Rule of nines: how adult burn TBSA percent is added up

The rule of nines splits an adult body into regions worth 9% or 18% of total body surface area (TBSA), plus 1% for the perineum. Adding the regions that are burned gives the percent TBSA. Magent's calculator takes a fraction for each region (none, half or full) and returns the sum.

Not a medical device. Not a diagnosis or dosing recommendation.

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Wallace AB · 1951
The rule of nines formula
Worked examples
Frequently asked questions

What is the rule of nines?

Burns are often described by how much of the skin surface they cover, written as a percent of total body surface area. Measuring that directly is slow, so a set of fixed percentages for each part of the body is used as a shortcut. In the adult version, the head and each arm are 9%, the front of the trunk, the back of the trunk and each leg are 18%, and the perineum is 1%. Together these come to 100%.

Magent's calculator uses those adult percentages as printed in the paper it cites, Wallace in The Lancet, 1951. For each of the eight regions, the person entering the data chooses 0 (not involved), 0.5 (half involved) or 1 (fully involved). The calculator multiplies and adds.

This page explains the arithmetic. It doesn't say what a percentage means for a particular person.

The rule of nines formula

region            percent when fully involved
head                    9
left arm                9
right arm               9
front of trunk         18
back of trunk          18
left leg               18
right leg              18
perineum                1

TBSA % = sum of (region fraction × region percent)    fraction is 0, 0.5 or 1

Exact expression the calculator uses:

tbsa_percent = head*9 + arm_left*9 + arm_right*9 + anterior_trunk*18 + posterior_trunk*18 + leg_left*18 + leg_right*18 + perineum*1 (Wallace 1951 adult percents; each factor is 0, 0.5, or 1)

Inputs

Input	Region	Percent when full	Values accepted
`head`	Head and neck	9	0, 0.5, 1
`arm_left`	Left arm	9	0, 0.5, 1
`arm_right`	Right arm	9	0, 0.5, 1
`anterior_trunk`	Front of the trunk	18	0, 0.5, 1
`posterior_trunk`	Back of the trunk	18	0, 0.5, 1
`leg_left`	Left leg	18	0, 0.5, 1
`leg_right`	Right leg	18	0, 0.5, 1
`perineum`	Perineum	1	0, 0.5, 1

All eight inputs are required. Any other value returns an invalid_region_fraction error.

How the calculation works

Assign a fraction to each region. 0 for none, 0.5 for half, 1 for full.
Multiply each fraction by its region percent. A full arm gives 9, a half arm gives 4.5.
Add the eight results. The total is the percent TBSA, from 0 up to 100.

With three allowed fractions, the calculator can't record a burn covering a smaller share of a region. The choice of fraction is made by the person entering the data. The calculator doesn't look at the burn.

Units

Each input is a fraction with no units. The result is a percent of total body surface area. The calculator doesn't use height, weight or body surface area in square metres.

Worked examples

These use Magent's own expected values for the calculator.

Example 1: every region involved

All eight regions set to 1.
9 + 9 + 9 + 18 + 18 + 18 + 18 + 1.
TBSA 100.0%.

Example 2: anterior trunk, nothing else

Front of the trunk set to 1. The other seven regions are 0.
TBSA 18.0%.

Example 3: half the head

Head set to 0.5. The other seven regions are 0.
Half of 9 gives TBSA 4.5%.

Example 4: half of each leg

Left leg 0.5 and right leg 0.5. The other six regions are 0.
Half of 18 twice gives TBSA 18.0%.

Example 5: perineum by itself

Perineum set to 1. The other seven regions are 0.
TBSA 1.0%.

History of the rule of nines

The calculator cites Wallace, The Lancet, 1951, a paper on managing burns by the exposure method. The adult percentages the calculator uses (9 for the head and each arm, 18 for each trunk surface and each leg, 1 for the perineum) are the ones it attributes to that paper. The calculator follows the adult chart and doesn't include age-adjusted charts for children.

When it's used

The rule of nines is used to put a quick number on how much skin surface a burn covers in an adult.

Where it's used

Because it needs no measuring equipment, the rule of nines can be applied anywhere an adult with a burn is first seen, from an emergency department to a burns unit.

Who uses it

The person using the calculator looks at the burn and chooses a fraction for each region. The calculator applies the adult percentages and adds them.

Limits of the calculation

It uses the adult chart. Children have different body proportions, and the calculator has no pediatric percentages.
It isn't the Lund-Browder chart.
Each region is none, half or full, so smaller patches are rounded into one of those three choices by the person entering them.
The calculator doesn't examine the burn or judge its depth.
It isn't a fluid calculation and doesn't give a fluid volume. This page doesn't say what to do with a result.

Clinical decisions belong to a qualified clinician who knows the patient.

Lund-Browder chart is the chart with pediatric percentages. The rule of nines calculator uses one fixed adult set and doesn't include Lund-Browder.
Burn depth scales describe how deep a burn goes. The rule of nines measures surface area, and depth isn't an input.

Glossary

TBSA: total body surface area, the whole skin surface, written as 100%.
Anterior trunk: the front of the chest and abdomen.
Posterior trunk: the back, from the shoulders to the buttocks.
Perineum: the area between the legs, counted as 1%.

Common mistakes

Using adult percentages for a child. The calculator's chart is for adults.
Entering a percent instead of a fraction. Enter 1 for a fully involved arm, not 9.
Entering a fraction other than 0, 0.5 or 1. Any other value is rejected with an error.
Counting the trunk once. The front and back of the trunk are separate inputs, each worth 18 when full.

Source

Wallace AB. The exposure treatment of burns. Lancet. 1951;1(6653):501-504. DOI: 10.1016/s0140-6736(51)91975-7. PMID: 14805109

Frequently asked questions

What are the rule of nines percentages for an adult?

Head 9, each arm 9, front of the trunk 18, back of the trunk 18, each leg 18 and perineum 1. They add up to 100.

How are half regions counted?

A region entered as 0.5 counts for half its percent. Half the head is 4.5, half a leg is 9.

Does the calculator work for children?

No. It uses the adult chart and has no pediatric percentages.

Does the calculator tell me burn depth?

No. It adds up surface area from the fractions entered. Depth isn't an input.

What is the highest total?

100.0%, when all eight regions are set to 1.