Tools · TRT
Free testosterone calculator
By An NayalLast reviewed 2 sources
A total testosterone number on its own hides how much is unbound. 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.
Worked example
The calculator opens with total testosterone of 600 ng/dL, SHBG of 30 nmol/L and albumin of 4.3 g/dL. Here is the path from those three numbers to the result.
- Total T to molar: 600 / 2.8 x 10-10 = 2.14 x 10-8 mol/L.
- SHBG to molar: 30 / 10 x 10-8 = 3.0 x 10-8 mol/L.
- Albumin factor: N = 36000 x 4.3 x 1.45 x 10-4 + 1 = 23.446.
- Solving the quadratic gives free testosterone of about 4.92 x 10-10 mol/L, which is 2.3% of the total.
- 2.3% of 600 ng/dL is 13.78 ng/dL free, shown as 137.8 pg/mL.
- Bioavailable is free times N: 13.78 x 23.446 = 323 ng/dL, or 53.8% of total.
If your lab reports in SI units, switch the toggle to nmol/L. Entering 20.8 nmol/L shows "= 599.9 ng/dL" under the box, and the rest of the math runs on that ng/dL figure.
How the math works
Most testosterone in blood is bound to SHBG or albumin. The Vermeulen method treats that as a binding balance and solves for the unbound (free) part from three inputs. The code follows the reference calculator step by step.
It first puts the inputs in mol/L. Total T in ng/dL is divided by 2.8 and multiplied by 10-10; SHBG in nmol/L is divided by 10 and multiplied by 10-8. The albumin term is:
N = 36000 x albumin (g/dL) x 1.45e-4 + 1
With the SHBG association constant Kt = 1e9 L/mol, free testosterone (FT) is the positive root of a quadratic:
a = N x Kt; b = Kt x (SHBG - TT) + N; c = -TT
FT = (-b + sqrt(b^2 - 4ac)) / 2a
From there, free percent is FT / TT x 100, free ng/dL is that percent of your total, and bioavailable ng/dL is free ng/dL x N. Free T in pg/mL is ng/dL x 10.
Two unit factors appear. The nmol/L toggle converts with 28.84 ng/dL per nmol/L (molecular weight 288.4). The internal molar step uses the reference calculator's factor of 2.8 as written, so the outputs match issam.ch. Free ng/dL shows to two decimals, pg/mL and bioavailable to one. If total T, SHBG or albumin is empty, zero or not a number, no result is shown.
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.
Questions people ask
How do I convert testosterone from nmol/L to ng/dL?
Multiply by 28.84, which comes from testosterone's molecular weight of 288.4 g/mol. 20.8 nmol/L is about 599.9 ng/dL. The calculator does this for you when you switch the unit toggle to nmol/L.
What is the difference between free and bioavailable testosterone?
Free testosterone is the small fraction not bound to SHBG or albumin. Bioavailable testosterone is the free part plus the loosely albumin-bound part. In this calculator, bioavailable equals free times the albumin factor N, so it is always the larger number.
What albumin value should I use if my lab did not measure it?
The calculator defaults to 4.3 g/dL, the value the Vermeulen reference calculator assumes when albumin is not measured. If your lab report has an albumin value in g/dL, enter that instead.
Why does a higher SHBG lower the free testosterone estimate?
In the equation, SHBG is a binding site for testosterone, so with the same total T more SHBG means more is counted as bound. Holding total T at 600 ng/dL and albumin at 4.3 g/dL, SHBG of 15, 30 and 60 nmol/L gives 18.47, 13.78 and 8.66 ng/dL free.
Is this the same as a lab-measured free testosterone?
No. It is a calculated estimate from three lab values. Free-testosterone results vary by method and assay, so a calculated number and a directly measured one can differ.
Pepteca is for education and information only. It does not provide medical advice, diagnosis, or treatment, and its calculators perform unit math only. Always verify with a licensed provider before starting, changing, or stopping any protocol. Many peptides are not approved for human use and may be regulated differently where you live.