Tools · TRT
Free testosterone calculator
A total testosterone number on its own hides what is actually available to your tissues. Enter total T, SHBG, and albumin and this estimates free and bioavailable testosterone by the Vermeulen 1999 method - the same math as the issam.ch reference calculator.
Free & bioavailable testosterone
13.78ng/dL free
= 137.8 pg/mL · 2.3% of total
Bioavailable T
323 ng/dL
53.8% of total
Free T percent
2.3%
of total T
Vermeulen 1999 method (PMID 10523012). This reproduces the issam.ch reference calculator; Free T in pg/mL is ng/dL times 10.
Math conversion only. This does not diagnose or tell you what to do with a result. Free-testosterone estimates vary by method and assay; always verify with a licensed provider. Testosterone is a controlled substance in many places.
How the math works
Most testosterone in blood is bound to SHBG and albumin; only a small fraction is free, and free plus the loosely albumin-bound portion is the bioavailable fraction. The Vermeulen method solves the binding equilibrium from three inputs - total testosterone, SHBG, and albumin - using a fixed SHBG association constant and an albumin term. If your lab reports total T in nmol/L, switch the unit toggle; the tool converts to ng/dL first (1 nmol/L = 28.84 ng/dL), exactly as the reference calculator does. Free T is also shown in pg/mL, which is ng/dL times ten, because many US labs report it that way.
Sources
- [1]Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab. 1999;84(10):3666-72.studyThe source equation this calculator implements (PMID 10523012).
- [2]ISSAM free & bioavailable testosterone calculator (issam.ch)referenceThe widely used online reference implementation of the Vermeulen method; this tool's outputs are pinned to match it.