Foremost. Using just one gallon of water, the All in One 1-gallon high efficiency toilet (HET) can flush twice the average amount of solid waste, up to 500 grams, the maker says. One gallon per flush (gpf) is 37% less than standard 1.6 gpf toilets on the market, and 21% less than 1.28 gpf HETs. The toilet comes in white or soft white; special order bone or biscuit. Round or elongated bowls are available. 800.443.1410.