Brushing with a Cup Instead of a Running Tap: Water Saved per Person and the Rinse Volume That Still Clears Paste

Toothbrushing advice assumes a running tap and never prices the water. This study compares a running-tap routine (6 litres per minute for 45 seconds, 4.5 litres per brushing) with a single 200 mL cup, and checks the cup against a stated dilution model of paste clearance in which the residual fraction after rinsing with volume V is exp(-V/60). The cup saves 4.3 litres per brushing, 3139.0 litres per person per year at two brushings daily, and 15695 litres per year for a household of five. The volume needed to reach a 5 percent residual is 179.7 mL, and a 200 mL cup leaves a residual of 3.57 percent. The cup method is a large saving with no modelled hygiene penalty. This study is simulated, not measured. SIMULATED, NOT MEASURED.