2. TO BECOME A NURSE:
YOU HAVE TO
LEARN DRUG
DOSAGE
CALCULATIONS
3. ARE THEY EASY?
Yes and No
Everybody wants to teach
you how.
Everybody teaches you a
different way.
Everybody says their way is
the best way.
But no one will say you don’t
need to understand basic
math.
4. A QUESTION TO ASK YOURSELF
Do I want to plug into a
formula?
Or do I want to
really understand?
5. SOME QUOTES FOR THOUGHT
• “Students who developed logical reasoning
skills reported improved confidence in drug
dosage calculations.”
• “Feedback and discussion reveal that
confidence in arithmetic skills can be low even
when students are able to pass the end of
semester drug dosage calculation exam.”
• “It is regarded as acceptable that a formula
card or mnemonic can be used to find the
correct dose even though this removes any
requirement for performing the underlying
computation.”
• “Emphasize students’ innate powers of logical
Europepmc.org/article/med/27138475
7. ONE GOOD WAY TO KNOW YOU
ARE REALLY THINKING: SEE
THAT ALL THE WAYS OF DOING
= SAME RESULT.
AND THE WAYS ARE:
The formula (“desired over have”)
Proportions
Unit cancellation (also called “dimensional analysis”)
Reasoning by logical step
AND THE PROS AND CONS ARE:
The formula (“desired over have”): can be very confusing – formulas stated and explained in
many different ways.
Proportions: no issues, somewhat an indirect approach but very logical.
Unit cancellation: virtually always works – no brainer once you understand it.
Reasoning by logical step: it perhaps can take more time in the short run.
8. TAKE A BASIC PROBLEM
Order: 4 mg Ativan by injection
Available: 2mg/ml solution
Problem: how many ml to draw up
And let’s start with the famous
“desired over have” method:
Ordered = “Desired” = D in the formula
Available = “Have” = H in the formula
“Quantity” = Q in the formula
“Amount” = A in the formula
𝐴𝑚𝑜𝑢𝑛𝑡 =
𝐷𝑒𝑠𝑖𝑟𝑒𝑑
𝐻𝑎𝑣𝑒
× 𝑄𝑢𝑎𝑛𝑡𝑖𝑡𝑦 =
𝐷
𝐻
× 𝑄 =
4 𝑚𝑔
2 𝑚𝑔
𝑥 1 𝑚𝑙 = 2 𝑚𝑙
(1 is understood)
9. DRAWBACKS TO
“DESIRED OVER HAVE”
You will see different versions of the formula, e.g. some will replace Q with V for “volume”
– fine for this particular problem (ml is a volume), but what about when its tablets or
capsules?
You have to correctly identify the D, H, and Q in each problem to get them plugged in
right.
You have to memorize the formula.
The units of the desired and have must match – if they don’t it throws a monkey wrench
into the method: you have to convert the units of either the numerator or denominator so
that they do match, and this must be done outside of the formula itself.
Finally, this way works in basic drug dosage calculations, but what about IV problems (ml
or drop rate, etc.) – NOPE.
10. LET’S TRY PROPORTIONS
1
5
=
Order: 4 mg Ativan by injection
Available: 2mg/ml solution
Problem: how many ml to draw up
2 𝑚𝑔
1 𝑚𝑙
=
4 𝑚𝑔
𝑥 𝑚𝑙
Cross multiplying and solving for the amount x:
2𝑥 = 4
2 2
𝑥 = 2
So the answer is draw up 2 ml.
NOTE: A shortcut that always works is to
multiply the numbers that are diagonal to
each other and then divide that result by
the 3rd number, as:
4 X 1 = 4 and 4 ÷ 2 = 2
11. PROS AND CONS OF PROPORTION
• You do have to think of what is happening to set it up (think: there
are 2 mg in each ml of solution, but I want 4 mg to go into the
patient).
• Remember same units on top and bottom of each ratio.
• Remember how to solve for x.
• Can also be used in more complex problems, but then more than
one proportion may be required. (With unit cancellation the extra
proportion(s) are worked right into the setup).
• For example, if unit conversion is necessary, just use another
proportion. For example:
I need to covert 4000 mg to g:
4000 𝑚𝑔
𝑥 𝑔
=
1000 𝑚𝑔
1 𝑔
4000 X 1 = 4000 and 4000 ÷ 1000 = 4 so 4g
12. HOW ABOUT UNIT
CANCELLATION?
Order: 4 mg Ativan by injection
Available: 2mg/ml solution
Problem: how many ml to draw up
4 𝑚𝑔 ×
1𝑚𝑙
2 𝑚𝑔
= 2 𝑚𝑙 4 𝑚𝑔
1
×
1 𝑚𝑙
2 𝑚𝑔
=
4 × 1
1 × 2
𝑚𝑙 = 2 𝑚𝑙
Or with everything explicit:
Calculator operation:
4 ÷ 1 X 1 ÷ 2 or ignoring the 1’s: 4 ÷ 2
Alternative calculator operations:
4 X 1 ÷ 1 ÷ 2 or ignoring the 1’s: 4 ÷ 2
4 x 1/(1 X 2) or ignoring the 1’s: 4 ÷ 2
When the units cancel you don’t want and the
unit you do want is left standing, you can be sure
the problem is set up correctly. Multiply key
before any number on the top (if a whole number
it is the top) and divide key before any number
on the bottom = the number part of the answer.
13. PROS AND CONS UNIT CANCELLATION
• It works for any problem.
• It is a no brainer – cancel the units, it is set up
correctly.
• The order doesn’t matter; I like to start with the
simple unit (mg) and then the complex (mg/ml)
You have to use the calculator correctly – just
remember: on top, multiply, on bottom, divide and OK
TO HIT DIVIDED BY KEY MORE THAN ONCE IN A
ROW.
Looks funny when you flip the complex unit (from g/ml
to ml/g – JUST MAKE THE UNITS CANCEL).
14. ONE MORE WAY: REASONING
BY LOGICAL STEP
Also known as: look Ma, no format!
Order: 4 mg Ativan by injection
Available: 2mg/ml solution
Problem: how many ml to draw up
I think: OK, I have a solution here with 2 mg of drug in every 1 ml.
I need to put 4 mg into the patient. So if every ml has 2 mg, the
number of times 2 goes into the 4 that is wanted would be the
number of ml needed. OR, more simply given the simple
numbers involved in this particular problem: I need 4 g to go into
the patient; each 1 ml has 2 g in it; I need twice as much, so 2 ml.
15. PROS AND CONS OF LOGICAL REASONING?
• You really understand the problem.
• But you do need to have the correct understanding, and sometimes
our understanding can be wrong – but with practice you can be
quite confident.
• As for every method we have described:
YOU HAVE TO UNDERSTAND BASIC MATH
When using logical reasoning the trick is to decide when to multiply,
divide, even add or subtract. When you know basic math, the correct
decision is intuitive and easy.
16. LET’S LOOK AT SOME
KINDS OF PROBLEMS
We will use unit cancellation to solve them.
Basic drug dosage problems
• Available: mass/volume
• Available: mass/tablet
Dosage by weight
IV rate calculations
• Desired volume/time given– wanted in mL/hr
• Desired volume/time and drop factor given – wanted in gtts/min
• Desired mass/time and available mass/volume given – wanted in mL/hr
Combination IV problems: e.g. given = desired mass drug/kg patient weight/hour,
patient weight, and available mass/volume – wanted in mL/hr
Other IV problems: calculate total infusion volume or time, adjust manual flow rate
Amounts of non-injectable fluids
17. BASIC DRUG DOSAGE:
AVAILABLE MASS/VOLUME
We did this in our example of 4 mg of Ativan by
injection and available was 2 mg/mL, and we wanted
to know how many mL to inject.
Here is another example (with an added twist of unit conversion needed):
Order: 0.1 g of drug by injection
Available: 30 mg/5 mL solution
Problem: how many mL to draw up
0.1 𝑔 ×
1000 𝑚𝑔
1 𝑔
×
5 𝑚𝐿
30 𝑚𝑔
= 16.666 … = 16.7 𝑚𝐿
Note: This problem had a number, 5 (not just an understood 1) in the volume. I like to start with the simple unit given (g).
This helps me to “follow my nose” through the calculation. The complex unit (mg/mL) is flipped so that the units cancel.
Order doesn’t matter, but if you start with the given 30 mg/5mL fact you need to be sure to write the relation with mL on top
(because that is the unit of your wanted answer – your answer must have mL “on top” and nothing on the bottom).
18. BASIC DRUG DOSAGE:
AVAILABLE IS
MASS/TABLET
Or capsule or whatever.
Order: 25 mg of drug PO
Available: 50 mg/tablet
Problem: how many tablets to administer
25 𝑚𝑔 ×
1 𝑡𝑎𝑏𝑙𝑒𝑡
50 𝑚𝑔
= 0.5 𝑡𝑎𝑏𝑙𝑒𝑡
Note: Looks like we are going to have to split that tablet! Again, note how I start with the simple unit given (mg).
It helps to “follow my nose,” because I know that unit will have to cancel (I want tablets). If I start with the complex
unit I have to think that tablets (the wanted unit) will have to be on the top, but when I start with mg it simply forces
me to write the complex unit the right way so that the mg unit cancels.
19. DOSAGE BY WEIGHT
Order: 1.5 mg of drug per kg of patient weigh
Given information: patient weighs 74.8 lb
Available: 125 mg/ 2 mL solution
Problem: how many mL to draw up
This brings in a famous conversion fact: 1 kg = 2.2 lb
74.8 𝑙𝑏 ×
1 𝑘𝑔
2.2 𝑙𝑏
×
1.5 𝑚𝑔
1 𝑘𝑔
×
2 𝑚𝐿
125 𝑚𝑔
= .816 = .82 𝑚𝐿
Note: Starting with the simple unit (lb, the patient’s weight) helps us to “follow our nose” canceling units – whatever unit
is on the top (and which is not the unit of our answer) must go on the bottom of the next fraction, so that it cancels. As
with all unit cancellation, you are done when all the units you don’t want cancel, and the unit you want in your answer
remains uncanceled. That is the unit of your answer, and you calculate the number. REMEMBER: numbers on top are
multiplied, numbers on the bottom are divided which may mean repeated use of the division key.
20. IV RATE: VOLUME/TIME IS GIVEN –
CALCULATE ML/HR
The flow rate wanted is in mL/hr. This is the usual case with the
“electronically regulated infusion pump.”
Order: 250 mL of solution to be infused over (per) the next 120 minutes
Available: the solution
Problem: What is the infusion rate in mL/hr?
250 𝑚𝐿
120 𝑚𝑖𝑛
×
60 𝑚𝑖𝑛
1 ℎ𝑟
=
125 𝑚𝐿
1 ℎ𝑟
Note: you are only given a complex unit (a “per expression”), so you have to start with that. Then you use the conversion
fact 1 hr = 60 min. Note in this case we cancel bottom to top (till now it has all been top to bottom). KEEP YOUR EYE
ON THE UNIT OF YOUR ANSWER: in this case a complex unit with mL on top and hr on the bottom. A conversion fact
is true written either way, but we write it so that min cancel. That way not only does mL remain uncanceled on the top,
but hr remains uncanceled on the bottom, which is where you want it!
= 125 mL/hr
21. IV RATE: VOLUME/TIME IS
GIVEN WITH A DROP FACTOR –
CALCULATE GTT/MIN
The flow rate wanted is gtt/min (gtt = “drops”). This is the usual
case with the “manually regulated IV.”
Order: 1000 mL of fluid in 10 hrs
Drop factor given: 15 gtt/mL
Problem: What is the infusion rate in gtt/min?
1000 𝑚𝐿
10 ℎ𝑟𝑠
×
1 ℎ𝑟
60 𝑚𝑖𝑛
×
15 𝑔𝑡𝑡
1 𝑚𝐿
=
25 𝑔𝑡𝑡
1 𝑚𝑖𝑛
= 25 gtt/min
Again, note where you are going: you want gtt on top and min on bottom in the answer. We didn’t have to write the
order or the drop factor flipped – we simply made sure both hours and mL cancel out and we are left with gtt on top and
min on bottom. We cancel both bottom to top and top to bottom as needed. Note that the beginning complex unit (the
order) has mL, a volume, on the top because our answer needs to have gtt, a volume, on the top.
22. IV RATE: MASS/TIME IS
GIVEN – CALCULATE ML/HR
Order: 20 mg of drug per hour
Available: 500 mg in 250 mL of solution
Problem: What is the infusion rate in mL/hr?
250 𝑚𝐿
500 𝑚𝑔
×
20 𝑚𝑔
1 ℎ𝑟
=
10 𝑚𝐿
1 ℎ𝑟
= 10 mL/hr
Note: we want mL on top in the answer, so we write the mass and volume relation with volume on
the top – mg (which is not in the answer) cancels bottom to top.
23. IV PROBLEM: COMBO
WITH PATIENT WEIGHT
Order: 6 mcg/kg/hr
Patient weight: 100 kg
To infuse: 12.5 mg drug in 250 mL of fluid
Problem: What is infusion rate in mL/hr?
100 𝑘𝑔 ×
6 𝑚𝑐𝑔
𝑘𝑔 × ℎ𝑟
×
1 𝑚𝑔
1000 𝑚𝑐𝑔
×
250 𝑚𝐿
12.5 𝑚𝑔
=
12 𝑚𝐿
ℎ𝑟
= 12 mL/hr
Note:
6 𝑚𝑐𝑔
𝑘𝑔
ℎ𝑟
1
=
6 𝑚𝑐𝑔
𝑘𝑔
×
1
ℎ𝑟
=
6 𝑚𝑐𝑔
𝑘𝑔 ×ℎ𝑟
Note: Start with the simple unit (kg) which needs to
cancel. The given per/per order of mass of drug/patient
weight/time is written with the patient weight unit
multiplied by the time unit in the denominator. The mass
of drug per volume relation is written with volume on the
top, because volume is on the top of the wanted answer.
24. MORE VARIATIONS ON IV
PROBLEMS – SAME METHOD
WORKS FINE
Infuse 500 mL at 20 gtt/min, factor = 20 gtt per mL.
What is the total time in hrs to infuse?
500 𝑚𝐿 ×
20 𝑔𝑡𝑡
𝑚𝐿
×
1 𝑚𝑖𝑛
20 𝑔𝑡𝑡
×
1 ℎ𝑟
60 𝑚𝑖𝑛
= 8.333… hr = 8 1/3 hr
Infuse the solution over 8 hr at 21 gtt/min, factor
= 10 gtt/mL. What is the total volume needed?
8 ℎ𝑟 ×
60 𝑚𝑖𝑛
ℎ𝑟
×
21 𝑔𝑡𝑡
𝑚𝑖𝑛
×
1 𝑚𝐿
10 𝑔𝑡𝑡
= 1008 mL
The order was 500 mL over 5 hrs, factor = 10 gtt/mL. After 1 hr of infusion, 300 mL remain in the bag.
Calculate the adjusted flow rate. Note, just a new problem with 300 mL over 4 hr.
300 𝑚𝐿
4 ℎ𝑟
×
1 ℎ𝑟
60 𝑚𝑖𝑛
×
10 𝑔𝑡𝑡
𝑚𝐿
= 12.5 gtt/min = 13 gtt/min (rounding to full drop).
25. CALCULATE AMOUNTS OF
NON-INJECTABLE FLUIDS
Ordered: wound care q2h for 3 da using 45 mL of 25%
solution (available is full strength solution = “medication”).
Problem: find the volume of medication and volume of
normal saline to mix to prepare enough.
First calculate total volume needed:
3 𝑑𝑎 ×
12 𝑑𝑜𝑠𝑒𝑠
𝑑𝑎
×
45 𝑚𝐿
𝑑𝑜𝑠𝑒
= 1620 mL
Then calculate the volume of medication in that total volume:
1620 𝑚𝐿 × .25 = 405 𝑚𝐿 𝑜𝑓 𝑚𝑒𝑑𝑖𝑐𝑎𝑡𝑖𝑜𝑛
Finally, subtract volume of medication from the total volume to get the volume of normal saline:
1620 𝑚𝐿 − 405 𝑚𝐿 = 1215 𝑚𝐿 𝑜𝑓 𝑛𝑜𝑟𝑚𝑎𝑙 𝑠𝑎𝑙𝑖𝑛𝑒
27. CONVENTIONS PER
DEPARTMENT PRACTICE TESTS
Answers over 1 mL: round to nearest tenth.
Answers under 1 mL: round to nearest hundredth.
All flow rates are rounded to the nearest whole number.
Decimals less than 1: always put a leading zero (e.g., 0.75 mL).
Partial tablets written as decimal (e.g., 1.5 tablets).
READ THE QUESTION: sometimes it will also tell you how to
round.
TIPS:
“Available” is the same as “On hand.”
Don’t type units of your answer if the units are already there after the blank to fill in.
Note that “dose” is the order and is usually in mg.
“Volume” can be used to indicate any kind of answer, not just mL, but also tablets or capsules.
• Capsules can’t be divided; a fractional answer or an answer more than three indicate a
possible error.
• Tablets can be divided, at least if “scored,” and an answer would usually range from ½ to 3.
NOTE: The mass/volume relations come from reading the label. Write so that units cancel.
28. Order: 50 mEq PO daily
Available: 25 mEq/5 mL
Wanted: how many mL to administer
ANSWER: 10 mL
50 𝑚𝐸𝑞 ×
5 𝑚𝐿
25 𝑚𝐸𝑞
= 10 𝑚𝐿
NOTE: The “PO daily” is not information used in the calculation. You just need to calculate how
many mL to draw for each day’s dose. Start with the simple unit (mEq) which you don’t want in
your answer, and write the mEq/volume (which is given) so that the units cancel. You want mL
for your answer, so it goes on top.
29. Order: 1.5 g PO 4X daily
Available: 500 mg scored tablets
Wanted: amount to administer
ANSWER: 3 tablets
1.5 𝑔 ×
1000 𝑚𝑔
1 𝑔
×
1 𝑡𝑎𝑏𝑙𝑒𝑡
500 𝑚𝑔
= 3 𝑡𝑎𝑏𝑙𝑒𝑡𝑠
NOTE: The “PO 4X daily” is not used in the calculation. Always interpret the “amount to
administer” as per dose, unless specifically asked for the daily total or some other total. Start
with simple unit (g) which needs to be converted to mg (because the tablets are given in mg).
That cancels g. Then write the mass per tablet so that the mg also cancels (your answer has
the units “tablets”). “Tablets” is left standing and that is the unit of your answer.
30. Order: 1.4 mg/kg of patient weight
Patient weight 34 lb
Available: 3 mg/mL
Wanted: volume to draw
ANSWER: 7.2121... = 7.2 mL
34 𝑙𝑏 ×
1 𝑘𝑔
2.2 𝑙𝑏
×
1.4 𝑚𝑔
1𝑘𝑔
×
1 𝑚𝐿
3 𝑚𝑔
= 7.2121 … = 7.2 𝑚𝐿
NOTE: A perfect “follow your nose” example. Start with the simple unit (lb). That unit must go in
the denominator to cancel. Then with each relation you have (knowing lb to kg is 1lb = 2.2 kg)
write so that the unit on top cancels top to bottom, until what you want, mL, is left standing.
31. Order: 100 mL to be infused every 15
min for 5 hours
Wanted: infusion pump rate in mL/hr
ANSWER: 400 mL/hr
100 𝑚𝐿
15 𝑚𝑖𝑛
×
60 𝑚𝑖𝑛
1 ℎ𝑟
=
400 𝑚𝐿
1 ℎ𝑟
= 400 𝑚𝐿/ℎ𝑟
NOTE: Some problems start with a “complex unit,” in this case mL/min. We want mL/hr for an answer,
so we write the relation with mL on top. We don’t want min, we want hr, so we make min cancel bottom
to top. That leaves us with mL on top and hr on the bottom for our mL/hr answer. We didn’t use the
total time of 5 hours.
But what if the order stated more simply: “infuse 2000 mL over 5 hours.” Then: 2000 mL divided by 5 hr = 400
mL/hr. Sometimes the answer is just reducing the fraction given!
32. Order: 1500 mL to infuse over 12 hr
Given: drop factor = 20 gtt/mL
Wanted: manual IV infusion rate in gtt/min
ANSWER: 42 gtt/min
1500 𝑚𝐿
12 ℎ𝑟
×
1 ℎ𝑟
60 𝑚𝑖𝑛
×
20 𝑔𝑡𝑡
1 𝑚𝐿
=
42 𝑔𝑡𝑡
1 𝑚𝑖𝑛
= 42 𝑔𝑡𝑡/𝑚𝑖𝑛
NOTE: There is no simple unit to start with, because 1500 mL over 12 hr is a rate of mL/hr. We start
with that, because that is volume over time which our answer will also be (just different units). Then
we use known and given relations to convert hr to min and mL to gtt (order does not matter, just so
what we don’t want cancels, and what we do want remains: gtt on top, and min on bottom).
(41.666… = 42)
33. Order: infuse a solution of 2g in 500 mL at 4 mg/min.
Wanted: flow rate for infusion pump in mL/hr
ANSWER: 60 mL/hr
500 𝑚𝐿
2 𝑔
×
1 𝑔
1000 𝑚𝑔
×
4 𝑚𝑔
1 𝑚𝑖𝑛
×
60 𝑚𝑖𝑛
1 ℎ𝑟
=
60 𝑚𝐿
1 ℎ𝑟
= 60 𝑚𝐿/ℎ𝑟
NOTE: No simple unit to start with, so we start with the solution relation with mL on top (for the top of our
answer). Then in any order, conversion of g to mg, cancel mg with the given mass to time rate, and conversion
of min to hr. However, I always use the natural order: whatever unit that is in what I have just written (a unit I
don’t want in my answer), I cancel it with the next thing I write. Sometimes one of the units may have to wait to
be cancelled by something later in the series. In this case, units always cancel bottom to top immediately.
34. Order: infuse a solution of 5 g in 1000 mL at a rate
of 1 mg/kg/hour
Patient weight: 50 lb
Wanted: flow rate of infusion pump in mL/hr
50 𝑙𝑏 ×
1 𝑘𝑔
2.2 𝑙𝑏
×
1 𝑚𝑔
𝑘𝑔 × ℎ𝑟
×
1 𝑔
1000 𝑚𝑔
×
1000 𝑚𝐿
5 𝑔
=
22 𝑚𝐿
1 ℎ𝑟
= 4.5454 … = 5 𝑚𝐿/ℎ𝑟
ANSWER: 4.5454… = 5 mL/hr
NOTE: We do have a simple unit to start with, the patient’s weight in lb. We first convert to kg and
then use the “per/per” expression so that kg cancels, then since our solution relation has g, we
convert to grams (cancelling the mg). Finally we write our solution relation with mL at the top (where
it is in the answer). Grams then cancels, and note the hr we want in our answer is still on the bottom
uncancelled. In this case all cancellations are top to bottom.
35. Order: 900 mL to infuse at 150 mL/hr
Wanted: if we start at 0730, when will the
infusion be done?
ANSWER: 1330
900 𝑚𝐿 ×
1 ℎ𝑟
150 𝑚𝐿
= 6 ℎ𝑟
0730 + 6 hr = 1330
Military time: after 1200 = 12N, instead of
going back to 0100 (which means 1 AM) we
just keep counting: 1 PM =1300, 8 PM =
2000, 11 PM = 2300, 12 midnight = 0000. So,
for afternoon time just add 12: 2 PM + 12 =
1400.
NOTE: The set up is wonderfully simple for this problem. Start with the simple unit (mL) and cancel the
mL which you don’t want leaving the hr that you do. Then, for a change, we ADD to get the ending
military time.
36. Order: 8.45% NS to infuse at 125 mL/hr over 32 hr
Wanted: the total volume needed
ANSWER: 4000 mL
32 ℎ𝑟 ×
125 𝑚𝐿
ℎ𝑟
= 4000 𝑚𝐿
NOTE: Again the setup is really simple. Start with that simple unit (hr) and then look at your rate which
has hours in it (and the mL you need). Make the hr cancel and you’re done.
37. Order: Infuse 50 mL of solution in 15 min
Drop factor: 15 gtt/mL
Wanted: infusion rate (manual IV) in gtt/mL
ANSWER: 50 gtt/min
50 𝑚𝐿
15 𝑚𝑖𝑛
×
15 𝑔𝑡𝑡
𝑚𝐿
=
50 𝑔𝑡𝑡
1 𝑚𝑖𝑛
= 50 𝑔𝑡𝑡/𝑚𝑖𝑛
Problem: After 5 min there are 40 mL left in the IV bag.
Wanted: Adjusted flow rate so that the drug is finished
infusing on time.
ANSWER: 60 gtt/mL
40 𝑚𝐿
10 𝑚𝑖𝑛
×
15 𝑔𝑡𝑡
𝑚𝐿
=
60 𝑔𝑡𝑡
1 𝑚𝑖𝑛
= 60 𝑔𝑡𝑡/𝑚𝑖𝑛
NOTE: We start with mL on the top, because that is a volume and we want a volume (gtt) on top in
our answer. We only need to cancel mL. For the adjusted flow rate problem, we of course have to
SUBTRACT 15 – 5 = 10 min remaining to put in the given (now) 40 mL.
38. Order: For wound care – 1 ½ oz of ¾ strength wound care solution applied
each hr for 24 hr.
Available: full strength solution (“medication”)
Wanted: volume of medication and volume of normal saline to prepare enough
ANSWER: 810 mL of medication and 270 mL of normal saline
Calculate total volume needed:
24 𝑑𝑜𝑠𝑒𝑠 ×
1.5 𝑜𝑧
𝑑𝑜𝑠𝑒
×
30 𝑚𝐿
1 𝑜𝑧
= 1080 𝑚𝐿
Calculate the volume of medication needed:
3
4
× 1080 𝑚𝐿 = 810 𝑚𝐿
Calculate the volume of normal saline needed:
1080 mL − 810 mL = 270 mL
NOTE: Start with the simple unit (doses), then use the
oz per dose, then convert oz to mL (memorize that
conversion). The strength of the solution is best done
as a separate calculation (the ¾ or .75 does not have
units). Finally, add or subtract needs to be done
separately from unit cancellation: unit cancellation is
only multiply and divide.
39. REVIEW: IMPORTANCE OF
CORRECT CALCULATOR
OPERATION
EXAMPLE:
74.8 𝑙𝑏 ×
1 𝑘𝑔
2.2 𝑙𝑏
×
1.5 𝑚𝑔
1 𝑘𝑔
×
2 𝑚𝐿
125 𝑚𝑔
= .816 = .82 𝑚𝐿
DO:
74.8 X 1 ÷ 2.2 X 1.5 ÷ 1 X 2 ÷ 125 = .816 or 74.8 ÷ 2.2 X 1.5 X 2 ÷ 125
OR
74.8 X 1 X 1.5 X 2 ÷ 2.2 ÷ 1 ÷ 125 or 74.8 X 1.5 X 2 ÷ 2.2 ÷ 125
DO NOT DO:
74.8 X 1 X 1.5 X 2 ÷ 2.2 X 1 X 125 = 8500 or 74.8 X 1.5 X 2 ÷ 2.2 X 125 = 8500
Unless you use PARENTHESES:
74.8 X 1 X 1.5 X 2 ÷ (2.2 X 1 X 125) = .816 or 74.8 X 1.5 X 2 ÷ (2.2 X 125) = .816
REMEMBER: It is fine to
use ÷ two or more times in a
row. Just be sure any
number on top gets X and
any number on the bottom
gets ÷ .
40. WRAP UP
• Memorize formulas – not recommended.
• Proportion – OK, but on more complicated problems (IV) may take 2 or 3 separate ones.
• Unit cancellation – you can’t go wrong.
o However: always check to see if the unit (uncancelled) that you end up with is the unit of the
wanted answer – otherwise you stopped too soon!
o Always look toward the units of the answer: what is on top of the answer will have to be
somewhere on top of the set-up (and not cancel), and what is on the bottom of the answer will
have to be somewhere on the bottom of the set-up (and not cancel).
o If a number has a single unit only – that is, it is simply a whole number with a unit, no fraction
involved (like lb or whatever), it is understood to be on the top. You may write it over a 1 if that
helps you.
o It helps to start with a simple unit number if possible, but sometimes you have to start with a
“complex” unit which is a ratio (“rate”) or “per expression.” For example:
50 mL
15 min
. Just be sure
what you want on the top of your answer is on the top of the ratio.
Try it! I did an hour practice test in 45 min, and I found myself using ALL unit cancellation. I got,
oops, 90%, but that was from 3 simple errors. One I stopped to soon (check the units when you’re
done!). One I put .75 instead of 0.75. And one was a rounding error (up instead of down).
GO FOR IT! PRACTICE! ALL THE BEST IN YOUR NURSING CAREER!
