Translate every claim into one sentence: “for each gallon purified, how many gallons go to drain?” That removes the ambiguity.
Pure-to-drain notation
If a manufacturer says 3:1 pure-to-drain, three units of purified water are produced for one unit sent to drain. That is relatively efficient.
Drain-to-pure notation
A traditional statement such as “3 gallons waste for 1 gallon product” describes the opposite relationship. Written casually as 3:1, it can look identical to the efficient claim even though it means something very different.
Convert to recovery percentage
A 1:1 pure-to-drain relationship is about 50% recovery. A 3:1 pure-to-drain relationship is about 75% recovery before considering flush cycles.
Use ratio as one buying factor
Higher recovery is useful, but contaminant performance, filter cost, membrane life and reliability still matter more than winning one efficiency metric.