Multiply the weight in teragrams by 10^(-15). There are 10^15 milligrams in one teragram. For example, if the weight of the material is 2.3 teragrams, then the weight in milligrams would be 2.3e-15.
Ensure that the volume of the solution is in liters. Use an online volume converter if it is not.
Divide the weight of the material in milligrams by the volume of the solution in liters to yield the concentration. For example, if the volume was .0003 liters, then the concentration would be 2.3e-15/.0003, or 7.66e-12 mg/L.