Every certified water operator on the continent can recite it half-asleep: $\text{lb/day} = \mathrm{mg/L} \times \mathrm{MGD} \times 8.34$ — read aloud: pounds per day equals milligrams per litre times M-G-D times eight-point-three-four. MGD is million US gallons per day, the flow unit of the trade, and 8.34 is what one gallon of water weighs in pounds.

Why it works is lesson one wearing work boots: mg/L is parts per million, a million gallons of water weighs 8.34 million pounds, so a dose of C ppm of that flow is C × 8.34 pounds — per MGD, per day. This formula lives natively in US units; that is not an accident, it is the certification exam's home address. Chant it forward for the feed, chant it backward for the dose.