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120 ppm = 1 lb/1000 gallons By James McDonald, PE, CWT Originally Published: CSTN – March 2005 Rule of Thumb: One pound of anything added to 1,000 gallons of water will give you 120 ppm of that substance. This is a very handy rule of thumb, and it is useful to understand why this is true. We first need to convert 1 pound in 1,000 gallons to parts per million or pounds per million pounds. The density of water is 8.335 pounds per gallon. 1,000 gallons * 8.335 lb/gallon = 8,335 lb of water Now we can restate 1 pound in 1,000 gallons as 1 pound in 8,335 pounds. How can we convert this to pounds per million pounds? We need a number that we can multiply 8,335 by to get 1,000,000. 1,000,000 ÷ 8,335 = 119.976 This 119.976 can be rounded up without much error to 120. With this, we can now multiply the numerator and denominator of our 1 pound in 8,335 pounds ratio by 120 to turn it into 120 pounds per 1,000,000 pounds. This is 120 ppm. (1 lb / 8,335 lb) * (120/120) = 120 lb/1,000,000 lb = 120 ppm We have just proven the rule of thumb that 1 pound of anything in 1,000 gallons of water gives 120 ppm of that substance. This relationship can be manipulated to give the equation: pounds = ppm 120 Volume 1,000 This is a very useful equation that can be also used to find the volume of the system if pounds of substance added and resultant ppm are known (e.g., adding nitrite to a system and measuring the change in nitrite concentration). Calculations 413