Reverse Osmosis System Size Calculator
Find the right RO membrane and tank size for your household in seconds — sized on real-world output, not lab numbers.
For 4 people (drinking & cooking), with a 3.2-gallon tank — sized so the tank stays ahead of peak use, not by daily volume.
| Typical daily RO water use | ~6 gallons |
|---|---|
| Why this size | tank refill speed at peak draw |
| Real membrane output | ~50% of the GPD rating (typical temp/pressure) |
| Recommended membrane | 75 GPD |
| Recommended storage tank | 3.2-gallon |
75 GPD RO System
Best for 4–5 people
A higher-output membrane that refills the tank faster, so a typical family isn't waiting between glasses. The most common size for a household of four.
How this is calculated
A household only uses a few gallons of RO water a day, so the membrane size isn't about daily volume — it's about how fast the tank refills at peak draw (several people filling bottles at once). Bigger households need a faster membrane. Note that a membrane's rated GPD is measured at 77°F and 60 psi; a typical home gets roughly half that. We map your household size and usage to the standard tiers (1–3 people → 50 GPD, 4–5 → 75, 6 → 100).
How RO sizing works
Reverse osmosis membranes are rated in gallons per day (GPD), but that figure is measured in lab conditions — 77°F water at 60 psi. A real home is usually cooler and lower-pressure, so you get roughly half the rated output. Size on the label number and you'll be waiting on an empty tank; size on real output with a little margin and the system keeps up.
This calculator maps your household size and usage to the right membrane tier — sized by how fast it must refill the tank at peak draw, then sanity-checked against real-world output (about half the rated GPD).
Common RO membrane sizes
| 50 GPD | 1–3 people · drinking & cooking |
|---|---|
| 75 GPD | 4–5 people |
| 100 GPD | 5–6 people · heavy use |
| 150 GPD | large homes · high demand |
Frequently asked questions
What size reverse osmosis system do I need?
Match the membrane's gallons-per-day (GPD) rating to your household's daily drinking and cooking water, with margin. As a rule of thumb: 1–3 people → 50 GPD, 4–5 people → 75 GPD, 6+ or heavy use → 100 GPD. The calculator above tailors it to your usage.
Does a 75 GPD membrane really make 75 gallons a day?
No — that rating is measured at 77°F and 60 psi. A typical home runs cooler and lower-pressure, so real output is roughly half the rating. That's why we size against real-world output, not the label number.
What size RO storage tank should I get?
Most under-sink systems use a 2.8–4 gallon tank (which holds ~2–3 gallons of usable water). Larger families or higher-GPD membranes benefit from a 4-gallon or dual-tank setup so you don't run the tank dry.
Do more people just mean a bigger membrane?
Mostly. A higher-GPD membrane refills the tank faster so it keeps up with demand. Above ~6 people or heavy use, a permeate pump or a second tank helps more than going to an oversized membrane alone.