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Dosage Calcs

Dosage Calculation Basics

Every dosage problem is built from the same few pieces. Once you can name them, any calculation method becomes easier to set up and check.

In this lesson

  • The ordered dose, available dose, quantity and concentration
  • How to spot what the question is asking for
  • Why units must match before you calculate
  • How to check that your final unit makes sense
  • The three methods used on Dosage Calcs

What you're solving for

Most basic dosage problems ask one question: how much of the product you have matches the amount that was ordered? The answer is usually a number of tablets, capsules or milliliters.

The parts of a dosage problem

Ordered (desired) dose
The amount written in the order, such as 500 mg. It is the amount you are working toward.
Available (on-hand) dose
The amount of drug in one unit of the product you have, read from the label, such as 250 mg.
Quantity
The amount of product that contains the available dose: 1 tablet, 1 capsule, or a volume like 5 mL.
Concentration
For liquids, the available dose and quantity together, such as 250 mg / 5 mL. It says how much drug is in a given volume.
Units
The labels on each number — g, mg, mcg, mL, tablet. Numbers without units can't be compared safely.

Step 1: Identify what the question asks for

Before any math, name the unit your answer must be in. “How many mL?” means the answer ends in mL. “How many tablets?” means tablets. Writing the target unit first keeps you from solving for the wrong thing.

Step 2: Make the units compatible

The ordered dose and available dose must be in the same unit before you compare them. If the order is in g and the label is in mg, convert one of them first. The nursing unit conversions lesson shows how.

Step 3: Calculate and check the final unit

After calculating, look at the unit that is left. If you wanted mL and ended with mg, something was set up incorrectly. Also ask whether the size of the answer is reasonable: if the order is larger than the available dose, you should need more than one unit of the product.

Worked example: putting the parts together

An order reads 500 mg. The liquid on hand is labeled 250 mg / 5 mL. How many mL match the order?

Ordered dose: 500 mg · Available dose: 250 mg · Quantity: 5 mL · Target unit: mL

  1. Step 1: Calculation

    500 mg ÷ 250 mg × 5 mL

    = 10 mL

  2. Step 2: Formula

    Desired ÷ Have × Quantity = Amount

  3. Step 3: Check your units

    mg cancels with mg, leaving mL.

Answer: 10 mL

Three ways to set up the math

Dosage Calcs teaches three common approaches. They are different ways of writing the same relationship, so on any problem they give the same answer. Nursing programs and students differ in which they prefer, and no single method is best for everyone.

Desired divided by Have, times Quantity, equals the amount

Desired Over Have uses a short formula. Learn Desired Over Have.

Dimensional Analysis chains conversion factors so unwanted units cancel. Learn Dimensional Analysis.

Ratio & Proportion compares the known relationship to the one you need. Learn Ratio and Proportion.

Quick self-check

An order reads 0.5 g and the tablets are labeled 250 mg. What must happen before you calculate?

Show the answer

Convert so both doses use the same unit: 0.5 g = 500 mg. Then compare 500 mg to 250 mg.

Key takeaway

Name the target unit, match the dose units, calculate, then check that the unit left over is the one you wanted.

Calculate and practice

Use the Calculator →: Dosage CalculatorPractice This Topic →: Desired Over Have practice

Related lessons

Sources

Explanations are original Dosage Calcs wording. See all sources and the methodology.

Educational Use & Safety

Educational content only. These lessons teach medication math using fictional values and do not determine an appropriate medication, dose, infusion rate, or treatment for an actual patient.

Educational Use & Safety