Take the required dosage and divide it by the stock dosage. For instance, say you have 100 mg of a substance that is being diluted in 200 mL of fluid.
100 mg / 200 mL = 0.5 mg/mL
Multiply the number obtained in Step 1 by 1000 to convert the amount in milligrams to micrograms. So, for example:
0.5 mg/mL x 1000 = 500 mcg/mL
Take the number obtained in Step 2 and divide it by 60. Because there are 60 minutes in 1 hour, this number gives you the amount administered per minute.
500 / 60 = 8.33
Take the number obtained in Step 3 and multiply it by the required volume per hour in milliliters. For instance, say you need to administer a solution at 6 mL per hour:
8.33 x 6 = 50 mcg/minute