If you are converting the mass of a substance from milligrams/kilograms (mg/kg) to parts per million (ppm), then the ratio is 1:1, and no conversion is necessary. Therefore, 1 mg/kg is the same as 1 ppm because 1 milligram is equal to 0.001 grams while 1 kilogram is equal to 1,000 grams. So, if you divide 0.001 by 1,000, then the result is 0.000001, which is a millionth of a gram, making it the same as 1 part per million.
This logic only works for mass conversions. If you are attempting to convert mg/kg of volume or moles to ppm, then the substances being used will change the outcome. In the case of both volume and moles, ppm is found by using the formula c/s where "c" is the volume or moles of the compound divided by "s," which is the volume or moles of the solution.