The question this checklist answers
Why can two sensible-looking estimates disagree? Usually they describe different use patterns, units or fixture modes. Before treating a smaller total as a product advantage, make the comparison conditions explicit.
1. Write down what is inside the estimate
The household tool includes showers and toilet flushes. A household meter also sees other uses. A difference between the tool and a bill does not prove that a source rating is wrong, nor does this limited model estimate all indoor demand.
2. Hold one set of conditions constant
Suppose A is 2.0 GPM for eight minutes and B is 1.8 GPM for ten minutes. A uses 16 gallons and B uses 18 gallons per shower. The lower rating produces a higher scenario total because the use duration changed. To examine the rating alone, set both durations to eight minutes: B then gives 14.4 gallons.
3. Label rating, assumption and observation separately
A directory maximum is a source fact. Applying it as constant installed flow is an assumption. A measured flow under stated conditions is an observation. Preserve these labels in your notes; do not report an assumed result as a field measurement.
4. Count the correct flush mode
For a full/reduced pair of 1.6/1.1 GPF, two full and three reduced uses give 6.5 gallons. Multiplying 1.1 by all five uses gives 5.5 gallons and omits one gallon from the full-mode uses. A dual label alone does not determine the daily mix.
5. Decide what remains unanswered
These calculations cannot establish comfort, pressure response, compatibility or current certification. If one of those is your decision criterion, obtain the relevant current manufacturer or certification evidence rather than extending a GPM comparison beyond its scope.
A comparison you can reproduce
Record the source model or chosen value, its unit, the duration or mode counts, the period multiplier and what is excluded. For a current listing, use the official EPA WaterSense product search; for the saved data and calculation rules used here, read our public source and method explanation. Run the two scenarios, then use the flow-rate research or the flush-volume research to check the meaning of the inputs.
Source: EPA WaterSense product directory, saved 30 October 2026; this analysis uses the fixed catalog release described in Sources and research methods. These historical observations do not establish current certification or product availability. Fixture Flow Atlas is independent of EPA; no affiliation or endorsement is claimed.