Dosage Calculation Formulas: The One-Page Sheet
Every formula a school dosage exam asks for, with the conversion factors and the rounding rules that decide whether your answer matches the key. These are the textbook values US nursing programs and the HESI, ATI and NCLEX exams use; if your syllabus gives a different value, your syllabus wins.
Conversion Factors
| This | equals | Used for |
|---|---|---|
| 1 g | 1,000 mg | grams to milligrams |
| 1 mg | 1,000 mcg | milligrams to micrograms |
| 1 kg | 1,000 g | kilograms to grams |
| 1 L | 1,000 mL | liters to milliliters |
| 1 kg | 2.2 lb | kilograms to pounds |
| 1 lb | 16 oz | pounds to ounces |
| 1 tsp | 5 mL | teaspoon |
| 1 tbsp | 15 mL | tablespoon |
| 1 fl oz | 30 mL | fluid ounce |
| 1 cup | 240 mL | cup |
| 1 hr | 60 min | hours to minutes |
More precise figures exist (1 kg = 2.205 lb, 1 fl oz = 29.57 mL), but using them can make your answer differ from the exam key after rounding.
The Method
Write what you were given, then multiply by one fraction per conversion, each arranged so the unit you do not want cancels out. Every fraction equals 1, so the value does not change, only the units. Finish with the unit the question asked for on top, then round once, at the end, by the rule for that kind of answer.
given × (conversion 1) × (conversion 2) × … = answer in the unit you want
Rounding Rules
- Tablets
- Whole or half tablets only, and only if the tablet is scored. Capsules are never split.
- Liquids in mL
- Round to the nearest tenth (one decimal place). Under 1 mL, round to the nearest hundredth.
- Weight in kg
- Convert pounds first and round to the nearest tenth before you use the weight.
- IV pump rate (mL/hr)
- Whole number. Pumps take whole milliliters per hour.
- Gravity drip (gtt/min)
- Whole number. You cannot count a fraction of a drop.
- Weight-based drips (mL/hr)
- Critical care pumps take tenths, so round to one decimal place unless the question says whole.
- Infusion time
- Give hours and minutes, and the completion time on a 24-hour clock.
- Writing the answer
- Never a trailing zero (5, not 5.0) and always a leading zero (0.5, not .5). Both are marked wrong on most exams.
Formulas by Calculation
- Lesson 1 · Metric Conversions
- 1 g = 1,000 mg1 mg = 1,000 mcg1 kg = 1,000 g1 L = 1,000 mL
- Each jump is three decimal places.
- Lesson 2 · Household to Metric
- kg = lb ÷ 2.21 tsp = 5 mL1 tbsp = 15 mL1 oz = 30 mL16 oz = 1 lb
- Divide pounds by 2.2 to get kilograms; multiply kilograms by 2.2 to get pounds.
- Lesson 3 · Dimensional Analysis
- given × (conversion 1) × (conversion 2) × … = answer in the unit you want
- Each fraction is worth 1, so the value does not change, only the units.
- Lesson 4 · Tablets and Capsules
- tablets = desired dose ÷ dose per tablet
- D ÷ H × Q, where Q is one tablet.
- Lesson 5 · Liquid Oral Doses
- mL to give = (desired mg ÷ mg on label) × mL on label
- D ÷ H × Q, where Q is the volume the label's dose comes in.
- Lesson 6 · Injections
- mL to draw up = (desired ÷ have) × vial volume
- Same desired-over-have idea; the volume is what is in the vial, not the whole vial.
- Lesson 7 · mg/kg Dosing
- dose = mg/kg × weight in kg(kg = lb ÷ 2.2)
- Per-day orders: divide the daily dose by how many times a day it is given.
- Lesson 8 · Safe Dose Range
- low = min mg/kg × kghigh = max mg/kg × kgsafe if low ≤ ordered ≤ high
- Per-day ranges are compared with the ordered dose multiplied by the number of doses a day.
- Lesson 9 · IV Pump Rate (mL/hr)
- mL/hr = total mL ÷ hoursmL/hr = (total mL ÷ minutes) × 60
- Pumps take whole numbers, so round to the nearest whole mL/hr unless your pump accepts tenths.
- Lesson 10 · IV Drip Rate (gtt/min)
- gtt/min = (total mL × drop factor) ÷ total minutes
- From a pump rate: gtt/min = mL/hr × drop factor ÷ 60.
- Lesson 11 · Infusion Time
- hours = total mL ÷ mL/hrminutes = decimal part × 60
- Completion time = start time + infusion time, in 24-hour clock.
- Lesson 12 · mcg/kg/min Drips
- mL/hr = (mcg/kg/min × kg × 60) ÷ (mcg per mL in the bag)
- Concentration: mg in bag × 1,000 ÷ mL in bag = mcg/mL.
dosagepractice.com · For exam practice only. Not clinical guidance. Always follow your institution's policies and a pharmacist's verification.
Knowing the Formula Is the Easy Part
The exam gives you numbers you have not seen and one chance to round right. The free practice round is ten problems from across this sheet, graded exactly as your school grades.