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Nursing Dosage Calculation Practice: D/H × Q, Worked Answers and the Rounding Rules
Most dosage calculation problems are one formula — D/H × Q — with a conversion in front of it. Get the setup and the rounding consistent and the numbers stop being scary. Here is the method, the conversion card, the rounding rules and worked problems, free. Forty practice problems with a full worked key are in the printable workbook.
Written by a 13-year ER night-shift nurse. Free to read here — no sign-up.
Dosage calculation is the one skill in nursing school where “close” is not a grade you want. It is also the skill most often missed for the same three reasons: the units did not match, the conversion happened in your head, or the answer got rounded twice. Practice with feedback fixes all three, because you can see which step broke.
The formula: D over H times Q
D is the dose desired — what is ordered. H is the dose on hand — what you have. Q is the quantity that H comes in. The units of D and H must match before you divide, so convert first, then calculate.
Worked problem — mL to give
Ordered: 750 mg. Available: 500 mg in 5 mL. How many mL will you give?
D/H × Q = 750 mg ÷ 500 mg × 5 mL = 1.5 × 5 mL = 7.5 mL
Units match, so nothing to convert. The answer is more than 1 mL, so it rounds to the nearest tenth — 7.5 mL stays 7.5 mL.
If the order had read 0.6 g and the vial was labeled 250 mg in 1 mL, the first line of your work would be the conversion, not the formula: 0.6 g = 600 mg, then 600 ÷ 250 × 1 = 2.4 mL. Writing that conversion down is not extra credit. It is the step that catches the decimal that moved.
The five formulas, one line each
| You want | Formula |
|---|---|
| mL to give | D ÷ H × Q (units of D and H must match) |
| mL/hr for a pump | total mL ÷ hours |
| gtt/min by gravity | (mL/hr × drop factor) ÷ 60 |
| hours to finish | total mL ÷ mL/hr |
| mL/hr from a concentration | units (or mcg/mg) per hour ÷ units (or mcg/mg) per mL |
The same five lines are printed on the conversion reference card in the practice workbook.
Worked problem — 1,000 mL over 8 hours
Pump rate, then gravity
mL/hr = total mL ÷ hours = 1,000 mL ÷ 8 h = 125 mL/hr
Now the same bag by gravity with 15 gtt/mL tubing: (125 × 15) ÷ 60 = 1,875 ÷ 60 = 31.25 gtt/min, which rounds to 31 gtt/min — whole drops, because you cannot count a quarter of one.
And the reverse question, which shows up on every exam: how long does 1,000 mL take at 125 mL/hr? 1,000 ÷ 125 = 8 h.
Rounding rules — pick one set and stay with it
Half of all “wrong” answers are right answers rounded differently. These are the rules the practice workbook’s answer key uses, stated once so your work can be marked against it:
| Answer type | Round to | Example |
|---|---|---|
| mL (more than 1 mL) | nearest tenth | 7.46 mL → 7.5 mL |
| mL (less than 1 mL) | nearest hundredth | 0.666 mL → 0.67 mL |
| gtt/min (drops) | nearest whole drop | 31.25 gtt/min → 31 gtt/min |
| mL/hr (pump) | nearest whole mL/hr | 187.5 mL/hr → 188 mL/hr |
| hours | nearest tenth of an hour | 1.666 h → 1.7 h |
| kg, converting a weight | nearest tenth | 60.0 kg stays 60 kg |
The conversions these problems actually use
| Metric weight | Volume | ||
|---|---|---|---|
| 1 g | 1,000 mg | 1 L | 1,000 mL |
| 1 mg | 1,000 mcg | 1 dL | 100 mL |
| 1 kg | 1,000 g | 1 tsp | 5 mL |
| 1 mcg | 0.001 mg | 1 tbsp | 15 mL |
| 0.5 mg | 500 mcg | 1 fl oz | 30 mL |
| 0.25 mg | 250 mcg | 1 cup | 240 mL |
| Weight | Drop factors (gtt/mL) | ||
|---|---|---|---|
| 1 kg | 2.2 lb | Microdrip | 60 |
| 1 lb | 0.454 kg | Macrodrip (regular) | 10, 15, 20 |
| 2.2 lb | 1 kg | Blood tubing | 10 |
| 60 kg | 132 lb | What the box says | always check |
Two checks before you call an answer done
Does the size make sense? A tablet is a whole number, an ampoule takes a realistic volume, and a pump rate you would question is usually a decimal in the wrong place. Did the units survive the trip? If your answer is in mL and the question wanted mL/hr, the setup was right and the last step was missing. Those two questions catch more errors than re-doing the arithmetic does.
One more habit worth building early: when a real order does not look like one of these problems, slow down and get it double-checked. That is normal practice, not a sign you are bad at this.
The printable version
Nursing Dosage Calculation Practice Sheets — $9
Twelve printable pages: forty problems in four graded sets, and the full working shown for every single one — setup, conversion, arithmetic, rounding.
- 40 problems in four sets: D/H × Q and conversions, mg/kg and mL/hr, gtt/min and infusion time, then units/hr, mcg/min, mg/min
- A worked answer key for all forty, with the setup written out — nothing in the key was typed by hand
- Conversion reference card: metric weight, volume, lb ↔ kg, drop factors, and the five formulas on one line each
- Score and review tracker, plus a short method for finding the pattern in what you missed
This is standard educational calculation, not drug-specific dosing advice — no maximum doses, no protocols. Rounding conventions vary: follow your course’s or your facility’s rules in practice, and verify any unusual-looking dose with your instructor or a pharmacist.