In short: RO membranes are rated in gallons per day (GPD) at a standard 77°F feed. Colder water produces less — a 75 GPD membrane at 68°F actually makes about 63 GPD. To size a system, multiply your daily demand by the temperature correction factor for your coldest expected water, then round up to a standard membrane size. Enter your own numbers above for a figure specific to your home.
This is a planning estimate from published temperature correction tables. It does not account for feed pressure, fouling or membrane age — your installer's spec sheet is the final word for a specific product.
How RO sizing is calculated
Every RO membrane is rated in gallons per day at a standard test condition — 77°F feed water at the manufacturer's rated pressure. Real feed water is rarely exactly 77°F, so the rated GPD has to be corrected for your actual temperature using a temperature correction factor (TCF) from the manufacturer's table:
actual GPD = rated GPD (at 77°F) ÷ TCF
To size a system, run it in reverse: minimum rated GPD = daily demand × TCF, then round up to a standard membrane size. Feed flow needed to produce that permeate is demand ÷ recovery; concentrate sent to the drain is feed − demand.
Worked example. A household needing 4 gallons a day of drinking water, with feed water at 68°F (TCF 1.19) and 25% recovery, needs a membrane rated at least 4.8 GPD — comfortably covered by the smallest standard size, 50 GPD. A 75 GPD membrane at that temperature actually produces about 63.0 GPD, and reaching 4 gallons of product takes 16 gallons of feed water, sending 12 gallons to the drain — a 3.0:1 ratio.
Temperature correction factors
All RO membrane elements are rated at 77°F (25°C). Divide the rated GPD by the correction factor at your actual feed temperature to estimate real output.
| °F | °C | TCF | °F | °C | TCF |
|---|---|---|---|---|---|
| 41.0 | 5 | 2.58 | 64.4 | 18 | 1.29 |
| 42.8 | 6 | 2.38 | 66.2 | 19 | 1.24 |
| 44.6 | 7 | 2.22 | 68.0 | 20 | 1.19 |
| 46.4 | 8 | 2.11 | 69.8 | 21 | 1.15 |
| 48.2 | 9 | 2.00 | 71.6 | 22 | 1.11 |
| 50.0 | 10 | 1.89 | 73.4 | 23 | 1.09 |
| 51.8 | 11 | 1.78 | 75.2 | 24 | 1.04 |
| 53.6 | 12 | 1.68 | 77.0 | 25 | 1.00 |
| 55.4 | 13 | 1.61 | 78.8 | 26 | 0.97 |
| 57.2 | 14 | 1.54 | 80.6 | 27 | 0.94 |
| 59.0 | 15 | 1.47 | 82.4 | 28 | 0.91 |
| 60.8 | 16 | 1.39 | 84.2 | 29 | 0.88 |
| 62.6 | 17 | 1.34 | 86.0 | 30 | 0.85 |
Source: representative manufacturer TCF table (Nu-Stream Filtration). Individual membrane products may publish slightly different factors — use your specific product's data sheet when one is available.
Standard membrane sizes & typical uses
| Rated GPD (at 77°F) | Typical use |
|---|---|
| 50 | Single-tap under-sink drinking water |
| 75 | Under-sink drinking water, most common residential size |
| 100–150 | Larger households, multiple taps, or slower feed pressure |
| 200 | Small whole-house or light-commercial use |
| 400+ | Larger whole-house and commercial systems |
What else affects your real-world output
Feed pressure matters too. This calculator corrects for temperature only. Low household water pressure also reduces output — most residential membranes are rated at 60 psi feed pressure, and output drops noticeably below that. If your pressure is under about 40–60 psi, check your membrane's pressure-flow curve or ask your installer whether a booster pump is worth adding.
A storage tank buffers demand. Under-sink systems don't need to produce your full daily demand in one continuous run — they refill a small pressurized tank between uses, so the system can run intermittently as long as its 24-hour capacity, corrected for temperature, exceeds your daily draw.
Fouling and age reduce output over time. Sediment, scale and membrane fouling gradually lower flow between filter changes. Sizing with some margin above your calculated minimum — and keeping filters on schedule — keeps output closer to rated capacity for longer.
Frequently asked questions
Why does cold water reduce RO output?
Colder water is more viscous and resists pushing through the membrane, so less permeate is produced at the same pressure. Membranes are rated at 77°F (25°C); a temperature correction factor from the manufacturer's table tells you the real output at your actual feed temperature.
What GPD rating do I actually need?
Multiply your daily demand by the temperature correction factor at your coldest expected feed temperature, then round up to the nearest standard membrane size (commonly 50, 75, 100, 150, 200 or 400 GPD). Sizing for your coldest season avoids a system that falls short in winter.
What's a normal drain-to-product ratio for RO?
Conventional under-sink systems typically run 3:1 to 4:1 (three to four gallons to the drain per gallon of product water), around 20–25% recovery. EPA WaterSense-labeled point-of-use systems and permeate-pump designs can reach a 2.3:1 ratio or better, sometimes approaching 1:1.
Does higher recovery always mean better water?
No — it's a trade-off. As recovery rises, rejected minerals concentrate more heavily in the reject stream, which raises scaling risk and can push permeate quality down if the membrane and pretreatment aren't matched to that recovery level.
Does this calculator account for feed water pressure?
No — it corrects for feed temperature only, using standard manufacturer temperature correction tables. Low supply pressure also reduces real-world output and isn't modeled here; check your membrane's pressure-flow curve or ask your installer if your household pressure is below about 40–60 psi.
Sources & references
- American Water Works Association (AWWA). Manual M46: Reverse Osmosis and Nanofiltration.
- Reverse osmosis membrane manufacturer temperature correction factor table, representative example via Nu-Stream Filtration.
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)