Reverse osmosis
Reverse osmosis membrane output chart: rated GPD vs real output
Quick answer
A reverse osmosis membrane rated 75 GPD is tested at 60 psi and 77°F; at a more typical 50 psi and 60°F household condition it produces roughly 31 GPD, and at 40 psi and 50°F on a cold well it can fall to about 10 GPD. Feed pressure below 40 psi starves most systems entirely.
Almost every complaint that a reverse osmosis system "does not make as much water as advertised" traces back to one fact printed nowhere near the headline gallons per day number: that number is a laboratory ceiling, measured at a specific pressure and temperature, not a household guarantee. Real feed water is very often colder and lower pressure than the test condition, and output falls in both directions at once.
This chart lays out the published test condition, a realistic output grid across household pressure and temperature, recovery and drain ratio, and what a "4 gallon" storage tank actually delivers, which is considerably less than 4 gallons.
None of the figures below are a substitute for your own feed water test. TDS reduction is what a certified laboratory test or a home TDS meter confirms; see the TDS chart for what that number does and does not tell you.
On this page
- The laboratory test condition every rated GPD number comes from
- What a rated membrane actually produces at household pressure and temperature
- Recovery and drain ratio: how much goes down the drain per gallon produced
- Why feed pressure below 40 psi starves most systems
- Tank drawdown: a "4 gallon" tank never delivers 4 gallons
The laboratory test condition every rated GPD number comes from
Every membrane's rated gallons per day figure is measured at 60 psi feed pressure and 77°F, with 250 ppm TDS feed water.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| Test variable | Standard test value |
|---|---|
| Feed pressure | 60 psi |
| Feed water temperature | 77°F |
| Feed water TDS | 250 ppm |
A residential membrane such as the FilmTec BW60-1812-75 rated 75 GPD is rated under exactly this condition. Household feed pressure and, especially in winter, feed water temperature are very often both below the test values, which is the entire reason real output runs lower than the rating.
What a rated membrane actually produces at household pressure and temperature
Output falls with pressure in a close to linear relationship, and falls further as feed water gets colder than the 77°F test temperature. The figures below apply the commonly used correction of roughly three percent output loss per degree below the test temperature, an approximation rather than a specification for any particular membrane, to a typical household condition and a colder well condition.
A 75 GPD rated membrane produces roughly 31 GPD at typical household conditions of 50 psi and 60°F, and can fall to about 10 GPD at 40 psi and 50°F.
Rule of thumb Water treatment trade convention rather than a published standard. Taught everywhere, useful for planning, and not a specification. No standards body publishes it.
| Rated GPD | Typical household (50 psi, 60°F) | Cold well (40 psi, 50°F) |
|---|---|---|
| 50 | 20 GPD | 7 GPD |
| 75 | 31 GPD | 10 GPD |
| 90 | 37 GPD | 12 GPD |
| 100 | 41 GPD | 13 GPD |
| 400 | 163 GPD | 53 GPD |
| 600 | 245 GPD | 80 GPD |
| 800 | 327 GPD | 107 GPD |
This grid applies a widely used approximation for the temperature effect, roughly three percent output loss per degree Fahrenheit below the 77°F test temperature, floored so output cannot go negative. Treat it as a planning estimate of direction and rough size rather than a specification for any particular membrane; every manufacturer publishes its own correction curve.
Recovery and drain ratio: how much goes down the drain per gallon produced
Recovery is the share of feed water that leaves as usable permeate rather than going to drain as concentrate carrying away what the membrane rejected. A conventional tank system runs a deliberately low recovery, since pushing a membrane to a higher recovery than it was designed for accelerates scaling and shortens its life.
A conventional tank system runs about 20 percent recovery, sending 4 gallons to drain for every gallon produced, while a tankless high recovery system cuts that to roughly 1 to 1.
Rule of thumb Water treatment trade convention rather than a published standard. Taught everywhere, useful for planning, and not a specification. No standards body publishes it.
| System type | Typical recovery | Drain ratio |
|---|---|---|
| Conventional tank system | 20% | 4 to 1 |
| Efficient tank system with permeate pump | 33% | 2 to 1 |
| Tankless high recovery system | 50% | 1 to 1 |
Drain water is not simply wasted in the sense people usually mean; it is the concentrate stream carrying away the dissolved solids the membrane rejected. A membrane run at a recovery higher than it was designed for scales and fails early rather than saving meaningful water.
Why feed pressure below 40 psi starves most systems
A conventional reverse osmosis system's automatic shutoff valve trips once tank pressure reaches roughly two thirds of feed pressure, which is a genuine mechanical constraint rather than a rule of thumb someone could ignore. Below about 40 psi of feed pressure, that shutoff point arrives so early that the tank barely fills at all, and output can drop to a trickle even though the membrane itself is working correctly. A booster pump, not a new membrane, is the usual fix for chronically low feed pressure.
Tank drawdown: a "4 gallon" tank never delivers 4 gallons
A reverse osmosis storage tank's nominal size describes the bladder's full volume, not the water it actually delivers before the shutoff valve trips. Usable drawdown runs at roughly 60 to 70 percent of the nominal size, and the air charge behind the bladder has to be set with the tank completely empty for that fraction to hold true.
A nominal 4 gallon tank such as the APEC Tank-4 delivers only about 2.6 gallons of usable drawdown, roughly 65 percent of its nominal size.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| Nominal tank size | Typical usable drawdown |
|---|---|
| 4 gallon | 2.6 gallons |
| 5.5 gallon | 3.6 gallons |
| 14 gallon | 9.1 gallons |
The precharge, the air pressure behind the bladder, has to be set to about 7 to 8 psi with the tank fully drained, not while it is full or partially full. A tank charged at the wrong pressure, or charged while it still holds water, is one of the most common and completely free to fix reasons a system delivers only a trickle.
Frequently asked questions
Why is my reverse osmosis system not making as much water as advertised?
The rated gallons per day figure is measured at 60 psi and 77°F feed water. Household feed pressure and temperature are almost always lower, especially in winter, and output falls with both. A 75 GPD membrane can realistically produce around 31 GPD at typical household conditions and as little as 10 GPD on a cold, low pressure well.
What feed pressure does a reverse osmosis system need to work well?
At least 40 psi for a conventional tank system. Below that, the automatic shutoff valve trips at a fraction of an already low feed pressure, so the tank barely fills regardless of what the membrane itself is capable of. A booster pump is the standard fix for chronically low household feed pressure.
How much water does a reverse osmosis system send to the drain?
It depends on recovery. A conventional tank system runs about 20 percent recovery, sending roughly 4 gallons to drain per gallon of permeate produced. An efficient system with a permeate pump reaches about 33 percent recovery, and a tankless high recovery system can reach close to 1 to 1.
Does a 4 gallon reverse osmosis tank actually hold 4 gallons of water?
No. A "4 gallon" tank describes the bladder's nominal full volume, but the automatic shutoff trips once tank pressure reaches roughly two thirds of feed pressure, leaving usable drawdown at around 60 to 70 percent of nominal, commonly about 2.6 gallons for a 4 gallon tank.
How do I set the air pressure on a reverse osmosis tank?
The precharge needs to be set to roughly 7 to 8 psi with the tank completely empty of water, not full or partially full. A tank charged at the wrong pressure, or charged while water is still inside it, is one of the most common and entirely free to fix reasons a system delivers only a trickle.
Does reverse osmosis remove lead, arsenic and nitrate?
Only when the specific system is certified for that specific claim under NSF/ANSI 58; TDS reduction alone is the core scope and does not automatically include any health claim. Confirming your water actually needs one of these claims, and confirming a system is still delivering it, needs a certified laboratory test, since lead, arsenic and nitrate are not do it yourself topics.
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Researched, not professional advice. This page is compiled from published standards, regulatory limits, manufacturer specifications and owner-review consensus, not hands-on testing. Nothing here diagnoses a water problem or says any product makes water safe to drink. Figures described as a rule of thumb are water treatment trade convention rather than published standards, and they are labeled that way wherever they appear. Test your water before you buy equipment: on city water start with the Consumer Confidence Report your utility publishes each year, and on a private well only a certified laboratory test tells you what you have. Bacteria, nitrate, lead, arsenic and PFAS are not do it yourself topics, they are invisible and tasteless, and they need a laboratory and usually a licensed professional rather than a product. Remember that a certification covers a specific claim under a specific standard, so "NSF certified" with no number attached tells you very little.