TDS Rejection Rate Calculator
What the TDS Rejection Rate Calculator does
The TDS Rejection Rate Calculator shows the percentage of total dissolved solids (TDS) rejected by a reverse osmosis membrane, based on readings from the water entering the system and the filtered water it produces. It gives you a way to compare the two readings and follow changes in your system over time.
TDS refers to dissolved substances in water. A TDS meter provides a reading in parts per million (ppm), typically by measuring electrical conductivity and converting it to an estimated TDS value. The calculator uses your readings to produce a percentage; it does not identify individual substances in the water.
A higher rejection rate means the permeate reading is lower relative to the feed-water reading. For example, a result of 95% means the permeate TDS reading is 95% lower than the feed TDS reading. The result describes the readings you entered at that point in time, so it should be considered alongside operating conditions and the guidance supplied with your reverse osmosis system.
How to use the TDS Rejection Rate Calculator
You need two TDS readings, both in ppm. Take them under stable operating conditions so the comparison reflects the system while it is running normally.
- Measure Feed Water TDS. This is the TDS reading for the water entering the reverse osmosis system. Enter a whole-number value from 1 to 100,000 ppm.
- Measure Permeate Water TDS. Permeate is the filtered water that has passed through the membrane. Enter a whole-number value from 0 to 100,000 ppm.
- Read the TDS Rejection Rate. The result is displayed as a percentage to two decimal places.
Use clean sampling containers and follow your TDS meter’s instructions. If you use the same meter for both samples, rinse it between readings as directed by the manufacturer to reduce the chance of carrying water from one sample into the next. Make sure both readings use the same units; the calculator’s input labels specify ppm.
The feed value must be greater than zero because it is the denominator in the formula. A permeate value of zero is accepted and produces a rejection rate of 100% for the readings entered.
How the TDS Rejection Rate Calculator formula works
The calculator subtracts the permeate TDS from the feed-water TDS. It then divides that difference by the feed-water TDS and multiplies by 100:
TDS rejection rate (%) = (Feed Water TDS − Permeate Water TDS) ÷ Feed Water TDS × 100
This calculation expresses the reduction in the TDS reading as a percentage of the starting feed-water reading. Both measurements must represent the same type of unit for the comparison to make sense.
Worked example
Assume a feed-water reading of 500 ppm and a permeate reading of 25 ppm. These are illustrative readings and match the calculator’s default inputs.
- Subtract 25 ppm from 500 ppm: 475 ppm.
- Divide 475 by the feed-water reading of 500: 0.95.
- Multiply 0.95 by 100: 95.00%.
The result means the measured permeate TDS is 95% lower than the measured feed-water TDS in this example. As another illustrative comparison, feed water at 200 ppm and permeate at 20 ppm produce a 90.00% rejection rate.
If both readings are equal, the result is 0.00%. If the permeate reading is greater than the feed reading, the result is negative. For instance, 100 ppm feed water and 110 ppm permeate produce −10.00%. A negative result is a signal to check the readings and sampling conditions; the formula alone cannot establish why the difference occurred.
Use cases for the TDS Rejection Rate Calculator
For readers comparing or maintaining a home reverse osmosis system, the calculator can make two TDS readings easier to interpret.
- Track changes over time. Record feed and permeate readings periodically, together with the date and relevant operating conditions. Comparing results taken in a consistent way can help you notice a change worth investigating.
- Check a new or serviced system. Once the system is operating under stable conditions, calculate a baseline from your own readings. Compare later measurements with that baseline and the system manufacturer’s guidance.
- Understand a TDS reduction. A drop from 500 ppm to 25 ppm may be easy to see, but the percentage puts that difference in context relative to the starting reading.
- Compare readings from different feed conditions. If your source-water TDS changes, a percentage can help you interpret the permeate reading alongside the feed reading. Keep in mind that changing operating conditions can also affect the comparison.
- Prepare for a maintenance discussion. A record of readings and calculated percentages can give a service professional useful context when you ask about system performance.
A single percentage is a starting point, not a complete assessment of a reverse osmosis system. Use it with the manufacturer’s instructions and any other information relevant to your equipment.
Other factors to consider when calculating TDS Rejection Rate
Stable operating conditions matter. Temperature and other operating conditions can affect readings and system performance. When tracking results, note any meaningful differences in conditions and try to take measurements consistently.
Sampling location matters. Feed water should represent the water entering the system, while permeate should represent water that has passed through the membrane. Follow your system’s instructions when choosing sampling points. Water collected after additional treatment or storage may not represent the membrane’s permeate in the same way as a sample taken at the appropriate point.
Meter readings have limitations. TDS meters estimate dissolved solids from conductivity, so results depend on the instrument and its conversion method. Check the meter’s instructions for calibration, use and maintenance. Small differences between readings can matter when you are comparing results over time.
TDS is not a list of contaminants. A rejection percentage does not tell you which substances are present or the rejection rate for any particular substance. It also does not establish whether water meets a specific drinking-water standard. A suitable water test is needed when you want to know about particular substances.
The result is based on supplied measurements. The calculator does not predict future membrane performance or diagnose a fault. If a result changes unexpectedly, recheck the samples and meter, then consult the system manufacturer or a qualified service professional for guidance.
Frequently asked questions about TDS rejection rate
What is a TDS rejection rate?
It is the percentage decrease in the TDS reading from feed water to reverse osmosis permeate. It is calculated by dividing the difference between the two readings by the feed-water reading and multiplying by 100.
What does a 95% TDS rejection rate mean?
It means the permeate TDS reading is 95% lower than the feed-water reading. For example, the calculator returns 95.00% for illustrative readings of 500 ppm feed water and 25 ppm permeate.
Why is my TDS rejection rate negative?
A negative result appears when the entered permeate reading is higher than the feed reading. Check that the samples were taken from the intended locations, that both readings use ppm, and that the meter and sampling conditions were appropriate. The calculation itself does not identify the cause.
Can I use a feed-water TDS reading of zero?
No. The formula divides by the feed-water reading, so zero cannot be used as the denominator. This calculator accepts feed-water values starting at 1 ppm. A permeate reading of zero is accepted.
Does a high TDS rejection rate prove my water is safe to drink?
No. The percentage compares two overall TDS readings and does not identify specific substances or establish compliance with a drinking-water standard. If you need to assess a particular water-quality concern, use an appropriate water test and consult a qualified source for interpreting the results.