# Dosage Calculations

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I am in a desperate situation. I thought I was getting my calculations correctly and today I got back my math results and i got a 70% I am so disappointed I dont know what to do. On my first exam I received an 80%.

My professor said that if i get above 95% on my final (which is next week) I may be able to average a b+ or a- , but it will all depend on the overall average, if he curves grades or not. I went and bought 2 self help books on calculations .... My question is ..... do you think it is humanly possible to ingest all this info by next week (exactly a week from today)! I am having problems with the drips and the three step conversions. Is there a website that can help me? To top it off my professor sucks, so what I have learned, I have learned on my own. I am so frustrated, so much so, that it is getting harder and harder for me to pick up a book .....sorry for venting

Specializes in Med-Surg, Tele, Vascular, Plastics.
lpnlpn said:
i am having trouble converting between systems of measurement, i am trying to teach myself the conversion factor method as well as the ratio proportion method but this problem is giving me a bit of a problem the problem is ---3g=gr_____ help!

Hello,

I can teach you both ways. Proportions are bullcrap. They take too much time. Alot of times you can simply figure it out in your head if you know what the conversion is. But if your school is making you do proportions... this is how you do it. conversion is simple if you know the measurements...

You should know by now that 1 gram = 15-16 grains cuz grains are not exact. 15 is an easier number to work with... so if you have 3 grams then you have 45 grains. Just multiply.

Now here is how you do proportion: they waste your time... but sometime useful if you are just learning or have a complicated measurement.

this is just one example: if you want another one, let me know. Hope it helps.

1 g (gram) = (grains) gr 15 *grains is always written before the value

so whenever you set up a proportion you do it like this

1 g : 15 gr :: 3 g : x gr *you can put grains after the number to keep it simple

so this is how you read a proportion

1 gram is to 15 grains as 3 grams is to x number of grains

now you multiple the outside numbers first

then you multiple the inside numbers second

so

1x = 45

then divide to solve for x

so

x = 45 / 1

x = gr 45 * remember gr goes before the number in the final answer.

hope that makes sense.

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Specializes in Med-Surg, Tele, Vascular, Plastics.
Im doing Pharm now and I did come across something that needed rounding and I couldn't figure out when to round. Thanks for the info...

Here's one that I just did:

Order: Ampicllin 500mg IM q.6h

On hand 1g...instructions: add 3.5mL diuent

500(D)

1000(H) times 3.5mL(Q) = 1.75 mL rounded to 2mL

Hello,

I read your post and I am pretty sure that you should not round that up.

You should keep your final answer at 1.75 mL. Because, if you are going to use a 3 mL syringe then you can easily draw up 1.75. It has a 1.5 marking and a 2 ml marking with .10 mL increments. So you would just draw up half way between 1.7 and 1.8 marking.

I'ts not much of a difference, but when you are dealing with cardiac drugs or drugs for peds... that much could make a hell of alot of difference.

How we were taught to round up is when doing IV drip rates... cuz obviously you cant have 31.1 drips... hehehe. You have to round that to 31 drips.

Here is one time that you can round up...

phenobarbital 70 mg SQ q8 h ( supplied in a 1-ml ampule containing 65 mg)

70 mg

65 mg x 1 mL = 14/13 when you divide it out it comes to 1.07

you could not draw that up with one 3 mL syringe... so you would round that to 1.1 which you could draw up on a 3 ml syrninge.

I hope this helps you.

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Specializes in Med-Surg, Tele, Vascular, Plastics.
EDRNMass said:
Is there an easy way to figure out the rounding? I understand that an IV pump only understands ml/hour.

I understand heparing and ? you don't round.

But some problems they have us rounding to the whole number and some are to the tenths place.

Is there an easy way to remember which we round and which we don't?

This part frustrates me. I believe on our test it's going to say round to whatever place. But how about in real life.

Hello,

Usually you round when doing mL/Hr to nearest whole one's. Cuz you couldnt program a pump to do 125.2 ml/hr so it would have to be 125 ml/Hr.

And when you are doing gtt/min you have to round that to the nearest ones place cuz you cant have 31.5 drops... you have to round to 32 drops.

As for injections... read the post by lil' girl... she stated corrected.

Here is an example... if you need 0.449 round that to the nearest hundreths. 0.45 ( it is less than 1 mL and you can use an insulin syringe to draw it up)

if it is. If it is more than 1 mL... round to the nearest tenths. so 2.45 would round to 2.5 mL.

Hope that helps for you

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Hi everyone,

I am still working on learning my equivalencies (sp?) and I came across this website, it was really helpful to me because it has a conversions calculator so maybe it can help some other people out!

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Specializes in med/surg, telemetry, IV therapy, mgmt.

http://www.manuelsweb.com/nrs_calculators.htm - you can double check your pencil on paper drug calculations with this handy tool! these are on-line working calculators to solve nursing math problems with formulas and examples included to show you how each calculator works. you can work problems for oral and iv medications.

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here is how I did those problems: remember to change 8 mg to mcg

8 mcg x 60 min. x 250 ml comes to: 480/8000 mcg =.06 x 250 ml = 15 ml

8000 mcg

then divide by 60 drop x 60 min= 15 gtt/min

Specializes in Cardiac/Telemetry.

We just learned Dimensional Analysis and I found it very helpful. Also, this formula D (order)/ H (what's on hand or supply) X V (vehicle in which the drug will be administered). D/H X V. For example,

order:Orinase 250mg po bid

supply:Orinase 0.25g tablets

As you can see the order for Orinase is 250mg. The pharmacy has supplied you with 0.25g. You must convert the g of the supply into the mg of the order. So, DA is a lot easier to use for this type of problem. The formula is V/H X C (conversion of H)/ C (conversion of D) X D/1. V (vehicle) will be in tablets. So, 1 tab/0.25g X 1g/1000mg (these are the conversions of the supply and the order) X 250mg/1.

I don't know how to put the numbers on top of each other to make it look more simple, but if you put the formulas on paper, they look a lot less confusing than what they look now :) .

Anyway, so you cross out the grams and the milligrams and you're left with just numbers and the V (vehicle). So, you multiply 1x1x250=250. Then, you do the bottom: multiply 0.25x1000x1=250. Then, divide 250/250 and you get 1 tablet. That is the dosage you will give the pt.

I hope I didn't confuse you, but DA or Dimensional Analysis has really helped me out. I hope I helped out a little bit and didn't make your life a lot more complicated. If you care, PM me so that way I can explain myself a little better. God bless and good luck!!

Mave.

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Specializes in med/surg, telemetry, IV therapy, mgmt.

Awesome, Mave! You got it! Let me just add one little touch. The bottom line is that you set these equations up so that when you are all done multiplying and cancelling out labels from the numerator and denominator, that the label(s) you are left with are the ones the problem is asking for. Good job, Woman!

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Please help. I am a first semester student and my very first test in Holistic was a dosage calculations test. The requirement is you must make a 90 by the 2nd try or your out of the course. I am a nontraditional student and math is not my strongest area. I made a 70 on my first try. In two weeks I have to make a 90. I have a couple of questions that I missed on my first test. I would appreciate any help you are willing to offer.

1. Over the next 4 hours, infuse 500mL D51/2NS w/ 20 mEq KCL. The drop factor is 10 gtts/mL. How many gtts/min will the IV infuse?

2. Ordered: Kantrex 15mg/kg/day given in two equally divided doses.

Supplied: Kantrez 75 mg/2mL. Childs weight: 11 lbs. How many mL will you administer?

Specializes in med/surg, telemetry, IV therapy, mgmt.

In setting up and working these problems by dimensional analysis you are creating fractions that are really ratios. Some of these ratios are the equivalent of the number 1 which you might remember from math can always be multiplied by another number to get that same number. In dimensional analysis you are tweaking this ratio to adjust for the labels you are putting with the numbers. When you have fractions with mgs in one of the numerators and mgs in one of the denominators, the mgs label cancels itself out and all you are left with are the numbers. The goal in setting up these equations of ratios is to obtain a final answer with the labels you want on it, such as gtts/min as in the first problem or mL/day in the second problem.

Quote
Over the next 4 hours, infuse 500mL D51/2NS w/ 20 mEq KCL. The drop factor is 10 gtts/mL. How many gtts/min will the IV infuse?

500 mL (amount to be given)/4 hours (over this time) x 1 hr / 60 min (conversion factor of hours to minutes) x 10 gtts / 1 mL (drip factor of tubing) = 20.833 gtts / 1 min = 21 gtts /min (rounded up)

Quote
Ordered: Kantrex 15mg/kg/day given in two equally divided doses.

Supplied: Kantrez 75 mg/2mL. Childs weight: 11 lbs. How many mL will you administer?

First of all, to set this up in ratios to do the dimensional analysis you should re-write the ratio of 15mg/1kg/1day, which is a complex fraction, as: 15mg / 1 kg x 1 / 1 day (multiply the top and bottom of the equation by the reciprocal of what is in the denominator which is 1day/1). I am going to use the kg to lb conversion of .454kg = 1lb. This is the equation of fractions (ratios) you need to set up to work this problem:

15 mg / 1 kg x 1 / 1 day (amount to be given) x 2 mL / 75 mg (dosage on hand--note how the fraction is set up. This is in order to cancel out the "mg" label.) x 11 lbs / 1 (kids weight) x .454 kg / 1 lb (lb to kg conversion factor)= 1.9976 mL / day.
However, you need to give this daily dose in two equally divided doses, so divide the answer by 2 to get:
1.9976 mL/day / 2 = .9988ml/day = 1 mL (rounded up)

This medication is given IV, so each 1mL dose of the Kantrex 75mg/2mL would be diluted in NS or D5W and administered over 30 or 60 minutes.

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Hello everyone !! Long time don't post, but i have a question

Ordered 300 cc D5W.45%N/S in 4 hours.

A. How many cc/hr.

I get this one 300cc/4hours = 75cc/hr

B. How many gtt/min if you use 10gtt/ml tubing?

please show me the work on this one. Thanks!!!

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Specializes in med/surg, telemetry, IV therapy, mgmt.
Quote
Ordered 300 cc D5W.45%N/S in 4 hours. How many gtt/min if you use 10gtt/ml tubing? Please show me the work on this one. Thanks!!!

300 cc / 4 hours (desired dosage) x 10 gtt / mL(cc) (drip factor of tubing) x 1 hour / 60 minutes (time conversion factor = 12.5 gtts/minute, or rounding up, 13 gtts/minute

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