By: Mike Caviston, Director of Fitness, Naval Special Warfare Center
Posted: March 17, 2021

Over the years, candidates have wondered what effect body fat (BF) percentage has on success in the training pipeline. Some believe that a little extra body fat in the early weeks of Basic Underwater Demolition/SEAL (BUD/S), including Hell Week (HW), would be a good thing – extra energy reserves and insulation against the cold! But food isn’t restricted during Hell Week, and the amount of fat needed to provide significant insulation during an evolution like surf immersion would limit mobility pretty severely (it wouldn’t be easy dragging your belly roll over the obstacle course a.k.a. O-course or running up and down the beach). Actual data from BUD/S students discourages excess fat as well. A sample of 1500 students from BUD/S Classes 319-331 whose % body fat was measured at the Naval Special Warfare (NSW) Prep Course was tracked through Hell Week. The leanest students completed Hell Week at a higher rate than students with more body fat.

Body fat breakdown

  1. 257 students averaged 8% BF/23% completed HW
  2. 532 students averaged 11% BF/21% completed HW
  3. 479 students averaged 14% BF/18% completed HW
  4. 185 students averaged 17% BF/17% completed HW
  5. 41 students averaged 20% BF/12% completed HW


This should not be interpreted as a message to prospective candidates to radically reduce body fat to values below 10% if you are not naturally lean. That would potentially be unsafe and counterproductive. But the evidence does not support the idea of arbitrarily adding fat for the selection process. Prior research on SEAL and SWCC Operators and SEAL Qualification Training/Charlie Phase students (University of Pittsburgh Naval Special Warfare Tactical Athlete Program Human Performance Research) has suggested that best performance and fewer injuries occur in the 10-15 % body fat range, with lower and higher values correlating with more injuries. This seems like a reasonable target for the typical BUD/S candidate as well.

For the sample of students sited above, height and weight both correlated with the odds of completing Hell Week. Taller and heavier students were more successful than shorter and lighter students. Those averaging around 6’ 2” and 200lbs were at least three times as likely to complete Hell Week as those averaging around 5’ 6” and 150lbs. Successful students ran and swam faster than those who dropped before completing Hell Week, regardless of size. But the gap was even wider for the smaller students – it seems that they compensated by swimming and especially running extremely fast. The benefit of height and weight was more apparent for these classes than as recently as BUD/S Classes 301-308. Still more evidence that BUD/S continues to become more competitive as candidates are more athletic than ever before – bigger, faster, and stronger. Something to remember when preparing to enter the training pipeline.


Naval Special Warfare Assessment Command