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), paired with a 3.2-gallon storage tank.
| Daily RO water demand | 12 gallons |
|---|---|
| Real-world membrane output | ~50% of rated (at 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
We estimate your household's daily reverse-osmosis water use (drinking, cooking, coffee, ice), then size the membrane against real-world output: a membrane's rated GPD is measured at 77°F and 60 psi, but a typical home gets roughly half that. A safety margin keeps the storage tank ahead of peak demand. We then pick the smallest standard membrane that comfortably keeps up.
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 estimates your household's daily drinking and cooking water, applies that real-world output factor plus a safety margin, and picks the smallest standard membrane that comfortably keeps the tank full.
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.