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Before you can calculate any dose, the order and the label must be in the same unit. An order in grams and a label in milligrams, a weight in pounds and a dose per kilogram, an infusion time in hours and a drip rate per minute: each one needs a conversion first.
This lesson covers every conversion nurses use: metric units, household measures, weight, length, temperature, 24-hour time, and intake and output. Every conversion works the same way, by multiplying or dividing by a known equivalent, so learn the method once and use it everywhere.
Already know some of this? Skip straight to what you need.
One method works for every conversion on this page.
A conversion factor is an equivalent written as a fraction, like 1,000 mg1 g or 1 kg2.2 lb. Because the top and bottom are equal, the fraction equals 1, so multiplying by it changes the units but not the amount.
Choose the version of the fraction that puts the unit you want to get rid of on the bottom, so it cancels.
Watch out: Using the fraction upside down. If the unit you wanted to cancel is still there, flip the fraction.
Almost every dose is ordered in metric units.
| Prefix | Meaning | Examples |
|---|---|---|
| kilo- (k) | 1,000 times the base unit | 1 kg = 1,000 g |
| (base unit) | gram (g) for weight, liter (L) for volume, meter (m) for length | 1 g, 1 L |
| centi- (c) | one hundredth | 100 cm = 1 m |
| milli- (m) | one thousandth | 1,000 mg = 1 g · 1,000 mL = 1 L |
| micro- (mcg) | one millionth | 1,000 mcg = 1 mg |
Neighboring weight units (kg, g, mg, mcg) are 1,000 apart. To go to a smaller unit, multiply by 1,000 (move the decimal point 3 places right). To go to a larger unit, divide by 1,000 (move it 3 places left). Skipping a unit means doing it twice.
Watch out: Mixing up mg and mcg. They differ by 1,000, so this mistake gives a 1,000-fold error. Write “mcg,” not “µg,” which can be misread as mg.
More practice: Metric units conversion for nurses
A milliliter (mL) and a cubic centimeter (cc) are the same amount, but write mL: “cc” can be misread as “u” (units).
Some medicines are measured in units (insulin, heparin) or milliequivalents (mEq, for potassium). These are not metric and can't be converted to mg; you calculate with them directly.
Used for oral liquids and for recording what patients drink.
| Household | Equals | Metric |
|---|---|---|
| 1 teaspoon (tsp) | 5 mL | |
| 1 tablespoon (Tbsp) | 3 tsp | 15 mL |
| 1 ounce (oz) | 2 Tbsp | 30 mL |
| 1 cup (C) | 8 oz | 240 mL |
| 1 pint (pt) | 2 cups = 16 oz | 480 mL |
Use the same method: multiply by the matching equivalent from the table. These equivalents are approximate, so many hospitals give oral liquids only in mL with an oral syringe or medicine cup.
Watch out: Using a kitchen spoon to measure medicine. Spoons vary widely in size.
When a patient will take a liquid medicine at home, convert the dose to the measuring device they will use, and show them the line to fill to.
Doses for children and many adult drugs depend on weight in kilograms.
1 kg = 2.2 lb. To change pounds to kilograms, divide by 2.2. To change kilograms to pounds, multiply by 2.2. A weight in kilograms is always the smaller number. Round kilograms to the nearest tenth unless your program says otherwise.
Watch out: Using the weight in pounds in a mg/kg calculation. That makes the dose 2.2 times too large.
Infant weights are often given in pounds and ounces. There are 16 ounces in a pound. Change the ounces to part of a pound first, then change pounds to kilograms.
1 inch = 2.54 cm. Height in centimeters is needed for body surface area (BSA) dosing.
Thermometers and orders may use either scale.
Subtract 32 first (that's why it's in parentheses), then multiply by 5/9. Multiplying by 5/9 is the same as multiplying by 5 and dividing by 9.
Watch out: Multiplying before subtracting 32.
Multiply by 9/5 (or by 1.8) first, then add 32.
Medication times are charted in 24-hour time, and infusions need hours and minutes.
The day runs from 0000 (midnight) to 2359. Write four digits with no colon. For AM times, keep the hour and add a leading zero if needed. For PM times from 1:00 PM on, add 12 to the hour. To change back, subtract 12 from hours 13 to 23.
Watch out: Writing 12:30 AM as 1230. Times in the first hour after midnight start with 00.
There are 60 minutes in an hour. Multiply hours by 60 to get minutes; divide minutes by 60 to get hours. A decimal part of an hour is not minutes: 1.5 hours is 1 hour 30 minutes, not 1 hour 50 minutes.
To find how long something lasts, subtract the start time from the end time, borrowing 60 minutes when needed. To find a finish time, add the hours to the start time; if the result passes 2400, subtract 2400 and it is the next day.
Measuring fluid balance over a shift or a day.
Intake is everything the patient takes in as fluid: drinks, IV fluids, tube feedings, and foods that are liquid at room temperature (like gelatin or ice cream). Output is everything that comes out: urine, vomit, drainage, liquid stool. Convert everything to mL, add each side, then subtract output from intake.
Ice chips are usually recorded at about half their volume, since they melt to roughly half. Follow your facility's policy.
Watch out: Forgetting to convert ounces to mL before adding.
Work each problem on paper first, then check the answers.
Next, use these conversions to calculate doses.
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