. The Salt Mix tab calculates exactly how many liters of RO/DI to add for any target.
Can I use this converter for freshwater or brackish water?
Yes for brackish (5–15 ppt) — the UNESCO equation covers 0–42 ppt. For true freshwater the conversions are trivially near 1.000 SG and 0 ppt, and conductivity no longer tracks salinity (it tracks whatever ions happen to be present), so a conductivity reading can't be converted to ppt meaningfully below ~2 ppt. This tool is built for marine and brackish ranges.
Is 1.025 or 1.026 the 'correct' specific gravity for a reef tank?
Both are fine — 1.025 is ~33.7 ppt and 1.026 is ~35 ppt at 25°C, and the entire 1.025–1.027 band (34–36 ppt) is natural seawater range. The debate is mostly noise: pick 1.026 (35 ppt) as your target and focus on keeping it stable. Daily evaporation raises salinity (salt doesn't evaporate), so an auto-top-off with RO/DI water does more for your reef than arguing over the third decimal.