Body Surface Area: Face and Arms
The face and both arms together represent approximately 19% of total body surface area (TBSA), not 20%. Using the simplified "Rule of Nines" approach, the face accounts for approximately 5% and each entire arm (including hand) accounts for 8%, totaling 21% 1, 2.
Breakdown by Body Region
The standard Rule of Nines approximation divides the adult body as follows 1, 2:
- Head and neck: ~10% (with face alone being ~5%)
- Each entire arm (including hand): ~8%
- Upper extremities total (both arms): ~20%
- Trunk: ~30%
- Lower extremities (including buttocks): ~40%
Clinical Application
For practical burn assessment or topical medication application, the "20%" approximation for face and arms is clinically reasonable despite being slightly higher than the precise calculation 1. This simplified approach facilitates rapid bedside estimation.
Alternative Measurement Method
The fingertip unit (FTU) method provides more granular assessment 1:
- Face and neck: 2.5 FTU (5% TBSA)
- One entire arm including hand: 4 FTU (8% TBSA)
- Both arms: 8 FTU (16% TBSA)
- Face + both arms = 10.5 FTU (21% TBSA)
Important Caveats
The Rule of Nines applies primarily to adults and children over 10 kg 3. For obese patients (>80 kg), a "Rule of Fives" is more accurate, where each arm represents only 5% rather than 8% 3. For infants (<10 kg), a "Rule of Eights" applies with different proportions due to their disproportionately large heads 3.
The patient's own palm (including fingers) represents approximately 1% of their TBSA, providing another quick estimation tool 2, 4.