RO Calculator (Reverse Osmosis)
Your expert tool for analyzing RO system performance metrics like salt rejection and recovery rate.
Total Dissolved Solids in the source water, measured in ppm (parts per million).
Total Dissolved Solids in the purified water (product), in ppm.
The rate at which water is fed to the RO system.
The rate of purified water production. Must be in the same unit as Feed Flow.
System Flow Distribution
Visual breakdown of feedwater into permeate and concentrate.
| Parameter | Value | Unit |
|---|---|---|
| Feed Water TDS | – | ppm |
| Permeate Water TDS | – | ppm |
| Salt Rejection | – | % |
| System Recovery | – | % |
| Concentration Factor | – | x |
What is a RO Calculator?
A ro calculator is a specialized tool designed to evaluate the performance of a Reverse Osmosis (RO) water purification system. It takes key operational data—such as flow rates and Total Dissolved Solids (TDS) levels—to compute critical performance indicators. These indicators, including salt rejection and recovery rate, tell you how efficiently your system is removing impurities and how much water is being purified versus sent to the drain. This calculator is essential for homeowners, water treatment professionals, and engineers who need to monitor, troubleshoot, or design an RO system.
RO Calculator Formula and Explanation
The core of any RO calculator relies on two primary formulas: Salt Rejection and Recovery Rate. These calculations provide a clear picture of the system’s effectiveness.
Salt Rejection Rate (%)
This formula measures how effectively the RO membrane is blocking contaminants.
Salt Rejection (%) = [1 - (Permeate TDS / Feed TDS)] * 100
Recovery Rate (%)
This formula shows the percentage of feed water that becomes purified water (permeate).
Recovery Rate (%) = (Permeate Flow / Feed Flow) * 100
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Feed TDS | TDS level of the source water before filtration. | ppm | 50 – 2000+ |
| Permeate TDS | TDS level of the purified water after filtration. | ppm | 5 – 100 |
| Feed Flow | Rate of water entering the RO system. | GPM or LPM | 0.1 – 100+ |
| Permeate Flow | Rate of purified water being produced. | GPM or LPM | 0.05 – 75+ |
Practical Examples
Example 1: Home Under-Sink RO System
A typical homeowner wants to check their kitchen RO system.
- Inputs:
- Feed Water TDS: 350 ppm
- Permeate Water TDS: 20 ppm
- Feed Flow: 0.3 GPM
- Permeate Flow: 0.1 GPM
- Results:
- Salt Rejection: [1 – (20 / 350)] * 100 = 94.3%
- Recovery Rate: (0.1 / 0.3) * 100 = 33.3%
Example 2: Commercial Car Wash System
A car wash uses a large RO system to prevent water spots.
- Inputs:
- Feed Water TDS: 600 ppm
- Permeate Water TDS: 30 ppm
- Feed Flow: 50 LPM
- Permeate Flow: 35 LPM
- Results:
- Salt Rejection: [1 – (30 / 600)] * 100 = 95.0%
- Recovery Rate: (35 / 50) * 100 = 70.0%
For more advanced modeling, consider using an RO system design tool.
How to Use This RO Calculator
- Enter Feed Water TDS: Measure the TDS of your tap or source water using a TDS meter and enter the value in ppm.
- Enter Permeate Water TDS: Measure the TDS of the purified water coming from the RO faucet.
- Enter Flow Rates: Measure or find the specification for your system’s feed and permeate flow rates. Ensure you select the correct unit (GPM or LPM) and use the same unit for both values.
- Interpret Results: The calculator will instantly provide the Salt Rejection and Recovery Rate. A high rejection rate (typically >95%) and a balanced recovery rate (e.g., 25-75% depending on the system) indicate good performance.
Key Factors That Affect RO Performance
The efficiency calculated by the ro calculator is influenced by several real-world conditions.
- Pressure: Higher feed pressure forces water through the membrane more effectively, increasing both permeate flow and rejection rate up to a certain point.
- Temperature: Warmer water is less viscous and passes through the membrane more easily, increasing permeate flow. However, very high temperatures can damage the membrane.
- Feed Water TDS: Higher feed water TDS requires more pressure to overcome the natural osmotic pressure, which can reduce flow and rejection if the system pressure is not increased accordingly.
- Membrane Condition: Fouling (from sediment, organic matter) or scaling (from mineral buildup) on the membrane surface will block water flow and reduce system efficiency. This is a primary factor in RO membrane performance.
- Recovery Rate: Running a system at a very high recovery rate increases the concentration of salts on the feed side, which can lead to increased scaling and lower rejection over time.
- pH Level: Most RO membranes have an optimal pH range for operation. pH levels that are too high or too low can cause the membrane material to degrade, affecting performance and lifespan.
Frequently Asked Questions (FAQ)
A good salt rejection rate for a modern RO system is typically 95% to 99%. Rates below 90% may indicate the membrane needs cleaning or replacement.
Home under-sink systems often have low recovery rates, around 20-35%. This means for every 1 gallon of pure water, 3-4 gallons go to the drain. More efficient systems can achieve higher rates. Check your reverse osmosis efficiency to learn more.
An increase in permeate TDS, which lowers your salt rejection, can be caused by membrane fouling, scaling, damage (e.g., from chlorine), or simply old age. It can also be affected by a significant drop in feed pressure or temperature.
No. The salt rejection calculation is based only on TDS levels. The flow unit is critical for calculating the recovery rate, but as long as you use the same unit for feed and permeate flow, the percentage will be correct.
The concentration factor (shown in our results table) indicates how much more concentrated the waste (concentrate) stream is compared to the feed water. It is directly related to the water recovery rate and is a key parameter in preventing scaling.
Yes, the formulas are the same. However, seawater and brackish water systems operate with much higher TDS levels (2,000 to 35,000+ ppm) and require significantly higher pressures. The principles of rejection and recovery remain the same.
It’s good practice to log your system’s performance monthly. A sudden drop in performance can alert you to a problem before it becomes severe. This is a core part of maintaining RO systems.
Rejection measures the *quality* of filtration (how many contaminants are removed). Recovery measures the *efficiency* of water use (how much water is purified versus wasted).
Related Tools and Internal Resources
Explore these resources for more in-depth analysis and information on water treatment.
- TDS and Water Hardness Calculator: Understand your source water quality before it enters the RO system.
- What is Reverse Osmosis?: A complete guide to the technology.
- Choosing the Right RO Membrane: Learn about different membrane types and their applications.
- Water Pressure Unit Converter: Easily convert between PSI, bar, and other pressure units.
- Guide to RO System Maintenance: Tips for extending the life of your system.
- Blog: Improving Water Quality at Home: Discover other methods and tips for better water.