TDS Calculator

Total Dissolved Solids (TDS) is a term used to express the total amount of charged ions, including minerals, salts or metals, dissolved in a given volume of water. TDS is usually expressed in mg/L or ppm. The HM DigitalĀ® TDS-EZ Water Quality Tester can read the TDS levels in your tap water and in your RO product water to determine the effectiveness of your RO water purification system.

Follow the steps below to have our online calculator determine the membrane rejection rate of your RO system. Click here for a quick tutorial on how to use the TDS meter.



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Rinse the TDS Water Quality Tester with reverse osmosis water.


Fill a small glass with RO water and use the TDS Water Quality Tester to read the TDS. Key in your results below.


Rinse the TDS Water Quality Tester with RO water again. Then test your tap water with your TDS Water Quality Tester and key in your results below.


Click on the calculate button below to view your results.


RO Water Result:


Tap Water Result:


Your result is:


Now that we have the membrane's rejection rate, what does this mean?

The rejection rate is the percentage of the TDS (Total Dissolved Solids) removed by the RO membrane in your water purification system. Example: A 90% rejection rate means that the RO product water from your RO system contains 90% less TDS than the water coming into the system.

If your RO membrane rejection rate is between 85-100% your RO membrane cartridge is operating within factory specifications.

If your RO membrane TDS rejection is less than 85%, then we recommend replacing the RO membrane cartridge. If your system includes pre or post filters, they should also be changed at this time.