The Student Success Commons at York County Community College: Dosage Calculations

Dosage Calculation

Basic medication dose conversion and calculation are essential skills for those entering the nursing or pharmacology fields. There are three different methods for dosage calculation:

• Ratio and proportion
• Formula (desired/have)
• Dimensional analysis

This page contains resources and examples for each of these calculation methods.

Types of Calculations

• Solid oral medication
• Liquid oral medication
• Injectable medication
• Correct doses by weight
• IV infusion rates

Standard Conversion Factors

• 1 mg = 1,000 mcg
• 1 g = 1,000 mg
• 1 kg = 1,000 g
• 1 oz = 30 mL
• 1 tsp = 5mL
• 1 tbsp = 15 mL
• 1 tbsp = 3 tsp
• 1 kg = 2.2 lb
• 1 gr = 60mg

General Rounding Guidelines

• For dosages less than 1.0 round to the nearest hundredth
• For dosages greater than 1.0 round to the nearest tenth

When performing calculations, do not round until calculating the final answer. Dosages of oral liquid medications for adults are typically rounded to the tenth for doses over 1 mL. For doses less than 1 mL the dosage is rounded to the hundredth.

For pediatric patients, it is important to be as precise as possible to avoid medication errors. Oral liquid medications less than 1 mL should be rounded to the hundredth.

When rounding, it is also important to use critical thinking to evaluate your final answer. For example, a drop cannot be administered as a fraction of a drop, so drops are rounded to the nearest whole number

For a refresher and some practice rounding, please check out the Student Success Commons' Rounding Rules Guide

Dose Calculation Examples and Explanations 

Mass:

mcg → mg → g → kg ( ÷ by 1,000 )

lb → kg ( ÷ by 2.2 )

Volume:

mcL → mL → L → kL ( ÷ by 1,000 )

Time:

min → hr ( ÷ by 60 )

Certain conversions appear frequently in the nursing and pharmacology fields; it can be useful to memorize the following ratios:

• 1 pound = 16 ounces
• 1 kilogram= 2.2 pounds
• 1 teaspoon = 5 milliliters
• 1 tablespoon = 3 teaspoons = 15 milliliters

Though these are some of the most common ones, more handy conversions are listed on this page, and on our Units and Conversions page.

BIG TO SMALL

KILO to BASE UNIT: Multiply by 1,000
KILO to CENTI: Multiply by 100,000
KILO to MILLI: Multiply by 1,000,000
KILO to MICRO: Multiply by 1,000,000,000

BASE UNIT to CENTI: Multiply by 100
BASE UNIT to MILLI: Multiply by 1,000
BASE UNIT to MICRO: Multiply by 1,000,000

CENTI to MILLI: Multiply by 10
CENTI to MICRO: Multiply by 10,000

MILLI to MICRO: Multiply by 1,000

SMALL TO BIG

MICRO to KILO: Divide by 1,000,000,000
MICRO to BASE UNIT: Divide by 1,000,000
MICRO to CENTI: Divide by 10,000
MICRO to MILLI: Divide by 1,000

MILLI to KILO: Divide by 1,000,000
MILLI to BASE UNIT: Divide by 1,000
MILLI to CENTI: Divide by 10

Be sure to check out our video on Units and Conversions: the Metric System for an overview of metric system base units and prefixes.

The metric system is a system of measurement that uses the meter, liter, and gram as base units of length (distance), capacity (volume), and weight (mass) respectively. 
 

Metric system prefixes tell you how much bigger or smaller a unit is than the base unit. These prefixes are all powers of 10, which makes converting from one metric measurement to another very simple. Here are some common prefixes.

KILO: 1,000 × Base
HECTO: 100 × Base
DECA: 10 × Base
[BASE]
DECI: 0.1 × Base
CENTI: 0.01 × Base
MILLI: 0.001 × Base
MICRO: 0.000001 × Base

Some abbreviations to understand:

D/H x Q → (desired or dose/have) x quantity

mL/hr → milliliters per hour

gtt/min → drop(s) per minute

Nifty note: A drop is abbreviated gtt, with gtts used for the plural, and is often seen on prescriptions. These abbreviations come from gutta (plural guttae), the Latin word for drop.

mg/kg/day → milligrams per kilogram [of body weight] per day

gtts/mL→ also known as the drop factor: the number of drops it takes to make up one mL of fluid

D→ Dextrose

W → Water

S → Saline

NS → Normal Saline (0.9%NaCI)

RL → Lactated Ringer's

PO → per os or by mouth