Dosage Calculation Conversions Practice

A milligram is 1,000 times larger than a microgram, so changing mg to mcg makes the number bigger: multiply by 1,000. 0.088 mg × 1,000 mcg/mg = 88 mcg. Moving the decimal point three places to the right gives the same result. The answer is exact, so it is written as 88 mcg with no trailing zero. Common errors are dividing by 1,000, which gives 0.000088, and moving the decimal only two places, which gives 8.8 mcg, a tenfold underdose. Sense check: levothyroxine tablets are labeled in mcg, and 88 mcg is a marketed tablet strength. Takeaway: mg to mcg goes from a larger unit to a smaller one, so multiply by 1,000 and expect the bigger number.

Sources: Open RN Nursing Skills (ACC adaptation), 5.4 Equivalencies; ISMP List of Error-Prone Abbreviations, Symbols, and Dose Designations; DailyMed: Levothyroxine sodium tablets label (88 mcg, 0.088 mg, tablet strength)

Pounds are changed to kilograms by dividing by 2.2, because 1 kg = 2.2 lb. 172 lb ÷ 2.2 lb/kg = 78.1818 kg. Body weight is rounded to the nearest tenth before it is used in a dose calculation, so the weight is 78.2 kg. Sense check: the kg number is always smaller than the lb number, a little less than half of it. The common error is multiplying by 2.2, which gives 378.4 kg. That is an impossible weight, and a weight-based dose calculated from it would be almost five times too large. Takeaway: divide pounds by 2.2, round the kg to the nearest tenth, and check that the kg number is the smaller one.

Sources: Open RN Nursing Skills (ACC adaptation), 5.4 Equivalencies; Open RN Nursing Skills (ACC adaptation), 5.11 Weight-Based Calculations

The prescribed volume is in liters and the pump reading is in milliliters, so put both in the same unit before subtracting. Step 1: 1.5 L × 1,000 mL/L = 1,500 mL prescribed. Step 2: 1,500 mL − 640 mL infused = 860 mL remaining. A common error is moving the decimal only two places, which turns 1.5 L into 150 mL and gives a negative volume. Another is subtracting 640 from 1.5 without converting. Sense check: 640 mL is a little under half of 1,500 mL, so a little over half should remain. Takeaway: change liters to milliliters by multiplying by 1,000 before adding or subtracting volumes.

Sources: Open RN Nursing Skills (ACC adaptation), 5.4 Equivalencies; Open RN Nursing Skills (ACC adaptation), 5.15 IV Infusion by Pump

Step 1: add every dose, whatever the route, in mg. 1,000 mg + 650 mg + 650 mg = 2,300 mg. Step 2: convert to grams. Going from a smaller unit to a larger one, divide by 1,000: 2,300 mg ÷ 1,000 = 2.3 g. This is below the adult maximum of 4 g (4,000 mg) of acetaminophen in 24 hours, a limit that counts all sources and routes, including combination products. Common errors are counting only the two oral doses, which gives 1.3 g, and dividing by 100 instead of 1,000, which gives 23 g. Takeaway: add the doses in one unit first, then divide mg by 1,000 to get grams, and expect the smaller number.

Sources: Open RN Nursing Skills (ACC adaptation), 5.4 Equivalencies; DailyMed: Acetaminophen injection label, Hikma (adults 50 kg or more: 1,000 mg every 6 hours, no more than 4,000 mg in 24 hours, counting every route and product); DailyMed: Acetaminophen injection label, Baxter (adults 50 kg or more: no more than 4,000 mg in 24 hours, counting every route and product)

Two conversions are needed before dividing. Step 1, weight: 145 lb ÷ 2.2 = 65.909 kg, rounded to 65.9 kg. Step 2, dose: 0.25 mg × 1,000 mcg/mg = 250 mcg per day. Step 3, dose per kg: 250 mcg ÷ 65.9 kg = 3.7936, rounded to 3.8 mcg/kg/day. This lies inside the usual adult maintenance range of 3.4 to 5.1 mcg/kg/day for clients with normal kidney function. Skipping the weight conversion gives 1.7 mcg/kg/day, and skipping the dose conversion gives 0.004, so both units must be changed first. Takeaway: convert every quantity to the units of the reference, here kg and mcg, before comparing a dose with it.

Sources: Open RN Nursing Skills (ACC adaptation), 5.4 Equivalencies; Open RN Nursing Skills (ACC adaptation), 5.11 Weight-Based Calculations; DailyMed: Digoxin tablets label (maintenance dose in mcg/kg/day, scored 125 mcg and 250 mcg tablets)