Specific gravity to ppt: salinity conversion chart for reef tanks
Specific gravity and ppt measure the same thing, how salty your water is, in different ways. Converting between them depends on the water temperature and on the temperature your instrument was calibrated at, so the charts below show each common case.
By ScapeMath · Updated · Every number on this page is computed with the same formulas as our calculators.
Want the exact figure for your tank? Enter any reading, your water temperature and your instrument's reference temperature, and the converter gives ppt and what your hydrometer should read. Open the free salinity converter.
Quick answer
Natural seawater, the usual reef target, is 35 ppt = SG 1.0264 on the 25/25 °C scale. SG 1.025 is 33.2 ppt and SG 1.026 is 34.5 ppt on that scale. If you use a traditional glass or swing-arm hydrometer calibrated at 60 °F, read the second or third column of the chart instead.
Specific gravity to ppt chart
| SG reading | ppt, 25/25 °C scale | ppt, 60 °F hydrometer in 77 °F water | ppt, 60 °F hydrometer in 80 °F water |
|---|---|---|---|
| 1.018 | 23.9 | 26.6 | 27.3 |
| 1.019 | 25.2 | 27.9 | 28.6 |
| 1.020 | 26.6 | 29.2 | 29.9 |
| 1.021 | 27.9 | 30.6 | 31.2 |
| 1.022 | 29.2 | 31.9 | 32.6 |
| 1.023 | 30.5 | 33.2 | 33.9 |
| 1.024 | 31.9 | 34.5 | 35.2 |
| 1.025 | 33.2 | 35.9 | 36.5 |
| 1.026 | 34.5 | 37.2 | 37.9 |
| 1.027 | 35.8 | 38.5 | 39.2 |
| 1.028 | 37.1 | 39.8 | 40.5 |
Computed with the UNESCO EOS-80 equation of state of seawater, ignoring the instrument's own thermal expansion. The equation is valid to 42 ppt.
ppt to specific gravity chart
| Salinity | SG 25/25 °C | SG 60/60 °F | 60 °F hydrometer should read at 77 °F | Salts per litre |
|---|---|---|---|---|
| 30 ppt | 1.0226 | 1.0230 | 1.0206 | 31 g |
| 32 ppt | 1.0241 | 1.0245 | 1.0221 | 33 g |
| 33 ppt | 1.0249 | 1.0253 | 1.0228 | 34 g |
| 34 ppt | 1.0256 | 1.0261 | 1.0236 | 35 g |
| 35 ppt | 1.0264 | 1.0269 | 1.0244 | 36 g |
| 36 ppt | 1.0271 | 1.0276 | 1.0251 | 37 g |
Why one SG reading can mean different salinities
Specific gravity is the density of your water divided by the density of pure water. Warm water expands, so the same salt mix reads lower when warm. A hydrometer is marked for one reference temperature: 60 °F (15.6 °C) for most traditional glass and swing-arm hydrometers, or 25 °C for many reef products. Read one away from its reference temperature and the number shifts. That's why 1.025 on a 60 °F hydrometer in a 77 °F tank is 35.9 ppt, while 1.025 on the 25/25 scale is only 33.2 ppt.
Salinity in ppt (grams of salt per kilogram of seawater) doesn't depend on temperature, which is why many reef keepers prefer it and why conductivity probes and refractometers often show ppt directly.
Refractometer, hydrometer or probe?
- Refractometer: measures how the water bends light, not density. Models with automatic temperature compensation (ATC) handle temperature for you. Calibrate with a 35 ppt seawater standard, not just RO water, so it's right where it matters.
- Swing-arm or glass hydrometer: cheap and fine for fish-only tanks, but check which reference temperature it uses and read it at the tank temperature it was designed for, or convert with the chart above.
- Conductivity probe: 35 ppt seawater is about 53 mS/cm. Probes need regular calibration too.
Raising or lowering salinity
To lower salinity, replace some tank water with fresh RO/DI water; to raise it, replace some with saltwater mixed a little higher. The amount follows the same mixing maths as a water change, so the water change calculator works it out if you enter ppt in each box. Change it gradually over several days rather than all at once, because corals and invertebrates are sensitive to fast swings.
Common questions
What is 1.026 specific gravity in ppt?
On the 25/25 °C scale used by many reef tables and converters, SG 1.026 is 34.5 ppt. The usual reef target of 35 ppt is SG 1.0264, so 1.026 is just under it.
What is 1.025 in ppt?
33.2 ppt on the 25/25 °C scale, a little low for a reef. On a glass hydrometer calibrated at 60 °F and read in 25 °C (77 °F) water, the same 1.025 reading means 35.9 ppt.
What is 35 ppt in specific gravity?
1.0264 on the 25/25 °C scale and 1.0269 on the 60/60 °F scale. A 60 °F hydrometer floating in 25 °C water should read about 1.0244.
What salinity should a reef tank be?
Most reef keepers aim for natural seawater: 35 ppt, which is SG 1.0264 (often rounded to 1.026). Keeping it steady matters more than the last decimal, so mix new salt water to the same reading every time.
What is 1.020 specific gravity in ppt?
26.6 ppt on the 25/25 °C scale, well below natural seawater. On a 60/60 °F hydrometer read at 25 °C it's 29.2 ppt.
Is ppt the same as psu or grams per litre?
ppt and psu (practical salinity units) are within about 0.5% of each other for seawater, so hobbyists treat them as the same. Grams per litre is a little higher: 35 ppt seawater at 25 °C holds about 35.8 g of salts per litre, because a litre of it weighs 1.02 kg.
References
- Millero, F.J. & Poisson, A. (1981). International one-atmosphere equation of state of seawater. Deep-Sea Research 28A: 625–629
- UNESCO (1981). Background papers and supporting data on the International Equation of State of Seawater 1980. Tech. Papers in Marine Science 38
- IOC, SCOR & IAPSO (2010). TEOS-10: The international thermodynamic equation of seawater (practical salinity vs absolute salinity)
The tables on this page are generated from the calculators' own code and checked by automated tests. Last reviewed November 3, 2026. Found an error? Tell us.
Calculators for this job
Salinity & specific gravity
Convert hydrometer SG to ppt with temperature correction (EOS-80), and see what your instrument should read.
Water change & dilution
What a water change does to nitrate, salinity or hardness, how much to change to hit a target, and RO/tap blends.
Hardness & alkalinity units
Convert KH between dKH, ppm and meq/L, and GH between dGH, ppm, mmol/L and grains per gallon, or work out GH from Ca and Mg.
Tank volume & water weight
How many gallons or litres your tank holds: rectangular, cylinder, bow-front, corner and hexagon, plus water weight.
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