Divide the number of milligrams by 1,000,000,000 to convert to metric tons. For example, if you have 500,000,000 mg, divide 500,000,000 by 1,000,000,000 to get 0.5 metric tons.
Multiply the number of milligrams by 0.000000001 to convert to metric tons. For this example, multiply 500,000,000 by 0.000000001 to confirm your answer of 0.5 metric tons.
Check your answer using an online milligrams-to-metric tons converter (see Resources). Enter the number of milligrams and press "Go."