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A dosage calculation answers one question: how much do I give? You take the dose ordered, the strength on the label, and the form the drug comes in, and turn them into a number of tablets, milliliters, or units.
This lesson covers the three calculation methods, then applies them to tablets, oral liquids, injections, powders that must be mixed, and drugs measured in units or mEq. If fractions, decimals, or conversions feel shaky, review the earlier lessons in the series first.
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Every problem uses the same three pieces of information.
D (desired) is the dose ordered by the provider, like 500 mg. H (have) is the strength on the label, like 250 mg. Q (quantity) is the amount that contains H, like 1 tablet or 5 mL.
On a liquid label, the strength is a ratio such as “250 mg per 5 mL”: H = 250 mg and Q = 5 mL. Ignore the total amount in the bottle (like “100 mL”); it doesn't change the dose.
Watch out: Using the total volume of the bottle or vial as Q.
D and H must be in the same unit before you calculate. If the order is in grams and the label is in milligrams, convert one of them first.
Learn more: Conversions for nurses
These rules prevent 10-fold and 1,000-fold dose errors.
Write 0.5 mg, never .5 mg. If the decimal point is missed, .5 mg looks like 5 mg.
Write 5 mg, never 5.0 mg. If the decimal point is missed, 5.0 mg looks like 50 mg.
Write 10 units, never 10 U, and mcg, never µg. U can look like 0, and µg can look like mg.
Most oral doses are ½ to 3 tablets or a few mL, and most injections are under 3 mL in one site. An answer of 20 tablets or 0.02 mL is a signal to recheck the setup and the units.
All three give the same answer. Pick the one you understand best, and use another to double-check.
Divide the dose you want by the dose you have, then multiply by the quantity it comes in.
Set the label ratio equal to the ordered ratio, with x for the unknown amount, and cross-multiply.
Start with the unit you want the answer in. Multiply by fractions so every other unit cancels. This method handles conversions in the same line, which makes it the favorite for multi-step problems.
Use the formula with Q = 1 tablet. Only scored tablets can be split, and usually only in half. Capsules, enteric-coated, and extended-release tablets must never be split or crushed. If the answer isn't a whole or half tablet, recheck the math, or ask the pharmacy for another strength.
More practice: Tablet dosage calculation
Use the formula with Q = the volume on the label. Measure with an oral syringe or a medicine cup, not a spoon.
More practice: Oral dosage solution calculation
Your answer has to match a line you can actually measure on the syringe.
| Syringe | Each small line | Use it for | Round answers to |
|---|---|---|---|
| 1 mL (tuberculin) | 0.01 mL | Amounts under 1 mL | hundredths (0.75 mL) |
| 3 mL | 0.1 mL | Most injections, 1 to 3 mL | tenths (1.3 mL) |
| 5 mL and 10 mL | usually 0.2 mL | Larger volumes, flushes | the nearest line |
| Insulin 30 or 50 units (0.3 or 0.5 mL) | 1 unit | Small insulin doses | whole units |
| Insulin 100 units (1 mL) | usually 2 units | Insulin doses over 50 units | even units on most brands |
Use the formula exactly as for liquids, with Q = the volume in the vial's strength. Then choose the smallest syringe that holds the dose, so the lines are as fine as possible.
Watch out: Picking a syringe whose lines can't show your answer, like trying to measure 0.75 mL on a 3 mL syringe.
Some drugs come as a powder and must be mixed with a liquid (the diluent) before you calculate.
The label or package insert tells you which diluent, how much to add, and the concentration you get after mixing, like 250 mg/mL. Use that final concentration as H and Q in your calculation.
The powder itself takes up space, so the final volume is a little more than the liquid you added. That's why you never use the amount of diluent as Q; always use the concentration the label gives.
Watch out: Dividing the vial strength by the diluent volume (1 g ÷ 3.6 mL). Use the concentration printed on the label.
Some drugs aren't measured in milligrams.
Insulin is measured in units. Standard U-100 insulin has 100 units in every mL. Give it with an insulin syringe, which is marked in units, so the units ordered are the units you draw up; no calculation in mL is needed.
Watch out: Using a regular mL syringe for insulin. Always use an insulin syringe that matches the insulin's concentration.
Calculate with units just like milligrams. Spell out “units,” and double-check heparin and insulin with a second nurse if your facility requires it.
Electrolytes such as potassium are measured in mEq. Calculate with mEq the same way.
A percent drug strength means grams per 100 mL. Change it to mg/mL by writing it as grams over 100 mL and converting grams to mg.
Common rules; your program's or facility's policy comes first.
Keep every digit while you work, and round only the final answer. Then round to something you can actually measure and give.
Use any method. Round as the problem says.
Next, doses based on body weight and the special math of pediatrics.
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