From what I've read (and I've read a lot), scientific studies seem pretty evenly divided on both NBP and BP. That said, just because someone attempts to be scientific doesn't mean their methodology is beyond criticism.
The
Brasil study (
Penile anthropometry in Brazilian ... [J Pediatr (Rio J). 2007 Sep-Oct] - PubMed - NCBI ) is by far the best scientific review of the different methodologies I've ever read, and their conclusion was that BPFSL is by far the best, most reliable measurement. BPEL is good also so long as the erection is induced chemically, and the results are typically ~0.5" longer (I think because the head grows a lot more in erection, though a maximally stretched shaft will be the same length as a maximally erect one.)
A few studies (e.g. Ponchietti et al, aka the Italian conscript study) clearly don't BP, but their measurement is closer to:
- put ruler at the base of penis
- pull penis as far as it will go
So they're not really 'bone-pressing', but it's a longer measurement than if you stretched first, then put the ruler on. Just going by my own dick, I'm not sure where the 'base' is after I've stretched it, since what was the base before gets stretched forward too.
For NBPFSL, it's typically ~13cm for the median + average.
Ponchietti was 12.5cm median
Orakwe et al (Nigerian study) was 13cm
Can physique and gluteal size predict... [West Afr J Med. 2006 Jul-Sep] - PubMed - NCBI
as a side point, it's interesting that when it's not in porn or urban legends, there's remarkably little ethnic variation.This is so conclusive, yet the myths persist...
For a BP length, it's generally closer to somewhere over 14cm. Brazil puts the BPFSL (they call it RSLmax) of a fully-developed adult penis at ~14.5cm.
If we convert these to inches, that's about 4.9" NBPFSL, and 5.7" BPFSL. If you add in the ~0.5" extra you get at maximal erection vs stretched, you get the exact same results Wessels et al found: around 5" for a NBPEL, and around 6" for a BPEL.