How do you convert percentage to mg per kg?

1 part per million (ppm) is the same as 1 mg/kg and mg/L

1 mg=0.0001 g

1 kg=1000 g

1 mg/ 1 kg = 0.0001/1000 = 1/1,000,000

and,

1% = 0.01 = 1/100

so,

to get from 1% (1/100) to 1 mg/kg (1/1,000,000), you must multiply by 10,000 (invert)

this means 1% = 10,000 ppm or mg/kg or mg/L