Navigate to the Lenntech website (see Resources). Lenntech created a free online converter for ppm conversions.
Select whether your measurement of ppm in volume, moles or weight in the first drop-down menu.
Select whether you measured ppm in air or water in the second drop-down menu.
Select your gas under "Molecular Weight" in the drop-down menu. The site automatically provides the molecular weight of your selected gas to the right of the menu.
Click the button next to "expressed in units of ppm." Next, enter the ppm you want to convert in the window to the right of the button.
Click "Convert." Your converted total in mg/m3 will appear in the "Converted Concentration" row.