# How to calculate ratio of ppm (parts per million)?

If have a container with a capacity of 5,000 cubic inches, how do I calculate the ratio of 50 ppm? I would describe the problem as,

1.) statements,

a/b = 50/1,000,000
a = ?
b = 5,000


Establish the ratio of the ppm ratio and "to be scaled"* volume

50/1,000,000 = a/5,000


Cross multiply,

a1,000,000 = 50*5,000
a = .25


The fraction of liquid (a) to liquid (b) is
0.25 cu. in. of liquid (a), to 5,000 cu. in. of liquid (b).


4.) Check my work,

(0.25/5,000) = 0.00005 = (50/1,000,000)


Is this correct? Is there a simpler way?

* I'm making this ratio fraction problem simple and not using the "total" volume.

-

Ah, but you are using the "total" volume, and in fact, what you have called $B$ is this total volume, and it is not, as you seem to be thinking, the volume of the second liquid. If it is, then you now have a total liquid volume in the container of 5000.25 cu.in., and have apparently have spilled a little.
However, I think your overall approach is correct, you've just mis-interpreted your results. There are 0.25 cu.in. of liquid $A$, 4999.75 cu.in of liquid $B$, for a total volume of 5000 cu.in (with liquid A occupying 50 ppm.)