EPA WaterSense is the federal voluntary water-efficiency certification program. A WaterSense-certified toilet uses 1.28 gallons per flush or less AND meets a minimum flush performance threshold (MaP score 350g+). Buying WaterSense-certified is the simplest way to ensure you're getting a real high-efficiency toilet, not just a label.
A 1.28 GPF toilet that won't reliably flush solid waste isn't actually water-saving — owners double-flush, defeating the savings. WaterSense certification requires the toilet to pass the MaP (Maximum Performance) test, which measures how much simulated solid waste the toilet clears in a single flush. The minimum threshold is 350g; most WaterSense-certified toilets test at 600–1,000g, well above the threshold.
This is the difference between "1.28 GPF marketed water-saving" (could be a poorly-flushing budget toilet) and "1.28 GPF WaterSense-certified" (verified to flush as well as a 1.6 GPF traditional toilet).
Within the WaterSense program, there's a higher tier: WaterSense Most Efficient. To qualify, a toilet must be 1.1 GPF or lower while still meeting the 350g MaP threshold. Models on the Most Efficient list typically include:
The Most Efficient certification often qualifies for higher utility rebates than standard WaterSense.
Look for the blue WaterSense label on the box, in the product description, or on the manufacturer's website. The label is a circular blue logo with a water-drop and the words "WaterSense." If you don't see the label, the toilet isn't certified — regardless of the GPF spec on the box.