In short: rejection rate is the share of dissolved solids your membrane blocks, calculated as (1 − permeate TDS ÷ feed TDS) × 100%. Most residential RO membranes reject 95–98% when new; enter your own readings above for your system's actual figure.
This is a planning and diagnostic estimate from your TDS readings. It doesn't assess water safety — use it to track membrane performance and maintenance timing.
How rejection rate is calculated
Reverse osmosis rejection rate compares how concentrated the water is before and after it passes through the membrane. The membrane lets water molecules through while blocking most dissolved ions, so the permeate (product water) ends up with far less total dissolved solids (TDS) than the feed water that went in:
rejection rate = (1 − permeate TDS ÷ feed TDS) × 100%
Salt passage is the complement: 100% − rejection rate. TDS removed is the simple difference, feed TDS − permeate TDS, in ppm.
Worked example. Feed water at 300 ppm TDS and permeate at 8 ppm gives (1 − 8÷300) × 100% = 97.3% rejection. That's 2.7% salt passage, 292 ppm of TDS removed, and permeate that's about 37.5× purer than the feed — matching the calculator's defaults above.
Typical rejection rates by membrane type
The table below gives typical ranges for common RO membrane applications. Manufacturer specs are usually tested on a fresh membrane under standard conditions (commonly 2,000 ppm NaCl at 25°C and rated pressure), so real-world numbers on an installed, aged system are often a little lower.
| Application | Typical rejection | Notes |
|---|---|---|
| Residential under-sink RO (TFC) | 95–98% | Common point-of-use systems |
| Whole-house / light commercial RO | 97–99% | Larger, multi-membrane systems |
| Brackish water RO (BWRO) | 99–99.5% | Municipal & commercial brackish supply |
| Seawater RO (SWRO) | 99.4–99.8% | High-pressure seawater membranes |
| Nanofiltration (NF) | 40–90% | Varies widely by ion; rejects multivalent ions best |
What else affects the result
Temperature. Membranes are usually rated at 25°C (77°F). Colder feed water reduces both permeate flow and rejection slightly; warmer water increases flow but can also let a bit more salt through, since dissolved ions diffuse across the membrane faster at higher temperatures.
Fouling and scaling. Particulates, biofilm, or mineral scale on the membrane surface change how water and ions move through it — this is the most common cause of a rejection rate that's dropped from its factory spec, alongside a worn or damaged membrane, or an O-ring that's no longer sealing properly.
Net driving pressure. Higher applied pressure relative to osmotic pressure generally improves both flux and rejection, up to the membrane's design limits; low supply pressure can push rejection down along with production rate. Use the reverse osmosis sizing calculator if you're specifying a new system.
Ion type. Rejection is ion-specific — RO membranes typically reject multivalent ions (like calcium or sulfate) and larger ions better than small monovalent ions (like sodium, chloride, or nitrate). A single TDS-based rejection number is an average across everything dissolved in the water, not a guarantee for any one contaminant.
Worked examples
Under-sink RO, city tap water
97.5%
320 ppm feed, 8 ppm permeate → (1 − 8÷320) × 100% = 97.5% rejection, 312 ppm removed.
Whole-house RO, well water
97.7%
650 ppm feed, 15 ppm permeate → (1 − 15÷650) × 100% = 97.7% rejection, 635 ppm removed.
Commercial BWRO system
98.9%
2,200 ppm feed, 25 ppm permeate → (1 − 25÷2200) × 100% = 98.9% rejection, 2,175 ppm removed.
Frequently asked questions
What is a good rejection rate for a reverse osmosis system?
Most residential and light-commercial RO membranes reject 95–98% of dissolved solids when new, and many well-maintained systems reach 97–99%. Brackish and seawater RO systems are typically rated higher, from about 99% to 99.8%. Anything below about 90% usually points to a problem with the membrane or its seals.
How is rejection rate different from recovery rate?
Rejection rate measures how much dissolved solids the membrane blocks, based on feed and permeate TDS. Recovery rate measures how much of the feed water becomes usable permeate rather than concentrate, based on flow rates. A membrane can have excellent rejection but poor recovery, or the reverse — use the RO recovery rate calculator to find that figure from your flow readings.
Why is my rejection rate lower than the membrane's rated spec?
Rejection typically declines with membrane fouling or scaling, a damaged or improperly seated O-ring, high feed water temperature, or simple membrane age. Manufacturer specs are usually measured on a fresh membrane under standard test conditions, so some drop-off over time is normal — a large or sudden drop is the signal to investigate.
Does water temperature affect the rejection rate?
Yes. Membrane specs are usually rated at 25°C (77°F). Colder feed water reduces both permeate flow and rejection slightly, since dissolved salts diffuse through the membrane more easily relative to water at lower temperatures. Warmer water increases flow but can also let slightly more salt pass, so avoid comparing readings taken at very different temperatures.
How do I get the feed and permeate TDS numbers?
Measure both with a TDS or conductivity meter, or use the readings from your RO system's built-in monitor if it has one. If you only have conductivity in µS/cm, convert it to TDS first with the conductivity to TDS calculator.
Can a declining rejection rate tell me when to replace the membrane?
Tracking rejection rate over time is one of the best early-warning signs. If it falls several percentage points below where a fresh membrane tested, and cleaning the system doesn't bring it back up, the membrane itself is usually degrading and due for replacement.
Sources & references
- American Water Works Association (AWWA). Manual M46: Reverse Osmosis and Nanofiltration.
- U.S. Environmental Protection Agency (EPA). Drinking Water Treatability Database: Reverse Osmosis/Nanofiltration.
- Water Quality Association (WQA). Reverse Osmosis Technology Fact Sheet.
Sarah Bennett, MS
Water Quality & Sustainability Specialist · Portland, Oregon
Environmental scientist (MS, Water Resources) with 9+ years in residential water quality, treatment, and usage efficiency, and 150+ completed assessments. Sarah writes and reviews the treatment, usage, and cost calculators on watercalculator.us. Read full bio
MS Environmental Science, Water Resources (2018) · BS Environmental Engineering (2016)