What Is RO Rejection Rate?
You bought a reverse osmosis system. Maybe it has been a year. The water tastes a little off. You start wondering: is it actually still working?
That single question is what RO rejection rate exists to answer. It is the one number that tells you whether your reverse osmosis membrane is doing its job, and it is something you can check yourself with a $20 meter and a calculator. No service call, no guessing.
This article walks through what rejection rate means, why it matters, what affects it, and how to calculate yours from the kitchen counter. By the end, you will know whether your system is healthy, where it is headed, and what to do if the number is not what it should be.
Rejection rate is the percentage of dissolved contaminants an RO membrane removes from your feed water. If 100 ppm of dissolved solids go in and 5 ppm come out, the membrane "rejected" 95 of them. That is a 95% rejection rate.
The formula is straightforward:
Rejection % = ((Feed TDS - Permeate TDS) / Feed TDS) × 100
Two terms worth defining up front:
- Feed TDS is the total dissolved solids (TDS) in the water entering the membrane, measured in parts per million (ppm)
- Permeate is the clean water that makes it through the membrane to your faucet
Think of rejection rate as the membrane's report card. It tells you, in one number, what percentage of the dissolved stuff your membrane is keeping out of your drinking water. The higher, the better.
How to Calculate Your RO System's Rejection Rate
The whole calculation takes about five minutes. You need a TDS meter (the digital pen-style ones run about $15 to $30 online) and a small glass.
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Run your RO faucet for 30 seconds first.
The first water out of the tank can read artificially high since it has been sitting. You want a fresh sample.
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Measure the permeate.
Catch some water from your RO faucet in a glass and dip the meter in. Write down the reading. That is your permeate TDS in ppm.
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Measure the feed water.
Pull water from a faucet that bypasses your RO system. The kitchen cold tap (if it is not connected to the RO) or the bathroom sink usually works. Measure that. That is your feed TDS.
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Plug the numbers into the formula.
A worked example below shows what a healthy reading looks like.
Say your feed water reads 250 ppm and your permeate reads 12 ppm. Here is the math, one step at a time:
Step 1. Subtract permeate from feed to find how much the membrane removed:
250 - 12 = 238 ppm removed
Step 2. Divide that by the feed to get the proportion removed:
238 ÷ 250 = 0.952
Step 3. Multiply by 100 to turn the proportion into a percentage:
0.952 × 100 = 95.2%
Your system has a 95.2% rejection rate. That is a healthy number for a residential RO membrane.
If you are new to TDS as a measurement, the Crystal Quest guide to TDS in water covers what the number actually represents and why it matters at the tap.
What Is a "Good" Rejection Rate?
A healthy residential RO membrane typically rejects between 95% and 99% of dissolved solids. Crystal Quest manufactures its RO membranes to deliver in that range, with peak performance approaching 99% on a well-maintained membrane.
Here is a rough guide for interpreting your number:
| Rejection % | What It Means | What to Do |
|---|---|---|
| 95% to 99% | Healthy membrane, working as designed | Nothing. Stay on your normal maintenance schedule |
| 90% to 94% | Acceptable but trending down | Check pre-filters, water pressure, and membrane age |
| 80% to 89% | Membrane is fouled or aging | Try cleaning it before replacing |
| Below 80% | Membrane is failing or has been bypassed | Inspect for leaks, then plan to replace |
These numbers assume a typical municipal feed of 100 to 500 ppm. If you are on well water or a high-TDS source above 1,000 ppm, the numbers shift slightly because the membrane has more work to do. Do not panic at 92% if your feed is 1,500 ppm.
A note on the "removes up to 99%" claim you will see in marketing. That is peak laboratory performance under ideal conditions. Real-world residential systems usually run a few points lower. The U.S. EPA recommends keeping total dissolved solids below 500 ppm for taste and aesthetic reasons in finished drinking water (EPA Secondary Drinking Water Standards), and a healthy RO membrane gets you well under that even on hard feed.
What Affects RO Rejection Rate
Several variables push your rejection rate up or down day to day. Understanding them helps you diagnose unexpected drops.
Membrane type
RO membranes come in three classes: freshwater (sometimes called tap water), brackish water, and desalination (seawater). Each is engineered for a different feed TDS range, and Crystal Quest manufactures and ships all three so the membrane matches the source.
A freshwater membrane forced to run on 5,000 ppm well water will look like it is failing when it is actually just being asked to do the wrong job.
Feed water TDS
The higher the TDS going in, the harder the membrane works. A membrane that hits 97% on city water at 200 ppm might drop to 92% on well water at 1,500 ppm. Same membrane, different conditions, different number.
Water pressure
RO membranes need pressure to push water through. Most residential systems are spec'd for 45 to 60 psi at the membrane. Drop below 40 psi and rejection slips because there is not enough force to drive the separation cleanly. A booster pump fixes this on low-pressure homes and most well setups.
Temperature
Cold water makes the membrane slower and slightly more efficient at rejection. Warm water is the opposite: faster flow, lower rejection. The effect is not huge in a kitchen since incoming water is usually 50 to 70 °F year-round, but it is measurable.
Membrane age and fouling
A new membrane performs at the top of its rated range. As scale, biofilm, or sediment build up on the membrane surface (a process called "fouling"), rejection drifts down. This is normal. Most residential RO membranes last 2 to 4 years before they need replacing, and cleaning can sometimes restore performance in between.
Pre-filter saturation
A clogged sediment or carbon pre-filter starves the membrane of pressure and exposes it to chlorine that should have been removed upstream. Both shorten membrane life. If your rejection drops suddenly, check the pre-filters before you suspect the membrane.
What Rejection Rate Does Not Tell You
Rejection rate is useful, but it is not the whole picture. The number is a single percentage based on TDS, and TDS is a bulk measurement of dissolved solids, not a contaminant-specific test.
Here is what rejection rate will not catch:
- Specific contaminants. A 96% rejection rate does not tell you whether lead, arsenic, or PFAS specifically were removed. The U.S. EPA sets enforceable limits for individual contaminants under its Primary Drinking Water Regulations, and the only way to confirm those are met is a lab test that measures the contaminants directly.
- Microbial issues. RO membranes block most bacteria and viruses, but the storage tank, post-filter, and faucet can still harbor microbes if they are not maintained. A high rejection rate does not mean the system is sterile.
- Dissolved gases. Carbon dioxide, hydrogen sulfide, and some volatile organic compounds slip through RO membranes. Carbon post-filters typically handle them, but rejection rate will not reflect their presence one way or the other.
- Mineral content for taste. RO removes calcium and magnesium along with the contaminants. If your water tastes flat after RO, that is by design, and a remineralization stage solves it.
This is why Crystal Quest engineers RO systems with multiple stages instead of relying on the membrane alone. Sediment and carbon pre-filters protect the membrane and catch chlorine and chemicals it cannot handle. Carbon post-filters polish the water and address dissolved gases. Optional remineralization adds back the minerals the membrane stripped.
How to Use Rejection Rate in the Real World
Knowing your rejection rate gives you a few practical superpowers.
Diagnosing a "tastes off" complaint. If the water suddenly tastes different, run the calculation. A drop from 96% to 88% explains it. A steady 96% means look elsewhere: storage tank, faucet, or post-filter.
Deciding when to clean versus replace. If rejection has slipped but the membrane is under three years old, cleaning the membrane often restores it. Past that, you are usually replacing it.
Comparing systems before you buy. When you are shopping, rejection rate is a real spec to compare across systems and membrane types. The Crystal Quest reverse osmosis buyer's guide shows which membrane class fits which water source.
Tracking membrane life over time. Log your rejection rate every six months and you will see the gradual drift before it becomes a taste problem. The complete RO maintenance guide covers the rest of the maintenance log.
The single biggest mistake homeowners make is testing rejection rate once and then never again. The number is not useful as a snapshot; it is useful as a trend.
Crystal Quest RO Membranes and What to Expect
With over 30 years of manufacturing reverse osmosis membranes in the USA, Crystal Quest's engineering team calibrates its residential and whole-house RO lineup to deliver 95% to 99% rejection on properly matched feed water.
Three classes of membrane are available, each designed for a different source:
- Freshwater (tap water) membranes for typical municipal supplies and most residential use, rated for feed TDS up to about 2,000 ppm
- Brackish water membranes for high-TDS well water and light industrial sources, rated for feed TDS roughly 2,000 to 10,000 ppm
- Desalination membranes for seawater (typically 32,000 to 35,000 ppm) and high-TDS industrial sources above 10,000 ppm
Match the membrane to the water and the rejection rate stays where it should be. Run a freshwater membrane on a brackish source and the number falls because the membrane was not engineered for that load. The Crystal Quest complete guide to reverse osmosis water filtration explains the differences in more depth, and the RO maintenance guide covers what to do as performance drifts.
Get the Right RO System for Your Water
The first step is knowing what you are working with. Pull a TDS reading from your tap. If you already have an RO system, run the rejection calculation. Numbers in hand, the next move is straightforward.
Ready to match your water to the right RO system?
Crystal Quest's reverse osmosis lineup covers under-sink, countertop, and whole-house options, each matched to a feed water class and built with the multi-stage design that keeps the membrane working at spec.
Frequently Asked Questions About RO Rejection Rate
What is a good rejection rate for a home RO system?
A healthy residential RO membrane should reject between 95% and 99% of dissolved solids. Above that range is rare in real-world conditions, and below 90% means the membrane is fouled, aging, or mismatched to the feed water.
Why is my RO rejection rate dropping?
The most common causes are saturated pre-filters, low feed pressure, scale or biofilm buildup on the membrane, or simple membrane age. Check pre-filters first since they are the easiest fix. If those are fresh, measure inlet pressure, which should be at least 45 psi. If both check out, the membrane is probably ready for cleaning or replacement.
Is 80% rejection rate bad?
Yes. Below 90% means something has changed since the system was installed. The membrane could be fouled, the pressure could be too low, or the wrong class of membrane could be installed for the source water. Investigate before assuming the membrane is dead.
How is rejection rate different from removal rate?
The two terms are interchangeable in most consumer contexts. "Removal rate" sometimes refers to a specific contaminant ("99.5% lead removal") while "rejection rate" usually refers to total dissolved solids as a bulk measurement. If you see a manufacturer claim, look for what the percentage applies to.
Does cold water affect RO rejection rate?
Slightly. Cold water improves rejection by a small margin since membranes are more selective at lower temperatures. The trade-off is slower water production, which is why RO output drops in winter for homes with unheated basements. The effect on rejection itself is small enough that most homeowners will not notice.
How often should I check my system's rejection rate?
Every 3 to 6 months is plenty for a residential system. Test more often if your water source is variable (well water after heavy rain, for example) or if you have recently replaced filters or the membrane. A quick log of the readings tells you when performance is drifting before it becomes a taste problem.
Does a higher feed TDS mean a worse rejection rate?
Higher feed TDS makes the membrane work harder, which can shave a few percentage points off the rejection number. A membrane rated at 97% on 200 ppm city water might run 93% on 1,500 ppm well water. Both are normal. Match the membrane class to your feed and the absolute rejection stays in spec.
What is the difference between rejection rate and recovery rate?
Rejection rate measures water quality (how much of the dissolved load gets removed). Recovery rate measures water efficiency (how much of the feed becomes clean water versus going to drain). They are different metrics. A residential RO might reject 96% of TDS while only recovering 25% of the feed water as permeate.
