Skip to content
DoseReady

Methods and notations · Published by Maksym Shevchuk · Updated

Dosage calculation formulas for nursing: one table for the common problems

Short answer

Most nursing dosage calculations use a few formulas: dose desired over dose on hand times quantity (D ÷ H × Q) for tablets, liquids and injections; volume over time for mL/hr; volume times drop factor over minutes for gtt/min; volume over rate for infusion time; and dose per kilogram times weight. Then round under your program's rule.

The formulas

Each row is one kind of problem on a dosage calculation exam. D is the dose desired (ordered), H the dose on hand (on the label), and Q the quantity the dose on hand comes in. Put D and H in the same unit before you divide.

Dosage calculation formulas by kind of problem
ProblemFormulaAnswer in
Tablets and capsulesD ÷ H × Q, where Q is one tablet or capsuletablets or capsules
Oral liquidsD ÷ H × Q, where Q is the volume the label's strength refers to, such as 5 mLmL
InjectionsD ÷ H × Q, where H and Q come from the vial's concentrationmL
ReconstitutionD ÷ the concentration after mixing, from the label, × its volumemL
Unit conversionsamount × a factor with the unit you are leaving on the bottomthe new unit
mL per hourvolume (mL) ÷ time (hr)mL/hr
mL per hour from a dose per hourdose per hour ÷ dose in each mLmL/hr
mL per hour from a dose per minutedose per minute × 60 ÷ dose in each mL, in the same drug unitmL/hr
mL per hour from a dose per kg per minutedose per kg per minute × weight (kg) × 60 ÷ dose in each mLmL/hr
Drops per minutevolume (mL) × drop factor (gtt/mL) ÷ time (min)gtt/min
Drops per minute from mL/hrmL/hr × drop factor ÷ 60gtt/min
Infusion timevolume (mL) ÷ rate (mL/hr)hr
Weight-based dosedose per kg × weight (kg), with kg = lb ÷ 2.2the order's unit, such as mg

Desired over have

D ÷ H × Q covers tablets, oral liquids and injections. StatPearls describes it as the formula method: the desired dose over the dose on hand, times the quantity it comes in. It only works when D and H are in the same unit, so an order in milligrams against a label in micrograms is converted first; the conversion chart lists the factors.

The worked example in this guide sets an oral suspension out in all three notations: dimensional analysis, ratio and proportion, and the formula. The desired over have guide and the ratio and proportion guide take each method further, and the reconstitution guide covers a dose made up from a powder.

Worked example · Oral liquids

The order reads 150 mg of phenytoin oral suspension by mouth every eight hours. The well-shaken suspension is labeled 125 mg per 5 mL. How many milliliters are given per dose?

Dimensional analysis

150 × 5 ÷ 125 = 6

Ratio and proportion

  1. 125 mg is to 5 mL as 150 mg is to x mL

    150 × 5 ÷ 125 = 6

    x = 6 mL

Formula method

D (desired)
150 mg
H (have)
125 mg
Q (quantity)
5 mL

D ÷ H × Q

150 × 5 ÷ 125 = 6

The answer

The answer under the stated rule

6 mL

Nearest tenth, half rounds up. The working comes to 6 mL exactly, so the rule leaves it as it is.

What each slip gives here

  • Decimal point one place out60 mL or 0.6 mL
  • Conversion factor inverted3,750 mL
  • Conversion skipped150 mL

What this problem is built from

Rates and times

An infusion pump runs in milliliters per hour, so a volume over a time in hours gives the rate, and a dose per hour divided by the dose in each milliliter gives it from an order such as units per hour. Gravity tubing runs in drops: the volume times the tubing's drop factor, over the time in minutes. The volume over the rate gives the time an infusion takes. The mL per hour guide, the IV drip rate guide and the infusion time guide work each one through.

Weight-based doses

An order written per kilogram is multiplied by the patient's weight in kilograms; a weight in pounds is divided by 2.2 first. Some programs round that weight to the tenth before using it; the rounding rules guide compares them. The weight-based dosing guide covers doses per day in divided doses as well.

One problem, three notations

The formula is one of three ways a program may teach. Dimensional analysis writes the same steps as a chain of factors whose units cancel, and ratio and proportion sets like units against like. They give the same answer; the dimensional analysis guide compares them, and your program may require one.

Practice each formula

Each practice page has five problems, each worked in every notation that fits it, with the answer under a stated rounding rule:

Practice this

Dosage calculation practice problems with answers

Each problem opens to its worked solution, the answer under a stated rounding rule and the sources it is built from.

Questions

What is the formula for dosage calculation in nursing?
For tablets, oral liquids and injections it is desired over have: the dose desired divided by the dose on hand, times the quantity the dose on hand comes in (D ÷ H × Q), with D and H in the same unit. Rates, times and weight-based doses have formulas of their own.

Sources

  1. [1]Toney-Butler TJ, Nicolas S, Wilcox L. Dose Calculation Desired Over Have Formula Method. StatPearls (2023)ncbi.nlm.nih.gov
  2. [2]Open RN, Nursing Skills 2e, 5.6 Using Dimensional Analysis (WisTech Open, 2023)wtcs.pressbooks.pub
  3. [3]Open RN, Nursing Skills 2e, 5.13 Intravenous Infusions (WisTech Open, 2023)wtcs.pressbooks.pub
  4. [4]Open RN, Nursing Skills 2e, 5.14 IV Infusion by Gravity (WisTech Open, 2023)wtcs.pressbooks.pub
  5. [5]WisTech Open, Nursing Dosage Calculations, Introduction to IV Flow Rates and Infusion Time (2024)wtcs.pressbooks.pub
  6. [6]Open RN, Nursing Skills 2e, 5.5 Rounding (WisTech Open, 2023)wtcs.pressbooks.pub

No licensed clinician has reviewed this guide. How the guides are made. Exam practice, never for patient care. Follow your program's rules.