Drug Dosage Calculations

1. DRUG DOSAGE CALCULATIONS
Dr. Binu Babu
Ph. D (N), MSN, MBA
Mrs. Jincy Ealias
MSN, RN

2. Terms & Terminology
• FLOW RATE: is the volume per unit time usually
expressed as ml/hr. can be ml/min or ml/sec
• DRIP RATE: is the number of drops per unit time usually
expressed as drops/min(dpm)
• DOSE: amount of drug per unit time given to the patient
expressed as mg/min, g/hr or IU/min
• CONCENTRATION: amount of the drug in a given
volume usually expressed as mg/ml, g/l or mIU/ml
• DROP FACTOR: is the number of drops per unit volume.
Expressed as drops/ml
• TITRATION: adjustment of the IV medication dosage
within prescribed parameters to achieve a desired
effects.

3. Measurement Systems
• Three main types
–Apothecary
–Household
–Metric

4. VOLUME Metric Apothecary Household
4000ml 1gal(4qt)
1L (1000ml) 1qt (2pt)
500ml 1pt (16fl oz)
240ml 8 oz 1 glass/measuring cup
180ml 6oz 1 teacup
30ml 1 oz (8 dr) 2 tbsp
15ml ½ oz (4 dr) 1 tbsp (tablespoon)
5ml 1 dr (60M) 1 tsp (Teaspoon)
1ml 15 M 15 gtt
1 M(minim) 1 gtt(drop)
180mL 6 oz 1 teacup
WEIGHT 60-65 mg 1 gr -
1 g (1000mg) 15gr -
1kg (1000g) - 2.2 ib
1mg (1000mcg) - -
LENGTH 2.5cm - 1 inch

5. CALCULATION METHODS
• Various methods are used for solving medication
calculation
• The most common methods are:
 Formula,
 Proportion
 Dimensional Analysis
• No one method is best for solving every type of
problem. Several good approaches are available,
however and one of the best is dimensional analysis
• Rule #1 in drug calculations - STICK TO
ONE METHOD!

6. Routes
• IM– Intramuscular
• IO– Intraosseous
• IV– Intravenous
• IVP– Intravenous Push
• ID – Intradermal
• IN – Intranasal
• IP – Intraperitoneal
• IT – Intrathecal
• IVPB – Intravenous piggyback
• p.o – By mouth
• SC / SubQ – Subcutaneous
• SL – Sublingual
• top. – Topical
• vag. – Vaginally

7. Measurements
• Kg – kilogram
• gm – gram
• mg – milligram
• mcg – microgram
• mEq - milliequivalent
• L – liter
• mL – milliliter
• µg – microgram
• gtt – drop
• µgtt – micro drop
• tbsp – tablespoon
• tsp – teaspoon
• mg/dL – milligrams per deciliter

8. BODY SURFACE AREA
• Use either a normogram chart to fine the
body surface area or
• Body surface area BSA=√𝑤𝑒𝑖𝑔ℎ𝑡 (𝑘𝑔) ×ℎ𝑒𝑖𝑔ℎ𝑡(𝑐𝑚)
3600

9. Recommended Volume for
Administration Per Sites:
• Intradermal = 0.1 - 0.5 ml (allergy testing)
• Subcutaneous injection = 0.5 - 1.0 ml per
site
• Intramuscular injection = 2.5 - 3.0 ml per
site (1 ml in the deltoid)
• IV injection (IV push) = 1 – 60 ml

10. Conversions
Solid Conversions
• gr 1 = 60 mg
gr 15 = 1 g
2.54 cm = 1 in
2.2 lb = 1 kg
Liquid Conversions
• 30 mL = 1 oz
1 tsp = 5 mL
1 standard measuring cup
= 240 mL

11. Conversions
Mass
• mcg ← mg ← g ← kg ( x
by 1,000 )
mcg → mg → g → kg ( ÷
by 1,000 )
lb ← kg ( x by 2.2 )
lb → kg ( ÷ by 2.2 )
Volume
• mcL ← mL ← L ← kL ( x
by 1,000 )
mcL → mL → L → kL ( ÷
by 1,000 )
Time
• min ← hr ( x by 60 )
min → hr ( ÷ by 60 )

12. Temperature Conversions
• Medication orders often use Centigrade temperature.
• The relationship of centigrade 0C and Fahrenheit 0F
degrees is
• Toconvert from Fahrenheit to Centigrade, use this formula:
 °C = (°F − 32) ÷ 1.8
 °C = (°F − 32) ÷ 9
5
• Toconvert from Centigrade to Fahrenheit, use this formula:
 °F = (°C × 1.8) + 32
9
5
 °F = (°C × ) + 32

13. Calculating Tablet Dosages
• In calculating tablet dosages, the following
formula is useful
Number of tablets =
𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑑𝑜𝑠𝑎𝑔𝑒
𝑠𝑡𝑜𝑐𝑘 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ
Example:-
Orders 1,500 mg of calcium carbonate for the
patient. The drug is available in 250 mg tablets. How
many tablets should be given to the patient?
Number of tablets =
1500 𝑚𝑔
250 𝑚𝑔
= 6 tablets

14. Calculating Mixtures and Solutions
The following formula is useful in calculating
mixtures and solutions:
Amount of solution to be given=
𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑑𝑜𝑠𝑎𝑔𝑒
𝑠𝑡𝑜𝑐𝑘 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ
x Stock volume
The orders 375 mg of cefuroxime for the patient. The drug is available
in 750 mg vial. You plan to dilute it in 10 mL of sterile water. How much
should you give to your patient?
• Amount of solution to be given =
375 𝑚𝑔
750 𝑚𝑔
x 10 ml
• = 0.5 mg x 10 ml = 5 ml

15. Calculate IV Rate – mL per hour
and minute
• It is easy to calculate the running rate of IV
fluids in terms of mL per hour or mL per
minute:
• ML per hour or minute =
Total IV Volume
Time (Hour or Minute)
• Start venoclysis with D5 0.9 NaCl 1 L to be
infused for 16 hours. How many mL of the IV
fluid should you infuse per hour?
• 1 L ÷ 16 hours = mL per hour
•
[1 L x 1,000]
16 hours = 62.5 mL per hour

16. Calculate IV Rate – drops per
minute
• Calculating for drops per minute is simple with the
following formula:
– Drops Per Minute =
Total IV Volume
Time (Minute)
x Drop Factor
Example
• The physician orders to start venoclysis with D5 0.3 NaCl 1 L
solution. The IV fluid will be infused for 14 hours and the drop
factor of the macrodrip used is 20. It should be regulated to how
many drops per minute?
– Drops Per Minute =
[1 L x 1,000]
[14 hours x 60]
x 20
– [1,000 mL ÷ 840 minutes] x 20 = drops per minute
– 1.19 x 20 = 23 to 24 drops per minute

17. Calculate IV Rate – Remaining
Time of Infusion
• Calculating for the remaining time of infusion for a certain IV
fluid is possible with the following formula:
• Minutes Remaining =
Volume remaining (in ml)
Drops per minute
x Drop Factor
Example
• A patient has 350 mL of Plain 0.9 NaCl solution as IV infusion
regulated at 20 drops per minute. Drop factor 10 was used for
the patient’s macrodrip set. Considering the IV fluid level of the
patient, for how many minutes will it run?
• Minutes Remaining =
350 mL
20 Drops per minute
x 10
= 175 minutes or 3 hours

18. 1. Proportion to age
A. Young’s formula
𝑑𝑜𝑠𝑒 𝑓𝑜𝑟 𝑎 𝑐ℎ𝑖𝑙𝑑
=
𝑎𝑔𝑒 (𝑦𝑒𝑎𝑟𝑠)
𝑎𝑔𝑒 + 12
× 𝑎𝑑𝑢𝑙𝑡 𝑑𝑜𝑠
𝑒
The above formula is used for calculating the doses for children less than 12
years of age (1-12years)
B. Dilling’s formula
𝑑𝑜𝑠𝑒 𝑜𝑓 𝑎 𝑐ℎ𝑖𝑙𝑑
=
𝑎𝑔𝑒(𝑦𝑒𝑎𝑟𝑠)
20
× 𝑎𝑑𝑢𝑙𝑡 𝑑𝑜𝑠𝑒
The above formula is used for calculating the doses of a child in between 4 -20
years of age
C. Fried’s formula
𝑑𝑜𝑠𝑒 𝑓𝑜𝑟 𝑎 𝑐ℎ𝑖𝑙𝑑
=
𝑎𝑔𝑒 𝑖𝑛 𝑚𝑜𝑛𝑡ℎ
𝑠 150
× 𝑎𝑑𝑢𝑙𝑡 𝑑𝑜𝑠
𝑒
Paediatrics Drug Calculation

19. 1. Calculation base on body surface area
A. Catzel rule:
𝐷𝑜𝑠𝑒 𝑓𝑜𝑟 𝑡ℎ𝑒 𝑐ℎ𝑖𝑙𝑑 =
𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑐ℎ𝑖𝑙𝑑
𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑎𝑑𝑢𝑙𝑡
× 𝑎𝑑𝑢𝑙𝑡 𝑑𝑜𝑠𝑒
The average body surface area for an adult=1.73m2
Hence
𝐷𝑜𝑠𝑒 𝑓𝑜𝑟 𝑡ℎ𝑒 𝑐ℎ𝑖𝑙𝑑 =
𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑐ℎ𝑖𝑙𝑑
1.73𝑚2
× 𝑎𝑑𝑢𝑙𝑡 𝑑𝑜𝑠𝑒
1. Calculation base on body weight:
A. Clark’s rule
𝐷𝑜𝑠𝑒 =
𝑤𝑡 𝑖𝑛 𝑙𝑏
150𝑙𝑏
× 𝑎𝑑𝑢𝑙𝑡 𝑑𝑜𝑠𝑒(𝑚𝑔)
𝐷𝑜𝑠𝑒 =
𝑤𝑡 𝑖𝑛 𝑘𝑔
70𝑘𝑔
× 𝑎𝑑𝑢𝑙𝑡 𝑑𝑜𝑠𝑒(𝑚𝑔)
The rule is applicable only when child dose is less than 150lb or 70kg

20. FLUID THERAPY
• In fluid therapy maintenance is calculated using 4, 2, 1 formula
(Holliday - Segar’s Formula): This was standard looking at
maintenance daily fluid requirement of health children
 4ml/kg/hr. for first 10 kg,
 2ml/kg/hr. for next 10 kg
 1ml/kg/hr. for remaining kilogram weight.
• This can be simplified by using formula
• Maintenance fluid requirement (ml/hr) = {60+ (Weight kg-20)}
ml/hr for weight ≥20 kg
• Example: 65 kg women who is nil per Os will require
 Fluid required= 60 + (65-20)
 60+45
 105ml/hr of intravenous infusion
• Similarly 70 kg who had fasted over (night) 8hrs for elective surgery
will have deficit of
• Fluid deficit = 60 + (70-20)
• =60 +50 ml
• =110 ml/hr
 Hence total fluid deficit is 110 ml/hr ×8 hr=880 ml

21. References
• Finkel, R., Clark, M. A., Cubeddu, L. X., Harvey, R.
A., & Champe, P. C. (2009). Lippincott’s
illustrated reviews : pharmacology. Lippincott
Williams & Wilkins, Cop.
• Rosenfeld, G. C., & Loose, D. S.
(2014). Pharmacology. Lippincott Williams &
Wilkins.
• Clayton, B. D., & Willihnganz, M. J. (2017). Basic
pharmacology for nurses. Elsevier.

22. THANK YOU