Everyone gets very confused when they try to learn how to convert from $$\textrm{g}/\textrm{l}$$ or $$\textrm{mg}/\textrm{l}$$, or $$\textrm{mg}/\textrm{g}$$, or something similar, to $$\textrm{ppm}$$. However, the process is almost identical to finding a percentage. To find a percentage (parts per hundred), you take the part, and the total and do the following:

$\textrm{percentage of part in total} = \frac{\textrm{part}}{\textrm{total}}\times 100~\%$

To find ppm (parts per million), you take the part, and the total and do the following:

$\textrm{parts per million} = \frac{\textrm{part}}{\textrm{total}}\times 1~000~000~\textrm{ppm}$

Notice any similarities there?

For example, suppose after a fire we have $$1~\textrm{g}$$ of carbon in every litre of water in some pond. (I don’t know if this is realistic… =P) We first convert everything to the same units. $$1~\textrm{litre}$$ of water is approximately $$1000~\textrm{g}$$. Now, we can find the percentage of carbon in the water:

$\textrm{percentage} = \frac{1~\textrm{g~C}} {1000~\textrm{g~H_{\textrm{2}}O}} \times 100~\% = 0.1~\%$

And the ppm of carbon in the water:

$\textrm{ppm} = \frac{1~\textrm{g~C}} {1000~\textrm{g~H_{\textrm{2}}O}} \times 1~000~000~\textrm{ppm} = 1000~\textrm{ppm}$

Simple right?