2 Medication Dosage Calculations

Learn to convert a prescribed medication dose into an amount to administer, check units and product instructions, and document the result safely.

Read the order and label

Medication calculations convert a prescribed into the amount available to administer, such as tablets or milliliters. Use the units on both the order and the product label, and arrange the calculation so units that are not needed cancel. is one reliable method: multiply the ordered by conversion factors until only the desired administration unit remains.

Identify the information and units

Before calculating, identify the medication, prescribed , route, and available strength or . For injectable products, also check the vial strength, route, diluent and directions, final , and relevant storage instructions.

Keep units explicit. Common examples include milligrams $(\text{mg})$ , micrograms $(\text{mcg})$ , grams $(\text{g})$ , milliliters $(\text{mL})$ , and tablets. Convert units before calculating if the order and label use different units. For example:

1\,\text{g} = 1{,}000\,\text{mg}

For a reconstituted medication, do not assume that the added diluent volume equals the final volume. Use the stated for the prepared product.

Set up and check the calculation

A repeatable method keeps the calculation focused on the amount and unit to administer:

State what you need to find, including its unit.
Write the prescribed and the or .
Set up the calculation so matching units cancel.
Calculate, then check that the answer has the requested unit and is reasonable.
Round only as directed by policy or the medication instructions. Do not split a tablet unless it is appropriate to split that product.

is one reliable way to arrange the and conversion factors. Multiply until only the desired administration unit remains.

Calculate tablet amounts

For a tablet order, multiply the prescribed by a conversion factor that relates the labeled amount to one tablet. The milligram units cancel, leaving tablets.

Example: The order is $500\,\text{mg}$ by mouth, and the label states $250\,\text{mg}$ per tablet.

500\,\text{mg} \times \frac{1\,\text{tablet}}{250\,\text{mg}} = 2\,\text{tablets}

The calculated amount is $2$ tablets per .

Calculate liquid amounts

A liquid label commonly gives a as an amount of medication per volume. Arrange the factors so the units cancel and the answer is in milliliters.

Example: The order is $250\,\text{mg}$ by mouth, and the label states $125\,\text{mg}$ per $5\,\text{mL}$ .

250\,\text{mg} \times \frac{5\,\text{mL}}{125\,\text{mg}} = 10\,\text{mL}

The calculated amount is $10\,\text{mL}$ per . Measure oral liquids with a metric dosing device; avoid household spoons because they can cause measurement errors.

Calculate injectable amounts

For a ready-to-use injection labeled $100\,\text{mg}/\text{mL}$ , an order for $75\,\text{mg}$ gives:

75\,\text{mg} \times \frac{1\,\text{mL}}{100\,\text{mg}} = 0.75\,\text{mL}

For a powder that must undergo , follow that exact product's instructions and use the resulting on the label. Do not calculate from the amount of diluent added alone.

Example: A hypothetical vial's instructions state that the prepared solution contains $250\,\text{mg}/\text{mL}$ . The order is $375\,\text{mg}$ IM.

375\,\text{mg} \times \frac{1\,\text{mL}}{250\,\text{mg}} = 1.5\,\text{mL}

The calculated volume is $1.5\,\text{mL}$ . This example is for calculation practice only; actual directions and concentrations vary by product.

Verify and document safely

A correct arithmetic result does not by itself establish that an order is clinically appropriate. Verify the order, patient, route, product, , and calculated amount against the medication label and applicable drug reference or protocol. If information is unclear, units do not match, or the result seems unusual, stop and clarify it with the appropriate clinician or pharmacist.

Document the amount with its unit, such as $2$ tablets, $10\,\text{mL}$ , or $0.75\,\text{mL}$ , and follow local documentation requirements. Use a leading zero for amounts below one, such as $0.5\,\text{mL}$ , and avoid a trailing zero: write $5\,\text{mg}$ , not $5.0\,\text{mg}$ , to reduce the risk of misreading.

Practice the calculations

Use the same unit-cancellation approach to solve these examples:

Order: $400\,\text{mg}$ . Available: $200\,\text{mg}$ per tablet. How many tablets?
Order: $300\,\text{mg}$ . Available: $150\,\text{mg}$ per $5\,\text{mL}$ . How many milliliters?
Order: $60\,\text{mg}$ by injection. Available: $30\,\text{mg}/\text{mL}$ . How many milliliters?

The answers are $2$ tablets, $10\,\text{mL}$ , and $2\,\text{mL}$ , respectively.