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Pharmaceutical CALCULATIONS

Hello every one ...... The very first blog I would like to place in this April is on calculations.

Generally most of the students are confused while dealing with calculations. During my course of time till now, here are few calculations which I have come across. Before proceeding into the calculation the pre requisite step is to read the data thoroughly, understand the given information and select the required procedure for calculation.

I would like to touch up few important concepts in these calculations.








Based on Age

Young Rule (Children under 12 yrs of age)

Child's dose = Age (yrs)/ Age (yrs) +12 x Adult dose

Example: 1. Let us consider a drug whose dose is 350 mg, then the dose for a three year old child will be:

Sol: Child dose = 3/3+12x 325 = 65mg

Dillings formula

Dose for the child = Age in years/20 x Adult dose

Fried's Formula

Dose for the child = Age in months/150 x AD

Example 2:

Consider that the dose of a drug for adult is 100 mg then the dose for a 9 month old infant and a boy of 16 yrs age can be calculated as below:

Ans i) Fried's Rule 9/150x100 Dose = 6 mg

ii) Dillings formula: Dose = 16/20 x 100 = 80 mg

Based on Body Weight:

Clarks's Rule

Dose for the child = Weight of the child/ 150 x Adult dose

Example 3: If the adult dose of a drug is 10mg the dose for a 40 lb child will be:

Child's Dose = 40/150 x 100 = 2.67mg

Based On the Body Surface

Child's Dose = Body surface area of a child/Body surface area of Adult x Adult dose

Average body surface area for adult = 1.73 m2

Example 4: Let us see how to calculate the dose for child who has a body surface equal to 0.57 sq.m. when an average adult dose of a drug is 50 mg

Child dose (approx) = 0.57/1.73 x 50 = 16.5 mg.


Alligation method

This method is used to find out the intermediate strength by mixing two preparations which are similar and having different strengths.

Example 5 Calculate the volume of 95% alcohol required to prepare 600 ml of 70% alcohol?

Volume required = 600ml

Percentage of alcohol required = 70

Percentage of alcohol used = 95

By allegation method Alcohol Water

95 0


70-0 95-70 =25

To produce required % of alcohol, 70 parts of 95% alcohol and 25% of water mixed.

Quantity of 95% alcohol required = 600x70/95 = 442.10ml

Quantity of water required = 600 x 25/95= 157.90ml.


Freezing point method or Cryoscopic method:

% w/v of adjusting substance needed = [0.52-a]/b

a= Freezing point of the unadjusted solution

b=Freezing point of a 1% W/V solution of the adjusting sub.

Example 6:

Using croscopic method how will you prepare 1% solution of boric acid, iso-osmotic with blood plasma. (Hints: freezing point of 1% w/v solution of boric acid is -0.288o C). The freezing point of 1% w/v solution of NaCl is - 0.576 o C)

Solution: % w/v of NaCl required = [0.52 -0.288]/0.576 = 0.402% w/v.

So, weigh 1 gm of Boric acid and transfer to a volumetric flask then weigh 0.4 g of NaCl and add it to the boric acid in flask. Then add small quantity of water, dissolve and make up the volume up to 100 ml. Now we will have an iso-osmotic solution.

NaCl Equivalent method:

Quantity of NaCl required to adjust tonicity = 0.9 - (n x E) n = number of grams of unadjusted substance E = NaCl equivalent (quantity of NaCl i.e. equivalent to 1 gm of drug)

White Vincet method:

V = w x E x 111.1

Where V = volume of isotonic solution in ml that can be prepared by dissolving drug in water w = wt. of drug

E = NaCl equivalent

Molecular Concentration4:

% w/v of adjusting substance required = 0.03M/N M= Gram molecular weight

N=Number of ions

Example 7:

How to prepare an iso-osmotic solution using dextrose.

Molecular wt of dextrose=180

Dextrose is non ionizing substance so no ions.

W= 0.03 x M = 0.03 x 180 = 5.4 g/100ml


Any alcoholic solution which has 57.1% v/v alcohol is proof spirit.

Steps which are involved in calculating.

Firstly let us known how 1.753 is obtained. 57.1 Volume of ethyl alcohol = 100 Volume of P.S Volume of ethyl alcohol= 100/57.1= 1.753 volume of PS

Step1: The % strength of alcohol is to be multiplied with 1.753 and substract100 from the value obtained after multiplying.

Step2. If value obtained is + then it is over proof

Step3. If value obtained is - then it is under proof.

Example 8:

What is the proof strength for 95% v/v alcohol?

% strength of alcohol x1.753 -100 = 95 x 1.753-100 = 166.53-100=

+66.53 o So 66.53 o OP


Number of parts by weight of drug that replaces one part by weight of base.

Displacement factor= 100 (E-G)/Gx + 1

E = Wt of pure suppositories

G = Wt of suppositories containing x % of drug

For preparation of suppositories in uniform and accurate weight this factor is used


9C = 5F - 160C

C = Degree centigrade

F = Degree Fahrenheit


1. Remington the science and practice of pharmacy, 21st edition, volume1 pg no 116 [accessed date: 12th march, 2010]

2. Remington the science and practice of pharmacy, 21st edition, volume1 pg no 122-123 [accessed date: 12th march, 2010]

3. Physical pharmacy and pharmaceutical sciences by Martin, Fourth edition, Pages-182-185 [accessed date: 13th march, 2010]

4. Dispensing Pharmacy by RM Mehta, 3rd edition, Page. No. 94 [accessed date: 13th march, 2010]

5. Theory and Practice of Pharmacy, 20th edition, Page No. 585 [accessed date: 14th march, 2010]

6.[accessed date: 14th march, 2010]

7. Image source:

"This blog does not contain any plagiarized material"


V.B.S. Aishwarya



About the Author

V.B.S.Aishwarya's picture
Author: V.B.S.Aishwarya


Tejaswi Padma's picture

Hi Aishwarya, Your blog is very good As you said calculations are really tough to solve and they seem to be very much dis-interesting especially for Bi.p.c students like us. Your blog in one way would serve as a mini collection reference for pharmaceutical calculations. Good job. Keep going....all the best...


Tejaswi Redla



V.B.S.Aishwarya's picture

Dear Teja, Very happy to see you commenting. Yeah you are right calculations are really tough ones but they are interesting and important. Keep commenting .... Regards,






Sirisha Pingali's picture

hi aishwarya.. good blog. Yeah these calculations are very important. Though they seem to be very simple they are even important in competitive exam mode. Thank you for sharing such valuable information with us.

Sirisha Pingali

Viswanadha Institute of Pharmaceutical Sciences.

V.B.S.Aishwarya's picture

Dear Sirisha, thanks a lot for your comment Your comments are really encouraging to me . Please keep on commenting






Prof. J. Vijaya Ratna's picture

Dear Aishwarya I am reminded of the following after reading your and Bhavani Prasad's blog also. Hope you don't mind. Take me for an old teacher! There is a old popular novel called Kanyasulkam. In that, Girisam (Tution teacher) and his student come to the student's house for a visit. The student's father is an illiterate and does not know anything. He fondly asks Girisam " Can my son now speak English?" Girisam " Ofcourse! He speaks beautifully." Father: " OK! Both of you, teacher and taught, converse in English so that I can hear and enjoy." Girisam and his student exchange glances, they are in a quandary as the student knows nothing. So this is what they do. Girisam: Twinkle, Twinkle little star. Student: How I wonder what you are! Hugely enjoying the father walks away. Nice. I have a huge suspicion that you all youngsters are taking us for a ride. OK! Let me enjoy it. All the best. Vijaya Ratna
V.B.S.Aishwarya's picture

Respected Mam,
First of all thanks for spending your time in giving comment to my blog. Always we will be waiting for comments from experienced teachers like you. Such comments will encourage, boost up, give proper guidance and more than let us know our mistakes such that we rectify them in further steps.
Even I could remember your speech given in our college as Chief Guest on the occasion of Valedictory Function, National Pharmacy Week Celebrations 2008 which inspired us a lot and even today I have the hand out provided by you during the speech.
Coming to this blog, I thought of presenting the data right from basics to the end of significance. Of course all these are of basics, but I am interested to just warm up all the readers bringing them back to their early calculations which we have to follow even we go to our PG studies.
Mam, Shortly we will provide a blog justifying these day's blogs from our team . So please consider the case and spend time in giving your valuable comments to our blogs.
Thanking you once again for such funny joke. We enjoyed it!






Prof. J. Vijaya Ratna's picture

Dear Aishwarya I find out from the comments made by all your friends that they are really benefiting from your blog. Very good . No problem with me. We are all jointly playing this game. And we will all of us 100%, everbody, be winners. At that moment it just struck me that there is a need for a pointer which just a bit tilts the intellectual level of the game a little bit higher. Then I wrote what I wrote. I like calculations maximum and I believe that when you do problems only you learn a lot of subject. Good. All the best. Vijaya Ratna
Uma Pratyusha's picture

Dear Aishwarya, can you tell me what are the systems of weights and measures?


Uma Prathyusha

V.B.S.Aishwarya's picture

Dear .... There are two systems 1)Imperial System a) Avoirdupois system b) Apothecaries system 2)Metric system Regards,






Jigard Patel's picture

Dear Aishwarya, Nice blog.I nearly not remember that all calculation . Thank you for brush up.. Regards Jigar Patel Team : " INVINCIBLE "
V.B.S.Aishwarya's picture

hello Jigar, Thank you for your comment. I am happy that my blog has helped you






Meghna Datta's picture

Topic chosen was really different. good work meghna Team Wisdom
V.B.S.Aishwarya's picture

dear , thanks a lot for your comment






D Shravani's picture

Hi Aishwarya, Really i must say you made me rewind my B.pharm life.... I hardly remember them.... Thanks for the brush up...


V.B.S.Aishwarya's picture

Hiii Shravani, Yeah you are right these things are a bit diffcuilt to remember but very very important Thanks for your comment






TK Indira's picture

You explained very important calculations in simple yet attractive way. Good work. Keep going...

T.K. Indira.

-- "Our greatest glory is not in never falling, but in rising every time we fall..." Team 'Char'minar.

V.B.S.Aishwarya's picture

Dear Indira, Thank you so much for your comment Please commenting Regards,






P.V.ABHIGNA's picture

Dear Aishwarya, It is very good to have all theses calculations at one place and as all said it is difficult to remember all because majority of pharmacy student are poor /scared of maths. Can you please explain the application part of each calculation in pharma research in brief? Regards, Team Sentinel


V.B.S.Aishwarya's picture

Dear Abhigna, Yeah you are right many of are afraid of maths. Coming to your query, DOSAGE CALCULATIONS These are used for calculating accurate doses in case of children. ALCOHOL DILUTIONS These calculations are used to prepare alcohol at required strenght. ISOTONICTY CALCULATIONS These calculations are used to find the concentration of a given solution(nacl)required to make other solution iso osmotic with blood plsma.The parenteral solutions and opthalmic solutions need adjustmant to isotonicty. PROOF SPIRIT CALCULATIONS These are used in case of beverages when they get exported some duties are imposed on them depending the type of proof. DOSAGE REPLACEMENT FACTOR For preparation of suppositories in uniform and accurate weight this factor is used. Regards,






P.V.ABHIGNA's picture

Dear Aishwarya, Thanks for your explanation. Regards, Team Sentinel


Niklesh Rao V's picture

Dear Aishwarya, Useful info at a single page. I needed all the formulas at one place to revise for GPAT. Can you please mention the DOSAGE calculations based on bio pharmaceutical models (like one compartment, two compartment etc) and dosage adjustment formulas for hepatic and renal failure etc. Please do consider my request.

Regards, Niklesh Rao V

V.B.S.Aishwarya's picture

Hello Niklesh, Very happy to see your comment,and also to know that my blog has helped you. Coming to your query , i am persuing my second year bpharm still so i just placed the calculations which i have come across so far. But i am very happy that through your question i will learn new things.I will search up and definately let you known very soon. Regards,






kranthi kumar's picture

Good way of representing the formulas all together. I would appreciate if you can provide some way of memorizing this formulas. Keep it up.
V.B.S.Aishwarya's picture

Dear sir, Thank you for your comment.






Pratibha Singh's picture

thanks for the useful info. it helped me and my friends.

pratibha singh

V.B.S.Aishwarya's picture

really very happy to know this pls read other blogs also






Hanumantha Rao's picture

The information provided is really fantastic... very much thankful for that. But I have a small doubt.. In Isotonicity calculations, in molecular concentration method' s Example you said dextrose is unionised and hence no ions means 0 na... then N value becomes zero in that formula...then the answer will be infinite na... how it can be?


Avinash Robin's picture

Dear madam, you said that,Volume of P.S Volume of ethyl alcohol= 100/57.1= 1.753 volume of PS. But on dividing 100/57, the value is 1.751. though it may be a minute value,while calculating the large volumes it's effect will be considarable. for example............. 30% over proof= 100+30/1.753= 70.16v/v 100+30/1.751= 74.24v/v so,please suggest me which value to be considered in case of compitetive exams.


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