10% plus or minus 2%.
12.5% plus or minus 2.5% is the lowest body fat percentage achievable without taking steroids. However, this number will vary depending on how lean an individual can get by dieting and what part of their body we measure. For example, men may have a higher variance than women due to more significant amounts of androgens and larger muscle mass in general. Furthermore, this number will also vary depending on the individual being examined (hard-training athlete versus recreational weight lifter). For example, women tend to hold less fat than men at any subcutaneous adiposity level, while athletes carry more fat due to greater muscularity. Therefore, it would be virtually impossible for an individual to achieve sub 8% body fat levels without the aid of exogenous hormones. To determine your level of adiposity, you could visit this page for a helpful calculator (http://www.simplyshredded.com/estimated-bf-percentage-calculator.html).
Sources:
1.) Kouri EM, Pope HG Jr, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sports Med 1999;9:222–7
2.) Kouri Box 1 – http://www.eroids.com/forum/steroids-qa/what-is-the-lowest-bodyfat-possible-without-steroids-43873.html
3.) http://simplyshredded.com/estimated-bf-percentage-calculator.html
4.) http://en.wikipedia.org/wiki/Fat_free_mass_index#Fat_free_mass_index_.28FFMI.29
5.) http://www.aapspharmacy.org/publications/positionpapers/ab109contents2012%20(WEB).pdf
6.) Hartgens F, Kuipers H, Wijnen JA, Reis MA . Body Composition and Endocrine Function in Athletes: Effects of Age, Gender and Athletic Status Drug Test Anal 2001;23:401–27
7.) Häkkinen K, Pakarinen A, Kraemer WJ, Hakkinen LM, Izquierdo M, Paavolainen L . Selective muscle hypertrophy, changes in EMG and force, and serum hormones during strength training in older women. J Appl Physiol 2001;91:244–51
8.) Friedl KE et al. “Age and gender comparisons of muscularity and somatotype in college-aged national Olympic committee-certified weightlifters.” Journal of Strength & Conditioning Research / National Strength & Conditioning Association 19.3 (2005): 550-557.
9.) https://www.muscleandstrength.com/articles/what-is-the-lowest-possible-bodyfat-percentage