Drug Dosage Calculations
Abbreviations Commonly Used in Dosage
Abbreviation Term Tips for Correct Calculations:
po by mouth (or orally) Make sure that all measurements are in the same units. If necessary,
convert
susp suspension between units.
pm • In most cases, it is best to convert a larger unit to smaller units (e.g.
as needed
convert
tab tablet grams to milligrams). This conversion uses multiplication and most
often keeps the
cap capsule
calculation in whole numbers.
q every • Always check for reasonableness of your answer. Does the drug
dosage make
bid twice a day sense? (Amounts less than half a tablet or greater than 3 tablets are not
tid three times a day common,
but still possible.)
qid four times a day
Formula: D/H * Q=X Desired/Have * Quantity=X
Term Symbol Meaning Example
"Give 500 milligrams" "
Dosage ordered The amount of medication Give grains/v" "
D
or desired dose that the physician prescribed Give 1.2 milliliters"
Dosage strength The amount of drug in a 250 milligrams
or supply on H specific unit of measure Grains/v
hand (what is available; in stock)
Unit of measure The unit of measure for the per 2 milliliters
or quantity of Q specific dosage strength or per capsule
unit supply on hand per tablet
Unknown S The dosage you are
Dosage Not applicable
trying to calculate
Dosage Calculations Conversions
1 kg = 2.2 lb 1 lb = 16 oz 1 mg = 1,000 mcg
1 gallon = 4 quart 1 tbsp = 3 tsp 1 cm = 10 mm
1 tsp = 5 mL 60 minute = 1 hour 1 tbsp = 15 mL
1 inch = 2.54 cm 1 cc = 1 mL 1 cup = 8 fl oz
1 L = 1,000 mL 2 pints = 1 qt 1 pint = 2 cups
1 kg = 1,000 g 8 oz = 240 mL = 1 glass 12 inches = 1 foot
1 oz = 30 mL = 2 tbsp 1 tsp = 60 gtt 1 L = 1.057 qt
1 g = 1,000 mg 1 pt = 500 mL = 16 oz 1 oz = 30 mL
Unit Conversions
Mass: Converting lb to kg and kg to lb
→ → →
mcg mg g kg ( ÷ by 1,000 ) lb = kg × 2.2
← ← ←
mcg mg g kg ( x by 1,000 ) kg = lb ÷ 2.2
→
lb kg ( ÷ by 2.2 ) Converting mL to L and L to mL
←
lb kg ( x by 2.2 ) mL = L × 1,000
Volume: L = mL ÷ 1,000
mcL → mL → L → kL ( ÷ by 1,000 ) Converting mg to g, g to mg, mg to mcg,
mcL ← mL ← L ← kL ( x by 1,000 ) and mcg to mg
Time: mg = g × 1,000
min → hr ( ÷ by 60 ) g = mg ÷ 1,000
min ← hr ( x by 60 ) mcg = mg × 1,000 mg = mcg ÷ 1,000
Figuring IV flow rate, infusion time, and total volume
flow rate (mL/hr) = total volume (mL) ÷ infusion time (hr)
infusion time (hr) = total volume (mL) ÷ flow rate (mL/hr)
total volume (mL) = flow rate (mL/hr) × infusion time (hr)
Example: if you must administer 1 L (1,000 mL) of fluid over 4 hours, use the first
formula to calculate the flow rate, like so:
flow rate (mL/hr) = total volume (mL) ÷ infusion time (hr)
flow rate (mL/hr) = 1,000 ÷ 4
flow rate (mL/hr) = 250
The flow rate is 250 mL/hr.
Drug Dosage Calculations Formula
Dosage By Weight Mass for Mass
Formula: Formula:
Weight in Kg * Dosage Per Kg = Y Ordered \ Have
(Required Dosage) = Y (Tablets Required)
Mass/Liquid For Liquid IV Fluid
Formula: Formula:
Ordered \ Have x Volume Per Have Volume (mL) \ Time (min) x Drop Factor
= Y (Liquid Required) (gtts/mL) = Y (Flow Rate in gtts/min)
Volume/Time - IV mL Volume/Time - IV Drop
Formula: Formula:
Volume (mL) \ Time (hr) Concentration % \ 100 x Volume (mL)
= Y (Flow Rate in mL/hr) = Y (Dosage Amount in g)
Volume/Time - IV mL Volume/Time - IV Drop
Formula: Formula:
Volume (mL) \ Time (hr) Volume (mL) \ Time (min) x Drop Factor
= Y (Flow Rate in mL/hr) (gtts/mL) = Y (Flow Rate in gtts/min)
Fluid Maintenance
Weight Range Required Daily Fluid
0-10 kg 100 mL per kg
10-20 kg 1,000 mL + 50 mL per each kg above 10 kg
20-70 kg 1,500 mL + 20 mL per each kg above 20 kg
Over 70 kg 2,500 mL (adult requirement)
TYPES OF DOSAGE FORMS
Classification
Physical state
Route of Administration
Based on
Site of Application
Uses
Based on Physical state
SOLID SEMI-SOLID LIQUID GAS
1. Monophasic liquid:
1. Unit solid:
Syrup
tablets
solution
capsules
2.Biphasic liquid:
2. Bulk dosage emulsion
form: suspension
powder
Inhaler
dusting powder
aerosols
Cream
paste
gel
suppositories
TYPES OF DOSAGE FORMS
Based on Physical state
Oral Parenteral Rectal Trans dermal
Powders Solutions Suppositories Ointments
Tablets Suspensions Enemas Powders
Capsules Emulsions Ointments Creams
Solutions Vaginal Solutions Pastes
Suspensions Suppositories Intranasal Lotions
Emulsions Tablets Solutions Plasters
Syrups Ointments Sprays Intra ocular
Elixirs Creams Inhalations Solutions
Magmas Douches Intra respiratory Ointments
Gels Sublingual Aerosols Suspensions
Cachets Tablets Conjuntival --
Pills Lozenges Ointments --
Site of Application Uses
1. Skin- Ointments, creams, lotions, liniments 1. Internal – all
2. Eye- Solutions, ointments, creams preparations except
3. Tooth- Tooth powder, tooth paste external
4. Hand- Hand creams, lotions, hand washings 2. External – Ointments,
5. Foot- Creams, ointments, dusting powders lotions, creams,
6. Hair- Hair cream, hair lotions, shampoos, hair powders, solutions,
fixing liniments, paste, hair
7. Nasal- Solutions, sprays, inhalations dyes, hair colorants
Metric System
9 KEY FACTS
Weight VOLUME
Kilogram (kg) Liter (L)
Killo-graham-cracker Liter-of-cola
In the metric system ,the gram (g/gm) is In the metric system, the liter (L) is
used to describe weight. 1 kilogram (kg)= used to describe volume.1 liter =
1,000 grams (g). 1,000 milliliters (mL).
Gram (g/gm)
Graham-cracker Milliliter (mL)
In the metric system, the gram (g/gm) is Millionaire-liter-of-cola
used to describe weight. 1 gram (g) = 1,000 In the metric system, the liter (L) is
milligrams (mg). used to describe volume. 1 liter =
Milligram (mg) 1,000 milliliters (mL).
Millionaire-graham-cracker
In the metric system, the gram (g/gm) is
used to describe weight. 1 milligram (mg) =
LENGTH
1,000 mcg.
Microgram (mcg) Meter (m)
Microphone-graham-cracker Meter-Pan
In the metric system, the gram (g/gm) is In the metric system, the meter is used
used to describe weight. 1,000 microgram to describe length.
(mcg) = 1 milligram (mg).
CONVERTING
Larger to Smaller: Multiply
Large Metro Car Turned into a Small Metro Car by Multiplying
The metric system applies the idea that units within the system get larger or smaller by
a power of 10. Conversions between the units in the metric system can be done by
multiplying or dividing by 1,000. To convert larger to smaller, multiply by 1,000, or
move the decimal point 3 places to the right.
Smaller to Larger: Divide
Small Metro Car Turned into a Large Metro Car by Dividing
The metric system applies the idea that units within the system get larger or smaller by
a power of 10. Conversions between the units in the metric system can be done by
multiplying or dividing by 1,000. To convert smaller to larger, divide by 1,000 or move
the decimal point 3 spaces to the left.
