Formulas for Calculating the Desired Dose

In the ideal world, the prescriber will write a medication prescription that has a dose that is available in the hospital. For example, the medication prescription is for a 15-mg tablet of Inderal and the hospital has on hand a 15-mg tablet of Inderal.

In the real world, the dose specified in the medical prescription may not be available. The hospital might have 10-mg tablets of Inderal and not the 15-mg tablets prescribed. Instead of asking the prescriber to change the medication order, the nurse calculates the proper medication to give the patient based on the medication order and the dose that is on hand.

In this example, the nurse calculates that the patient should receive 1.5 tablets of Inderal. This means that the nurse must divide one tablet into halves.

There are two methods nurses can use to calculate the desired dose. These are the formula method or the ratio-proportion method. Either method will produce same result. When applying either method, make sure that all the terms are in the same units before calculating the desired dose. For example, the medication order might be in grams and the dose on hand might be in milligrams. The nurse will need to convert the grams to milligrams before calculating the desired dose to give. Always convert to the unit of the "have" dose.

The formula method uses the following formula to determine the correct dose.

x V = A Quantity (Desired dose divided by dose you have multiplied by vehicle of drug you have equals the amount calculated to be given to the patient)

D = desired dose

H = dose you have

V = vehicle you have (tablets or liquids)

A = amount calculated to be given to the patient

Ratio and proportion method

Means Extremes

Means Extremes

H is the drug on hand (available) V is the vehicle or drug form (tablet, capsule, liquid) D is the desired dose (as prescribed) x is the unknown amount to give, and :: stands for "as" or "equal to."

Multiply the means and the extremes. Solve for x; x is the divisor.

Example: Give 500 mg of ampicillin sodium by mouth when the dose on hand is in capsules containing 250 mg.

500 mg divided by 250 mg multiplied by 1 capsule = 2 capsules

Formula method:

250 mg 1

Example: Give 375 mg of ampicillin when it is supplied as 250 mg/5mL. 375 mg divided by 250 mg multiplied by 5 mL = 7.5 mL

Example: Diltiazem (Cardizem) 60 mg PO b.i.d. On hand are 30 mg tablets.

60 mg

30 mg

Ratio and Proportion Method: Prescribed: Amoxicillin 100 mg PO q.i.d. Available: Amoxil (amoxicillin) 80 mL

Extremes

Extremes

Answer: Amoxicillin 100 mg = 2 mL

Prescribed: Aspirin/ASA gr x, q4h, PRN for headache

Available: Aspirin 325 mg/tablet a. Convert to one system and unit of measure. Change grains to milligrams

x = 1.8 tablets or 2 tablets (round up or use 650 instead of 600)

Be sure to label all the terms in the formula with the appropriate unit. This will help to prevent errors. For example, use mg following a value in the formula if the value is in milligrams.

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