Desired over have calculator
Enter the ordered dose, then read the other two numbers off the medication label. On a label that says 250 mg in 5 mL, the dose on hand is 250 mg and the volume on hand is 5 mL.
To try it, set the ordered dose unit to g and enter 0.5, then enter 250 for the dose on hand and 5 for the volume on hand. The answer is 10 mL.
Read D from the order, and H and Q from the label
D is the desired dose, the one the prescriber ordered. H is the dose on hand: how much drug the label puts in one tablet or in the stated volume. Q is the quantity that holds H: one tablet, or the volume on the label.[1] D ÷ H × Q gives one dose, as the Illinois Community College Board's healthcare math module teaches it.[1]
On a bottle labeled 250 mg in 5 mL, H is 250 mg and Q is 5 mL.
With tablets, Q is one tablet, so D ÷ H is the number of tablets: an order for 30 mg with 60 mg tablets is half a tablet.[1] This calculator answers in mL, so use it for liquids.
Put both doses in the same unit before you divide
The ordered dose and the dose on hand must be in the same unit,[1] so an order in grams and a label in milligrams need a conversion first. The metric steps are 1 g = 1,000 mg and 1 mg = 1,000 mcg.[2] The calculator converts the ordered dose into the dose on hand's unit and shows that step first.
If the order is in units, the label has to be in units too: the calculator doesn't convert units to mcg, mg, or g. Write the word "units" in full. ISMP lists "U" as error-prone because it has been misread as a zero or a 4.[3]
Five worked examples you can enter in the calculator
These are made-up exercise values, with the steps the calculator shows; in the steps, / means divide and x means multiply. Type any row into the calculator and you get the same hundredths answer.
| Ordered | On hand | Steps | Answer |
|---|---|---|---|
| 500 mg | 250 mg in 5 mL | 500 mg / 250 mg = 2; 2 x 5 mL = 10 mL | 10 mL |
| 0.5 g | 250 mg in 5 mL | 0.5 g = 500 mg; 500 mg / 250 mg = 2; 2 x 5 mL = 10 mL | 10 mL |
| 250 mcg | 0.5 mg in 2 mL | 250 mcg = 0.25 mg; 0.25 mg / 0.5 mg = 0.5; 0.5 x 2 mL = 1 mL | 1 mL |
| 40 units | 100 units in 1 mL | 40 units / 100 units = 0.4; 0.4 x 1 mL = 0.4 mL | 0.4 mL |
| 2 mg | 3 mg in 1 mL | 2 mg / 3 mg = 0.666666666667; 0.666666666667 x 1 mL = 0.666666666667 mL | 0.67 mL (under 1 mL, so hundredths) |
Round once, at the final answer
Don't round until the final answer, and follow your agency's rounding policy.[2] Open RN says an adult's oral liquid dose over 1 mL is typically rounded to the tenth. A dose under 1 mL is rounded to the hundredth.[2]
The calculator shows both answers, so pick the right one. Under 1 mL, use the hundredths answer: 0.67 mL, not 0.7 mL. Over 1 mL, an adult's oral liquid dose typically uses the tenths answer. For a child, Open RN asks for as much precision as possible, and its worked examples for children use the hundredths answer.[2]
Answers keep a leading zero and drop trailing zeros, so the calculator shows 0.5 mL (never .5 mL) and 1 mL (never 1.0 mL), as Open RN's decimal rules ask.[2] ISMP lists both as error-prone: a missed decimal point turns .5 mg into 5 mg and 1.0 mg into 10 mg.[3]
Three mistakes that give the wrong volume, and one check that catches them
All three use the same problem: 500 mg ordered, and a label that says 250 mg in 5 mL. The right answer is 10 mL.
- If the dose on hand goes on top, 250 ÷ 500 × 5 mL gives 2.5 mL, a quarter of the right answer. The desired dose always goes on top.
- If the same order is written 0.5 g and you don't convert it, 0.5 ÷ 250 × 5 mL gives 0.01 mL, a thousandth of the right answer. Convert 0.5 g to 500 mg first.
- If you stop at D ÷ H, you get 2, which would be the answer for 250 mg tablets. This is a liquid, so multiply by the 5 mL on the label.
To check any answer, ask whether it makes sense. The order is twice the 250 mg in each 5 mL, so the answer should be twice 5 mL, which is 10 mL. Open RN sets evaluating the final answer as a learning goal for safe medication administration.[2]
Practice dose math in MagstoneRN
MagstoneRN includes dose math with units at every step, 5,000+ flashcards with scheduled reviews, and 4,000+ practice questions with an explanation for every answer choice. See everything that's included or browse subjects and question types.
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