Free Testosterone Calculator — with Bioavailable Levels

Enter total testosterone, SHBG, and albumin from a blood test to calculate free and bioavailable testosterone using the Vermeulen equation, the standard clinical method for estimating unbound hormone levels.

Quick Facts

Formula
Vermeulen equation (1999)
Solves a quadratic derived from the mass-action binding of testosterone to SHBG and albumin.
Binding affinity
SHBG binds ~28,000x tighter than albumin
Association constants: SHBG ~1x10^9 L/mol vs albumin ~3.6x10^4 L/mol - small SHBG changes shift free T noticeably.

Your Results

Calculated
Free testosterone
-
Unbound hormone (pg/mL)
Bioavailable testosterone
-
Free + albumin-bound (ng/dL)
% Free
-
Free T as share of total
% Bioavailable
-
Non-SHBG-bound share of total

Ready

Enter total testosterone, SHBG, and albumin, then calculate.

About the Free Testosterone Calculator — with Bioavailable Levels

Most testosterone in the blood is not free to act on tissue. It is carried by two proteins: sex hormone-binding globulin (SHBG), which binds it very tightly, and albumin, which binds it loosely. Only the small unbound fraction — free testosterone — plus the loosely-bound albumin fraction — together called bioavailable testosterone — is generally considered available to tissues. This calculator applies the Vermeulen equation, published by Vermeulen, Verdonck and Kaufman in 1999, to estimate free and bioavailable testosterone from a standard total testosterone, SHBG, and albumin blood panel.

How the calculation works

  • Total testosterone, SHBG, and albumin are converted to a common concentration unit (nmol/L) using standard molecular-weight conversions.
  • The equation models testosterone binding to SHBG using the law of mass action (SHBG capacity is limited and comparable to testosterone concentration), while albumin binding is treated as effectively unsaturated because albumin is present in large excess.
  • Combining these relationships produces a quadratic equation in free testosterone, which is solved directly for the free concentration; bioavailable testosterone is then free testosterone plus the albumin-bound fraction.

Putting the result in context

SHBG rises with age, hyperthyroidism, and estrogen exposure, and falls with obesity, hypothyroidism, and some liver conditions — that is exactly why two people with the same total testosterone can have different free testosterone. A total testosterone result alone can miss this; calculated free and bioavailable testosterone accounts for it. Reference ranges for free testosterone vary by laboratory, assay, and age, so compare your result to the range your lab provides alongside its total testosterone measurement.

When to consult a professional

This tool performs the standard Vermeulen arithmetic for education and reference; it is not a diagnosis and does not direct treatment. Testosterone assays, especially in women and in men near the lower end of the range, can vary between laboratories. Any decision about hormone therapy or further testing should be made with a licensed healthcare provider who can interpret the result alongside symptoms and other labs.

Frequently Asked Questions

What formula does this calculator use?
It uses the Vermeulen equation (Vermeulen, Verdonck and Kaufman, 1999), the standard method for calculating free testosterone from total testosterone, SHBG, and albumin. The equation models how testosterone distributes between a free form, an SHBG-bound form, and an albumin-bound form based on each protein's binding affinity, then solves a quadratic for the free fraction. Bioavailable testosterone is the free fraction plus the loosely bound albumin fraction.
Why is free testosterone only a small percentage of total testosterone?
Most circulating testosterone is bound to carrier proteins. SHBG binds testosterone very tightly and holds roughly 40 to 60 percent of it, albumin binds it loosely and holds most of the rest, and only a small remainder, typically around 1 to 4 percent in adult men, circulates unbound as free testosterone. Because SHBG's affinity for testosterone is far higher than albumin's, even modest changes in SHBG concentration shift the free fraction noticeably.
How does calculated free testosterone compare to a direct lab assay?
Calculated free testosterone from total testosterone, SHBG, and albumin correlates well with equilibrium dialysis, the reference laboratory method, and is generally considered more reliable than older direct analog immunoassay kits for free testosterone. This calculator performs the standard arithmetic; it does not replace a laboratory measurement or a clinician's interpretation of your specific case.

Related calculators