According to Rule of Nine , burns involving perineum are
BURNS in % BSA

Determination of burn size estimates the extent of injury.
Burn size is assessed by Wallace rule of nines (By Alfred Russel Wallace)

Wallace Rule of Nines 

In adults:

- Each upper extremity: 9%
- Head and neck: 9%
- Lower extremities: 18%
- Anterior and posterior aspects of the trunk: 18%
- Perineum and genitalia: 1%

Children have a relatively larger proportion of body surface area in their head and neck, which is compensated by a relatively smaller surface area in the lower extremities.

- In infants: Head and neck- 21%; Each leg-13%

Berkow formula is used to accurately determine burn size in children.
For estimating smaller burns: Area of open hand (including palm and extended fingers) of the patient is approximately 1% of TBSA.
This method is helpful in evaluating splash burns and burns of mixed distribution.
1%
9%
18%
27%