RO Water Check

RO Water Check is a free reverse-osmosis TDS rejection calculator and a short set of guides for people who own an under-sink or similar RO system and want to interpret a handheld TDS meter before they buy a new membrane.The calculator uses feed-water TDS and product-water TDS to estimate percent rejection. It lets you note a mineral or alkaline stage and a first-cup versus flushed sample, because those two choices change the reading without meaning the membrane has failed. A first glass often reads high because of TDS creep after the system sits unused. A remineralization cartridge can add 15 to 80 or more ppm on purpose. Treating those numbers as "the membrane is dead" is a common and expensive mistake.Companion pages walk through the usual follow-up questions. One explains TDS creep and remineralization, including why a coffee-water target can be higher than a straight RO reading. Others cover continuous drain flow and weak faucet pressure, filter and membrane replacement cost over a year and five years, membrane versus pre-filter decisions, fridge hookups and air-gap leaks, and how a softener or well changes what TDS can and cannot tell you. A methodology page states the rejection formula, meter conversion notes, and the difference between an NSF/ANSI 58 75 percent floor and a 90 percent-plus field benchmark, with sources.RO Water Check does not sell filters and does not diagnose a specific install. It is an independent educational tool, free to use in the browser with no account, and it offers an optional embed for a related guide or shop page. The site is maintained by Jerry Chen and updated as its sources change.Who it is for: owners of under-sink RO systems who want to know whether a high TDS reading is a real problem, buyers comparing membrane and filter costs, and anyone using a handheld meter for the first time. The guides are short, plain-language and linked to published standards and water-treatment sources so readers can check every figure themselves.