Clean the prism, add a few drops of a seawater calibration standard, wait about 45 seconds, and turn the calibration screw until it reads the standard’s value. Do this about once a month. Bulk Reef Supply recommends a seawater standard rather than RODI water, because most hobby refractometers are made for salt-in-water brine.
Why calibrate with a seawater standard
Bulk Reef Supply explains that most aquarium refractometers are designed around sodium chloride brine, which bends light slightly differently from real seawater. Calibrating with a seawater standard corrects for that. It advises against relying only on the zero point with RODI or distilled water, as some manuals suggest.
Most standards are made at 35 ppt (SG about 1.026). Check the label of yours.
Step by step
Bulk Reef Supply’s method:
- Wipe the prism with a soft cloth and rinse with RODI water if needed
- Add a few drops of calibration standard and close the cover
- Wait about 45 seconds for the automatic temperature compensation to settle
- Look through the eyepiece and read the line where blue meets white
- If it doesn’t match the standard, turn the calibration screw until it does
- Rinse and dry the prism
Reading your tank
Use the same steps with a sample of tank water, with no air bubbles under the cover. Read the ppt scale if there is one; ppt doesn’t change with temperature, unlike specific gravity. Our salinity converter converts between the two.
How often
Bulk Reef Supply suggests calibrating about once a month. Do it sooner if a reading surprises you, or if the refractometer has been dropped. Bulk Reef Supply lists uncalibrated instruments among the common testing mistakes.
Common questions
Can I calibrate my refractometer with tap water?
Use RODI or distilled water for the zero point, and a seawater standard for the reading that matters.
How long does calibration fluid last?
Check the label. Keep it sealed and cool, and replace it when it’s past its date.
Why does my refractometer read differently from my meter?
One of them may need calibrating. Check both against the same standard.
Sources and limits
Last checked 13 October 2026. Follow your equipment maker’s own instructions where they differ from this guide.
Keep going
Convert specific gravity to ppt · Free Reef Companion · All calculators