Reverse Osmosis Rejection Rate Calculator
Calculate the rejection rate (removal efficiency) of a reverse osmosis membrane by comparing the solute concentration in the feed water versus the permeate (filtered output).
mg/L (or any consistent unit)
mg/L (or any consistent unit)
Fill in the fields above and click Calculate.
Formula
Rejection Rate (R) = (1 − Cp / Cf) × 100
Salt Passage (%) = (Cp / Cf) × 100
Where:
- Cf = Feed water concentration (mg/L or ppm)
- Cp = Permeate (product water) concentration (mg/L or ppm)
- R = Rejection rate expressed as a percentage (%)
Note: Salt Passage + Rejection Rate = 100%
Assumptions & References
- Both feed and permeate concentrations must be measured in the same units (e.g., mg/L, ppm, or μS/cm for conductivity-based estimates).
- The formula assumes steady-state operation with no concentration polarization correction applied.
- High-quality RO membranes typically achieve rejection rates of 95–99.5% for dissolved salts (NaCl, TDS).
- Rejection rates below 85% generally indicate membrane fouling, scaling, physical damage, or O-ring failure.
- Conductivity (in μS/cm) can be substituted for concentration as a practical field measurement proxy.
- Temperature, pressure, and recovery rate affect actual membrane performance but are not included in this simplified calculation.
- Reference: AWWA Manual M61 — Desalination of Seawater; DOW FILMTEC™ Membrane Technical Manual.
- Reference: ASTM D3923 — Standard Practices for Detecting Leaks in Reverse Osmosis Devices.