120 ppm = 1 lb/1000 gallons

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
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 =
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).