What GPD do I need for a family of 4?
Typically 600–800 GPD for drinking and cooking. The honest test is flow at your faucet: if a glass fills in seconds, you have enough.
Four things matter. Everything else is marketing.
Most RO buying guides are written by someone selling you an RO. This one isn't trying to close you — here's what actually matters, in order:
The single most expensive part of owning an RO isn't the machine — it's the replacement filters you'll buy for the next decade. Systems that use standard 10-inch × 2.5-inch cartridges let you buy refills from any supplier, typically at a third of branded prices. Systems with proprietary cartridges lock you into one seller at whatever price they choose.
Ask one question before anything else: "Can I buy replacement filters from someone other than you?" If the answer is no, keep shopping.
GPD (gallons per day) is the membrane's rated output under test conditions. Real-world guidance:
| Household | Sensible GPD |
|---|---|
| 1–2 people, drinking + cooking | 400–600 GPD |
| 3–5 people, heavy cooking | 600–800 GPD |
| Small office / light commercial | 800+ GPD |
Bigger isn't automatically better — an oversized membrane on low feed pressure underperforms. Match the membrane to your water pressure (see our booster pump guide).
Two NSF labels matter for RO:
A filter can work fine without certification, but certification is the only independent proof of a claim. Treat uncertified performance claims as opinions.
Add it up: machine + (yearly filter cost × 5). A $150 machine with $120/year proprietary filters costs $750 over five years. A $189 machine with $40/year universal filters costs $389. The sticker price is the least informative number on the page.
We won't tell you our system is "the best" — best depends on your water, your pressure, and your budget. We will tell you exactly what ours costs to run for five years, and you can verify every number. That's the whole pitch.
Typically 600–800 GPD for drinking and cooking. The honest test is flow at your faucet: if a glass fills in seconds, you have enough.
The filtration media (spun polypropylene, carbon block, TFC membrane) comes from the same handful of manufacturers worldwide. What differs is the plastic housing's branding and the price. Certification (NSF/58, NSF/42) is the way to compare claims objectively.
It's the only independent proof of performance claims. Uncertified filters can work well, but you're taking the seller's word for it.
Tankless (like high-GPD direct-flow systems) gives fresher water and saves under-sink space, but needs good feed pressure. Tank systems work on lower pressure but the stored water can taste stale.
With universal 10-inch cartridges, typically $30–60/year depending on your water. With proprietary cartridges, often $100+/year. Multiply by five years before you buy.