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The Vermeulen equation estimates free and bioavailable testosterone from three numbers a laboratory has already reported: total testosterone, SHBG and albumin. Albumin is prefilled at the default used by the reference implementation, labelled as an assumption rather than as your value. No reference range is shown — ranges are set by the laboratory that ran the assay, against its own method and population.

Last reviewed: September 21, 2026

The Vermeulen equation estimates free and bioavailable testosterone from a total testosterone, an SHBG and an albumin. It is arithmetic on three numbers your laboratory already reported.

Prefilled at 4.3 g/dL — the default on ISSAM's own Vermeulen calculator, not a value from your panel. Replace it with yours if you have one.

How the calculation works

Most circulating testosterone is bound to sex hormone-binding globulin (SHBG) and, more loosely, to albumin; only a small fraction is unbound (“free”). Vermeulen, Verdonck and Kaufman (1999) published a method for calculating the free fraction from three lab values — total testosterone, SHBG and albumin — using the known binding affinities of testosterone to each protein. Bioavailable testosterone adds back the loosely albumin-bound fraction, which dissociates readily enough to also be considered available to tissue.

The published paper describes the method; the specific binding-affinity constants it relies on are not printed in the paper’s own abstract. This page’s constants were read from two independent open implementations of the equation — ISSAM’s calculator and a published laboratory information system implementation — which agree with each other on every constant but one.

Why this figure differs from ISSAM’s by about 3%. ISSAM’s calculator divides by 2.8 to convert testosterone’s units, where testosterone’s actual molar mass is 288.4; this page uses the molar mass. That single difference puts this page’s calculated free testosterone about 3% below ISSAM’s for identical inputs. A reader comparing the two calculators will see this difference themselves, so it is explained here rather than left for them to puzzle out.

Worked example

Total testosterone 600 ng/dL, SHBG 30 nmol/L, albumin at the prefilled default of 4.3 g/dL:

Free testosterone13.708 ng/dL (2.28% of total)
Bioavailable testosterone321.25 ng/dL (53.54% of total)

Enter the same three numbers into the calculator above and it returns the same figures. Neither figure is labelled low, normal or high — see the next section.

Reference ranges belong to your laboratory

This page reports a calculated number and nothing else. It does not state whether a result is low, typical or high, and it carries no colour, arrow or verdict. A reference range is set by the laboratory that ran the assay, against its own method and its own tested population — ranges differ meaningfully between labs using different assay platforms. Compare this calculated figure against the range printed on your own report, not against a range stated here.

FAQ

What is the Vermeulen equation for free testosterone?
A method published by Vermeulen, Verdonck and Kaufman (J Clin Endocrinol Metab, 1999) for estimating free and bioavailable testosterone from total testosterone, SHBG and albumin, using the known binding affinities of testosterone to SHBG and albumin. This page implements that method's constants, read from two independent open implementations since the original paper's abstract does not print them.
free testosterone calculator vermeulen
This page's calculator takes total testosterone (ng/dL), SHBG (nmol/L) and albumin (g/dL, prefilled at a default of 4.3 you can replace with your own), and returns calculated free and bioavailable testosterone — as a figure, with no reference range attached.
Why doesn't this calculator tell me if my testosterone is low?
Because that determination belongs to the laboratory that ran the assay, against its own method and its own reference population — not to a calculator applying a formula to three numbers. This page reports the Vermeulen equation's calculated output and nothing else; comparing that figure against a range is a separate step using the range on your own lab report.
Why is albumin prefilled?
4.3 g/dL is the default value on ISSAM's own reference implementation of this equation, included so the calculator still returns a figure for the many lab panels that don't report albumin. It is labelled as an assumed value, not read from any specific panel — replace it with your own albumin result if your panel reported one.
Why does this calculator give a different number than other free testosterone calculators?
Most differ by a small amount depending on which constants they use for the same published equation. This calculator uses testosterone's molar mass (288.4 g/mol) in its unit conversion; ISSAM's widely used calculator uses a rounded divisor of 2.8 in the equivalent step. That difference alone puts this page's free testosterone figure about 3% below ISSAM's for identical inputs — not a disagreement about the equation, but about how precisely one conversion step is carried out.

References

  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 Oct;84(10):3666-72. https://pubmed.ncbi.nlm.nih.gov/10523012/
  2. The equation’s constants (SHBG and albumin association constants, molar masses) are not printed in the 1999 abstract above; this page’s implementation reads them from ISSAM’s published calculator (issam.ch/freetesto.htm) and cross-checked them against a second, independently published laboratory implementation.

For a testosterone dose’s own volume and syringe arithmetic, see the TRT dose calculator.