Weight-Based Calculations

mg/kg or mcg/kg

Infants and children can't safely take adult doses of most medications as they don't have the body volume or metabolism of an adult. Regular adult doses would quickly become toxic and could cause great harm or death. Therefore, we dose per kilogram of body weight for our smallest patients. At times, we also may dose an adult on a weight basis for certain medications or due to obesity.

This method of calculation is quite simple, just as the formula method is. But... along with this method is the recommended maximum or minimum doses per kg. In other words, we will need to calculate the proper dose and then compare the result with the safe ranges of the given medication. Well, let's get to work...

Let's learn from examples

Easy

The patient weighs 20 kg and ordered is a diuretic, furosemide 2 mg/kg as a single dose PO. Available is furosemide oral solution 10 mg/ml... the safe range for furosemide is 2 - 6 mg/kg. What will the nurse administer?

Is the dose safe? Ordered is 2mg/kg. Safe is 2 to 6 mg/kg. Therefore the dose is safe
Calculate the desired dose by multiplying 20 kg and 2 mg/kg... this gives us 40 mg as our desired dose
Use the formula method... 40/10 x 1 ml = 4 ml PO

Moderate

A 3 kg patient is ordered to receive filgrastim 20 mcg SQ daily to stimulate neutrophil production. The safe range for filgrastim is 5 to 10 mcg/kg daily. Supplied is filgrastim 300 mcg/ml solution for injection. What will the nurse administer?

Is the dose safe? In this case the order only gives the total dose, but the safe range is in mcg/kg. So we must find the low and high limits. 3 kg times 5 mg/kg = 15 mg- this is the lowest safe dose; 3 kg times 10 mg/kg = 30 mg- this is the highest safe dose. Does the ordered dose of 20 mcg fall between 15 mcg and 30 mcg? Yes, it does... the dose is safe
Since the total dose is already figured by the physician, we can go straight to the formula method
20/300 x 1 ml = 0.0666667 ml... we round to the hundredth because the total is less than one... the answer is 0.07 ml SQ

Challenging

The patient weighs 20 pounds (lbs). Order for promethazine 4 mg IM for nausea every 4 hours. Safe range for promethazine is 0.25 to 1 mg/kg every 4 hours. Supplied from the pharmacy is promethazine 25 mg/ml injectable solution. What will the nurse administer?

Is the dose safe? This time the given weight is in pounds. Let's convert to kilograms first... 20 divided by 2.2 = 9.1 kg (round to the tenths). Again the order is for the total dose, so we must make sure the total dose falls into the safe range. Take the weight in kg and multiply by the safe range for low and high doses... 9.1 x 0.25mg = 2.275 mg- this is the lowest dose; and 9.1 x 1 mg = 9. 1 mg- this is the highest safe dose. Does 4 mg fall between 2.275 and 9.1? Yes. The dose is safe.
The desired is already calculated by the doctor, so move to the formula
4/25 x 1 = 0.16 ml IM

Wait a second... I thought you said this was easy!? Well, I am trying to keep a positive attitude.

The patient with high blood pressure has a weight of 50 pounds. Ordered is hydralazine 10 mg PO every 12 hours for hypertension. The safe range for hydralazine is 0.75 to 1 mg/kg daily in 2 to 4 divided doses with a maximum of 25 mg per dose. Supplied by pharmacy is hydralazine 10 mg tablets. What will the nurse administer?

Is the dose safe? We must convert to kilograms... 50 divided by 2.2 = 22.7 kg (we round to the tenths for weights); the safe low dose is 22.7 x 0.75 = 17.025 mg; the safe high dose is 22.7 x 1 = 22.7 mg. But wait... this is the daily dose minimum and max. Your patient is taking the dose every 12 hours, or twice a day. So... divide the safe daily range by two for your safe range per dose. 17.025/2 = 8.5125 mg for safe low dose and 22.7/2 = 11.35 mg for safe high dose. Does the ordered 10 mg fall between 8.5125mg and 11.35 mg? Yes... the dose is safe
Again the calculation for the total dose is already done
10/10 x 1 tab = 1 tab PO

Just remember the steps:

Is the dose safe?
- Change weight to kilograms if needed (round to the nearest tenth)
- Calculate safe lowest dose and highest dose- keeping in mind daily vs per dose limits
- Compare these to the ordered dose- if the ordered dose falls out of range, then it is not safe
Calculate the total desired dose if not done already- pt weight in kg times the dose per kg
Plug in total dose into the formula

The best way to learn is to practice, practice, practice

Patient weighs 35 pounds and is ordered 300 mg amoxicilllin PO three times a day for an infection. The safe range of amoxicillin is 25 to 50 mg/kg/day in divided doses administered every 8 hr. The pharmacy provided amoxicillin 400 mg/5 ml. What would the nurse administer?

Do Not Give

First, convert weight to kilograms- 35 divided by 2.2 = 15.9 kg (round to tenth)

Second, figure safe low and high daily range- 15.9 x 25 = 397.5 mg and 15.9 x 50 = 795 mg

Now figure per dose range by dividing by 3 because every eight hours is the same as 3 times per day- 397.5/3 = 132.5 mg and 795.5/3 = 265 mg

Does the dose of 300 mg fall in the safe range of 132.58 mg to 265.17 mg? No

Do Not Give

Children's Tylenol 200 mg PO is ordered for a 6 year-old girl who weighs 40 pounds. The recommended dose is 10 to 15 mg/kg/dose. Supplied is acetaminophen (Tylenol) 160 mg/5 ml.

6.25 ml PO

First convert weight- 40/2.2 = 18.2 kg (round to hundredth)

Second determine safe dose- 18.2 x 10 = 182 mg and 18.2 x 15 = 273 mg

Is the dose safe? 200 mg falls between 182 mg and 273 mg- safe

Formula- 200/160 x 5 = 6.25 ml

A child with leukemia is ordered morphine 7.5 mg PO for pain. The child weighs 40 pounds. The safe range for morphine is 0.2 to 0.5 mg/kg every 4 to 6 hours. The pharmacy supplies morphine 15 mg tab. What will the nurse administer?

0.5 tab PO

First convert weight- 40/2.2 = 18.2 kg

Then determine safe dose- 18.2 x 0.2 = 3.64 mg and 18.2 x 0.5 = 9.1 mg

Dose the dose of 7.5 mg fall in the safe range of 3.64 mg and 9.1 mg? Yes

Formula- 7.5/15 x 1 tab = 0.5 tab

This same child is prescribed chlorpromazine to treat nausea. The physician orders 0.55mg/kg PO every 4 hours as needed for nausea. The safe dose is 0.55 mg/kg every 4 to 6 hours. The pharmacy supplies chlorpromazine 10 mg tab. What. will the nurse administer?

1 tab PO

As determined, weight is 18.2 kg

Dose ordered and safe dose are the same- 18.2 x 0.55 mg = 10.01 mg.... round to nearest tenth, in this case we end up with 10 mg

Formula- 10/10 x 1 tab = 1 tab

A newborn is needing epinephrine for severe allergic reaction. She weighs 8 pounds. Ordered is 0.08 mg IV every 3 minutes prn anaphylaxis. The safe range is 0.01 to 0.03 mg/kg every 3 to 5 minutes as needed. Supplied is epinephrine for injection 1 mg/ml. What will the nurse administer?

0.08 ml IV

Convert weight- 8/2.2 = 3.6 kg (round to the hundredth)

Next calculate safe range- 3.6 x 0.01 mg = 0.036 mg and 3.6 x 0.03 mg = 0.108 mg

Does ordered dose of 0.08 mg fall in the safe range? Yes

Formula- 0.08/1 x 1 = 0.08 ml

A patient weighing 440 pounds is ordered metoprolol 1 mg/kg twice daily PO. The upper limit for metoprolol is 450 mg/day. Supplied is metoprolol 100 mg tab. What will the nurse administer?

2 tab PO

Convert weight- 440/2.2 = 200 kg

Upper limit is 450 mg/day

Determine per dose limit (ordered twice daily)- 450/2 = 225 mg

Determine actual dose- 200 x 1 = 200 mg twice daily

Does ordered dose fall under the safe limit?- Yes

Formula- 200/100 x 1 tab = 2 tab

A neonate weighing 3.4 kg is ordered cefepime IV for Pseudomonas infection, pharmacy to dose. The safe dose is 50 mg/kg. What dose will the nurse expect from the pharmacy?

170 mg IV

Weight is already in kg, nice

Safe dose is 3.4 x 50 mg = 170 mg

Pharmacy should send a dose of 170 mg IV

A patient weighing 26 pounds is ordered erythromycin 30 mg IV every 6 hours for infection. The safe range is 15 to 50 mg/kg/day in 4 divided doses. Supplied from the pharmacy is erythromycin 30 mg/5 ml. What will the nurse administer?

Do Not Give

Convert weight- 26/2.2 = 11.8 kg

Safe daily dose is 11.8 x 15 mg = 177.3 mg and 11.8 x 50 mg = 591.05 mg

Daily dose is divided into 4 partial doses- 177.3/4 = 44.325 mg and 591.05/4 = 147.7625 mg

Does ordered dose of 30 mg fall in the safe range? No

Do Not Give

A patient undergoing sedation is ordered fentanyl 60 mcg IV. She weighs 175 pounds. The safe range is 0.5 to 1 mcg/kg/dose. Supplied by pharmacy is sufentanil 50 mcg/ml. What will the nurse administer?

Do Not Give

Fentanyl and sufentanil are not the same drug

Do Not Give

A patient weighing 350 pounds is ordered valproic acid 500 mg PO every 6 hours to prevent seizures. The safe range is 10 to 15 mg/kg/day in 1 to 4 divided doses. Supplied by pharmacy is valproic acid 250 mg capsules. What will the nurse administer?

2 cap PO

Convert weight- 350/2.2 = 159.1 kg

Determine safe range- 159.1 x 10 mg = 1,590.9 mg and 159.1 x 15 mg = 2,386.35 mg

Determine per dose safe range (4 times a day)- 1,590.9/4 = 397.725 mg and 2,386.35/4 = 596.5875 mg

Does ordered dose of 500 mg fall in the safe range? Yes

Formula- 500/250 x 1 cap = 2 cap

Great work so far! You almost have all the information you need. Next is drug labels and how to read them

Medication Labels

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