Medication Dosage Calculation

Often, when the doctor orders a medication and requests that the patient receives the initial, or whole dose before leaving the facility it is the medical assistant’s responsibility to it draw up, or count out the right amount of pills or drops and administer it, followed by charting the process into the patient’s medical chart.

Also, there are patients that come to the medical office for the sole purpose of receiving a single, or regular scheduled dose of a prescribed medication, such as allergy shots, Vitamin B 12 injections, flu shots, without seeing the doctor that day. This will require a standing prescription order in the chart form the doctor, and again, is usually carried out by the medical assistant.

Always check a medication’s actions, side effects and ask the patient about any allergies prior to administering or dispensing it. If you, in your role as a medical assistant administer or dispense it you may be held partially or fully responsible if something goes wrong, even though the supervising physician under whom you work ordered it. Always make sure that the bottles are labeled clearly and that you understand the dose, frequency and duration of the drug. Also, don’t forget to check the expiration date.

drawing from vial
drawing 1 cc from vial


If you are trying to find out what a milligram actually might look like, take a raisin, cut it into 1000 equal parts. There! Each little part will weigh about 1 milligram. There are 453,592.37 milligrams in a pound. The fact that most drugs are measured in milligrams should alert you to realize that sometimes the most miniscule amounts of a substance can be very powerful. Label instructions should be followed very carefully.

Factors that Affect a Drug’s Effect:

The two primary factors that determine or influence the dose are age and weight; but there are more:

  • Age
  • Weight
  • Gender
  • Time of administration
  • Immune response
  • Tolerance
  • Accumulation
  • Pathological factors
  • Psychological factors

Other factors:

  • Genetic make-up (nationality, ethnic heritage)
  • Occupation
  • Habitual use
  • Frequency of administration
  • Mode of administration


Problem is, sometimes the dosage or medication’s strength is not the same as the strength that you have on hand. In other words the doctor orders 500 mg of a certain medication in tablet form, you go to get the medication storage area and find it on the shelf, but when you check the label it is not the exact same strength as ordered. The only tablets you have on hand are 250 mg strength.

What to do next? The answer is rather simple, you must calculate the correct dose! To calculate the correct dose, you need to use the correct formula. The following dosage calculation formula  is easy to remember and if used properly it delivers the correct result every time:

Desired dose

x quantity of on-hand dose = desired dose (That’s: Desired dose divided by on-hand strength, multiplied by the quantity of on-hand dose (e.g.. 1 tablet), equals desired dose)

On-hand strength

For example, if a physician orders 500 mg of ibuprofen for a patient (which is the desired dose,) and you have 250 mg tablets (1 tablet = 250 mg) on-hand, the medical caregiver needs to dispense two tablets, because 500 divided by 250, then multiplied by 1 (one tablet) equals the desired dose, which comes to 2 (two tablets).


x 1 (tablet)
= 2 tablets
(That’s: 500 divided by 250, multiplied by 1 tablet, equals 2 tablets)


This formula works with any other type and strength of medication, whether tablets, suppository, or liquid (drops, suspensions, syrups etc.)

Had it been the other way around and the doctor had ordered 250 mg of ibuprofen but all you had on hand was 500 mg tablets, your calculation would look like this:


x 1 (tablet)
= 0.5 tablets
(That’s: 250 divided by 500, multiplied by 1 tablet, equals half of a tablet)


It even works for a liquid medication, for example where 1 cc (liquid) delivers 500 mg of a drug. If the doctor ordered 1500 mg of the drug you calculate:


x 1 cc = 3 cc (That’s: 1500 divided by 500, multiplied by 1 cc, equals three cc liquid)


 Remember: 1 cc is the exact same amount as 1 ml!

Primark Rotterdam

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