Reverse Osmosis Waste Calculator
Estimate drain water, feed water, and efficiency from product flow and waste ratio.
What it does
Reverse osmosis pushes water through a membrane that traps dissolved salts, metals, and many contaminants. Some water does not pass through and is sent to the drain to carry the concentrated waste away. The ratio of drain water to product water is the number most people mean when they talk about how much a reverse osmosis system wastes.
A 2:1 ratio is common for older residential systems: two gallons to the drain for each gallon of drinking water. Newer systems with a pump or a permeate pump can run closer to 1:1, and some designs claim less than 1:1. The ratio depends on water temperature, pressure, membrane condition, and the system design, so the label figure is a starting point rather than a guarantee.
This calculator converts the ratio into recovery efficiency, daily drain water, daily feed water, and annual drain water. Efficiency is product divided by feed, which is the same as 100 / (1 + ratio). A 2:1 system recovers 33.3 percent of the feed water. A 1:1 system recovers 50 percent.
The annual figure is useful for comparing a new system with an old one or for estimating the cost of water in a metered household. It does not include the water used for flushing or the concentrate that may be recycled by a zero waste accessory. Those features change the real number, sometimes by a lot.
The result is an estimate. A pressure gauge, a flow meter, and a temperature reading give a more accurate picture of a specific installation. Use the calculator to set expectations and to spot a system that is wasting far more than its design should.
How to use
- Choose gallons or liters per day.
- Enter the volume of product water the system makes in a day.
- Enter the waste ratio from the manual, the label, or a measured test.
- Read the recovery efficiency first.
- Check the daily drain and feed volumes.
- Use the annual drain figure to estimate water use or cost.
Formula
- P
- Product water in gallons or liters per day.
- r
- Waste ratio, drain volume per unit of product.
- E
- Recovery efficiency as a percentage.
- W
- Drain water per day.
- F
- Feed water per day.
A ratio of 0 means no drain water is counted, which gives 100 percent recovery and is not realistic for a real membrane. A ratio of 20 means 20 gallons to the drain for each gallon of product. The tool accepts 0 to 20 so it can match a measured test or a manufacturer claim.
Worked example
Example: 3 gal per day at a 2:1 waste ratio
Imperial defaults: 3 gal of product water per day and a 2:1 waste ratio. The metric mirror is 11.36 L per day.
- Efficiency: 100 / (1 + 2) = 33.3 percent.
- Drain water: 3 x 2 = 6.00 gal per day.
- Feed water: 3 + 6 = 9.00 gal per day.
- Annual drain: 6 x 365 = 2,190 gal per year.
- The tool shows 33.3 percent efficiency, 6.00 gal/day drain, 9.00 gal/day feed, and 2,190 gal per year.
- In metric mode, 11.36 L per day gives 22.72 L/day drain, 34.08 L/day feed, and 8,293 L per year.
- A 4:1 ratio on 4 gal per day would give 20 percent efficiency and 16.00 gal/day of drain water.
Reference table
| Waste ratio | Efficiency | Drain water | Feed water |
|---|---|---|---|
| 1:1 | 50 % | 3.00 gal | 6.00 gal |
| 2:1 | 33.3 % | 6.00 gal | 9.00 gal |
| 3:1 | 25 % | 9.00 gal | 12.00 gal |
| 4:1 | 20 % | 12.00 gal | 15.00 gal |
| 5:1 | 16.7 % | 15.00 gal | 18.00 gal |
Limits
- The ratio you enter may not match the system's real ratio. Water temperature, pressure, and membrane age all change the split between product and drain.
- The calculator does not include flush cycles, tank refills, or zero waste accessories that send concentrate back to the hot water line.
- It reports water volume only, not the cost of water, sewer, or replacement filters.
- A very low ratio can save water but may shorten membrane life if the system is not designed for it.
Common mistakes
- Confusing the waste ratio with the recovery percentage. A 2:1 ratio is 33.3 percent recovery, not 50 percent.
- Entering the system's rated product flow instead of the measured flow.
- Forgetting that cold feed water lowers output and can increase the waste ratio.
- Ignoring the extra water used by a booster pump or a flush valve.
- Assuming a lower ratio is always better without checking the membrane manufacturer's minimum concentrate flow.
Try it yourself
Quick checks. Answers sit tucked under each question.
A system makes 4 gal per day at a 4:1 ratio. What are the efficiency, drain, and feed volumes?
20 percent efficiency, 16.00 gal/day drain, 20.00 gal/day feed, and 5,840 gal per year of drain water.
A system makes 5 gal per day at a 3:1 ratio. What are the efficiency and drain volumes?
25 percent efficiency, 15.00 gal/day drain, 20.00 gal/day feed, and 5,475 gal per year of drain water.
A system makes 2 gal per day at a 1:1 ratio. What are the efficiency and feed volumes?
50 percent efficiency, 2.00 gal/day drain, 4.00 gal/day feed, and 730 gal per year of drain water.
Questions
What is a good reverse osmosis waste ratio?
Newer residential systems often target 1:1 or better. Older systems commonly run 2:1 to 4:1. The right figure depends on the membrane, the feed water, and the system design.
Does a lower waste ratio always save money?
It saves water, but a system pushed below its design concentrate flow can foul the membrane faster. Replacement membranes can cost more than the water saved.
Why does my system waste more in winter?
Cold water is more viscous and passes through the membrane more slowly. Many systems then send a larger share to the drain to protect the membrane.
Can I reuse the drain water?
Some homes route it to a garden, a toilet tank, or a laundry line. Check local plumbing rules and avoid using it for drinking or cooking.
How do I measure the ratio at home?
Run the system for a few minutes, then measure the product flow and the drain flow with a container and a timer. Divide drain volume by product volume.
Does the calculator include the water in the storage tank?
No. It estimates flow at the drain and the feed. Tank size affects how often the system runs, not the ratio while it is running.
Sources
- EPA WaterSense, Reverse osmosis systems specification
- EPA WaterSense, Reverse osmosis systems mini report
- NSF, NSF/ANSI 58 reverse osmosis systems