Enter Total Testosterone, Sex Hormone–Binding Globulin (SHBG), and Albumin into the calculator to estimate Free Testosterone using a standard binding-equilibrium (Vermeulen/Södergård) calculation. A separate tab is also included to compute the Free Androgen Index (FAI).
Medical note: Educational use only; not medical advice. Do not use this calculator to diagnose, treat, or change medications. “Calculated” free testosterone and FAI are model-based estimates and may differ from your lab’s measurement method and reference ranges. If you have symptoms or abnormal results, discuss them with a licensed clinician.
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Free Testosterone Level Formula
The calculator above estimates free testosterone using a commonly used binding-equilibrium method (often referred to as the Vermeulen/Södergård calculated free testosterone model). Because testosterone binding depends on association constants, the calculation must be done in molar units (mol/L).
\begin{aligned}
N &= 1 + K_a A \\
F &= \frac{-(N+K_s S-K_s T) + \sqrt{(N+K_s S-K_s T)^2 + 4 N K_s T}}{2 N K_s}
\end{aligned}Where:
- F is free testosterone (mol/L)
- T is total testosterone (mol/L)
- S is SHBG (mol/L)
- A is albumin (mol/L)
- Ka is the albumin association constant (commonly approximated as 3.6×104 L/mol)
- Ks is the SHBG association constant (commonly approximated as 1.0×109 L/mol)
In practice, the calculator converts your inputs (e.g., ng/dL, nmol/L, g/dL) into mol/L, applies the equation above, and then converts the free testosterone result back into your selected output unit.
Sources & limitations
- This calculator implements a Vermeulen/Södergård-style binding-equilibrium approach using commonly cited association constants (Ks ≈ 1×109 L/mol; Ka ≈ 3.6×104 L/mol) and an albumin molecular weight assumption (~66,500 g/mol).
- Calculated free testosterone is an estimate and may not match equilibrium dialysis/ultrafiltration or your lab’s reported “free testosterone” method.
- FAI is a rough surrogate and can be misleading when SHBG is altered (e.g., thyroid/liver conditions, obesity, pregnancy, and certain medications).
- For clinical interpretation and method details, consult the original Vermeulen and Södergård publications and current endocrinology society/laboratory guidance (and your lab’s reference ranges).
What is a Free Testosterone Level?
Free testosterone is the small fraction of circulating testosterone that is not bound to proteins (mainly SHBG and albumin). Because only the unbound portion is considered immediately bioavailable to tissues, free testosterone is sometimes evaluated when total testosterone does not match symptoms or when SHBG is abnormal. Free testosterone can be measured directly by specialized lab methods or estimated (“calculated free testosterone”) from total testosterone, SHBG, and albumin.
How to Calculate Free Testosterone Level?
The following steps outline how calculated free testosterone is estimated using the model implemented in the calculator:
- Determine total testosterone (TT) and its unit (commonly ng/dL or nmol/L).
- Determine SHBG (commonly nmol/L).
- Determine albumin (commonly g/dL or g/L).
- Convert TT, SHBG, and albumin into molar units (mol/L).
- Apply the binding-equilibrium equation (using association constants for SHBG and albumin) to solve for free testosterone.
- Convert the free testosterone result into your desired reporting unit (e.g., ng/dL or pg/mL).
Example Problem:
Use the following variables as an example problem to test your knowledge:
Total Testosterone (TT) = 500 ng/dL
Sex Hormone Binding Globulin (SHBG) = 20 nmol/L
Albumin = 4.3 g/dL
Using the calculator’s binding-equilibrium method, the estimated Free Testosterone is approximately 13.15 ng/dL (≈ 131.5 pg/mL), which is about 2.63% of the total.
