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Today we’re going to go over the rule of 9s which is used in burn
patients as a quick way to estimate the total body surface area involved.
This also pops on medical and nursing exams, so it’s good to know this topic.
We’ll go through a simple table, and there will be examples at the end,
so make sure to watch the entire video.
As always, you can find the notes and study guide for the video linked below.
All of the videos come with notes, so make sure to subscribe to not miss out on those.
As we mention before, the rule of 9s is used in burn patients to calculate
the total body surface area affected by 2nd-degree and 3rd-degree burns.
This helps guide the patient’s care, estimate fluid resuscitation,
and determine if the patient requires care at a specialized burn center.
The rule of 9s only applies to partial thickness and full thickness burns,
which are 2nd and 3rd-degree burns.
It does not apply to superficial burns, which are 1st-degree burns.
Let’s walk through the table and diagram,
and you’ll quickly see why it’s called the rule of 9s and how it works.
The rule of 9s provides you with an estimated percentage
of the body surface area for different body parts.
So what do we mean by that?
Well, let’s take a look at the table.
The entire head and neck make up 9% of the total body surface area.
The entire right arm and left arm each make up 9% of the total body surface area.
The entire trunk has a body surface area of 36%.
The groin has a body surface area of 1%,
and the entire right leg and left leg each make up 18% of the total body surface area.
And this totals 100%.
The easy way to remember this is to go in alphabetical order.
First we have the “A” and “H” for arms and head which are each 9%.
Next we have the “L” for legs which has a higher body surface area of 18%.
Finally, we have the “T” for trunk which has an even higher body surface area of 36%.
Once you have committed that to memory, the rest becomes easy.
With the exception of the groin, you can see how
the percentages are multiples of 9 which is why the rule gets its name.
The rule of 9s is used in adult patients who are not obese.
When it comes to pediatric and obese patients,
the accuracy of the rule of 9s can be affected, which we’ll cover at the end.
Now you can see the table shows the percentages of entire body parts,
so lets apply it to the front and back of the body.
All we’re going to do is divide each percentage in half.
We know the entire head and neck is 9%, so the face and anterior neck make up 4.5%
and the posterior head and neck make up 4.5%.
The entire right arm is 9%, so the anterior right arm has a body surface area of 4.5%,
and the posterior right arm has a body surface area of 4.5%.
The left arm is the same where the anterior and posterior sides each make up 4.5%.
The entire trunk is 36%, so the anterior trunk is 18% and the posterior trunk is 18%.
You can break down the anterior trunk even more where the chest is 9% and the abdomen is 9%.
The groin is 1%.
The entire right leg has a body surface area of 18%,
so the anterior side is 9%, and the posterior side is 9%.
The left leg is the same where the anterior and posterior sides each make up 9%.
Again you can see how the percentages for the anterior and posterior sides are multiples of 9,
hence the name rule of 9s.
Let’s look at a couple examples now.
Try to figure out the total body surface area for each burn.
The first patient has partial thickness burns involving the entire anterior chest
and the entire anterior right arm.
Hit pause in the video and figure out the total body surface area involved.
We know the anterior trunk is 18%.
Remember the entire trunk is 36%, so just the anterior side is 18%.
This burn only involves half of the anterior
trunk because it only involves the chest and not the abdomen.
So the burn to the anterior chest has a body surface area of 9%.
Next we have the burn involving the entire anterior right arm.
We know the entire right arm has a body surface area of 9%.
Since this burn only involves the anterior aspect, then it’s 4.5%.
So this patient with partial thickness burns to the anterior chest
and anterior right arm involves a total body surface area of 13.5%.
Let’s look at one more example.
This patient has a full thickness burn to the distal anterior
left arm and a partial thickness burn to the proximal anterior left leg.
Hit pause in the video and try to figure out the total body surface area involved.
We know the entire left arm is 9%, and the anterior side is 4.5%
Well this burn only involves half of the anterior aspect,
which would give it a body surface area of approximately 2%.
Next we have the burn to the proximal anterior left leg.
We know the entire left leg is 18% with the anterior aspect totaling 9%.
Since this burn involves half of the anterior aspect,
the body surface area would be approximately 4.5%.
So this patient’s burns involve a total body surface area of about 6.5%.
There are a few things you should know about the rule of 9s.
Remember the rule of 9s only applies to partial thickness and full thickness burns.
Superficial burns are not included in the total body surface area assessment.
The patient’s palm, not including their digits, is approximately 0.5% body surface area.
And their entire hand including their digits is roughly 0.8-1% body surface area.
So for smaller burns that might only be 1-2% total body surface area,
you can use the size of the patient's hand to approximate the area involved.
Burn patients can have fluid losses from damage to the skin and losing that skin barrier,
as well as capillary leak.
So figuring out the total body surface area involved
is important in estimating fluid resuscitation, along with the
patient’s overall care and potential need for treatment at a designated burn center
There are equations such as the Parkland formula that help you figure out how
much fluid resuscitation the patient requires using their total body surface area involved.
We’ll go into that more in the next video.
As we mentioned before, the rule of 9s can be applied to non-obese adults.
Patients that are defined as obese based on their BMI have disproportionately larger trunks,
and the rule of 9s is typically inaccurate.
The body surface area of the trunk in obese patients can be closer to 50%
compared to their non-obese counterparts with a trunk of 36%.
Due to their disproportionate trunk size, the other body surface area percentages
are slightly different in obese patients as you can see in the table and diagram.
The numbers you see on the diagram are the totals for both the front and back.
These percentages are approximations as they can vary among obese patients.
We can see the entire head and neck is 6%, each arm is 7%, the groin is 1%, and each leg is 15%.
The other patient population in which the rule of 9s is inaccurate is pediatrics.
Pediatric patients have disproportionately larger heads and shorter legs.
This table and diagram will give you approximate percentages for infants and young children.
These numbers will change in older pediatric patients.
We can see the head is disproportionately larger at 21% instead of 9%.
Each arm is about 10%, the entire trunk is about 26%,
the buttocks and groin area are about 6%, and each leg is about 13.5%.
Now it’s your turn to fill in the table and diagram.
Hit pause in the video and try to fill in the numbers
for an adult on your own using the rule of 9s.
Once you think you have the answers, hit play again to reveal the correct numbers.
The answers are going to be revealed in a couple seconds.
Here you can see the correct responses.
Hopefully, you were able to get them all correct.
Hopefully this gave you an easy way to remember the rule of 9s for burn assessments
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