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Free Testosterone Calculator (Vermeulen)

Enter your total testosterone, SHBG and albumin to estimate free and bioavailable testosterone with the Vermeulen equation, the method most laboratories use when they report a calculated free T. Results in ng/dL and pg/mL, with the unit conversions and the reference ranges by age explained.

  • Calculator
  • About 3 min
  • Vermeulen 1999 equation
  • Runs in your browser, nothing is sent
  • Medical review: Medical review pending
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What this calculator works out

Total testosterone counts every molecule in the blood, but most of it is bound to SHBG and unavailable to tissues. The calculation estimates how much is free, how much is loosely bound to albumin, and the share of the total that is free.

  1. 1

    Free testosterone

    The unbound fraction, reported in ng/dL and pg/mL. This is the number clinicians compare against a laboratory's calculated free T range, and the pg/mL figure is the one most US reports print.

  2. 2

    Bioavailable testosterone

    Free testosterone plus the part loosely bound to albumin, which can still reach tissues. Reported in ng/dL.

  3. 3

    Free fraction

    Free testosterone as a percentage of the total. Around two percent is typical; a high SHBG lowers it and a low SHBG raises it.

How this compares with a typical online quiz

FeatureTypical free quizRegenerated.com
Published equation namedRarelyVermeulen, Verdonck and Kaufman 1999
Binding constants shownNoYes, in the method note
Albumin includedOften fixedYour own value, or the 4.3 g/dL default
Bioavailable T reportedSometimesYes
Units handledng/dL onlyng/dL, nmol/L and pg/mL, with the conversions
Reference range by ageOne cut-offPublished equilibrium dialysis intervals by age, with their limits
Answers sent to a serverOftenNever; scored in your browser, counts-only events

Reference range by age

There is no single free testosterone range that every laboratory uses, so the fairest comparison is a published interval measured by the reference method. In 2023 Jasuja and colleagues measured free testosterone by a standardized equilibrium dialysis procedure in 145 healthy, nonobese men aged 19 and over. Across all ages the 2.5th to 97.5th percentile interval was 66 to 309 pg/mL (6.6 to 30.9 ng/dL), with a median of 141 pg/mL. In men aged 19 to 39 the interval was 120 to 368 pg/mL (12.0 to 36.8 ng/dL), with a median of 190 pg/mL. Free testosterone fell with age in the expected way and was lower with higher body mass index and higher SHBG.

The decline is gradual rather than a cliff. In the Massachusetts Male Aging Study, Feldman and colleagues followed men aged 40 to 70 and found total testosterone fell by about 1.6 percent per year, with free and bioavailable testosterone falling faster, around 2 to 3 percent per year, because SHBG rises with age and binds a growing share of the total. That is the reason a man in his sixties with a normal total can have a calculated free T below the young-adult interval.

Two cautions before you compare your number. The intervals above were measured by equilibrium dialysis, and the Vermeulen calculation tracks that method closely but not perfectly; your laboratory's own calculated free T range is the one its clinicians use. And the 6.5 ng/dL (65 pg/mL, 225 pmol/L) line used in the result band is an investigation threshold from the guidelines, close to the lower limit of the all-ages interval, not a definition of normal for a man of 25.

Units: ng/dL, nmol/L and pg/mL

The commonest reason a free testosterone result looks wrong is a unit mix-up. Total testosterone is reported in ng/dL by most US laboratories and in nmol/L in Europe, Canada and Australia; multiply nmol/L by 28.8 to get ng/dL (a total of 20 nmol/L is 576 ng/dL). SHBG is reported in nmol/L almost everywhere. Albumin is g/dL in the US and g/L elsewhere; divide g/L by 10.

Free testosterone itself is printed three ways. 1 ng/dL equals 10 pg/mL, and 1 ng/dL equals 34.7 pmol/L. So a calculated free T of 6.5 ng/dL is 65 pg/mL or about 225 pmol/L. The calculator shows ng/dL and pg/mL side by side so you can match whichever your report uses.

Methodology and sources

The calculator solves the mass-action equation published by Vermeulen, Verdonck and Kaufman in 1999, derived in adult men and validated against equilibrium dialysis. It assumes an SHBG binding constant of 1.0 x 10^9 L/mol and an albumin binding constant of 3.6 x 10^4 L/mol, converts albumin from g/dL to mol/L using a molecular weight of 69,000, and converts total testosterone from ng/dL to nmol/L with the factor 0.0347. The free fraction is found by iterating the binding equation until it stops changing, which gives the same answer as the closed-form quadratic in the paper. Bioavailable testosterone is the free plus albumin-bound share.

A worked example: a total testosterone of 600 ng/dL, SHBG of 40 nmol/L and albumin of 4.3 g/dL give a free testosterone of about 11.5 ng/dL (115 pg/mL), which is 1.92 percent of the total, with about 270 ng/dL bioavailable. Raising SHBG to 80 nmol/L with the same total cuts the free fraction by about 40 percent, from 1.92 to roughly 1.1 percent, which is why two men with the same total can sit on opposite sides of a threshold.

Calculated free testosterone agrees well with equilibrium dialysis in the published comparison, but it is still an estimate. Laboratories use slightly different constants and reference ranges, and a single morning sample can vary by 20 percent or more from day to day; two readings beat one. The 6.5 ng/dL (225 pmol/L) threshold shown in the result is the level that several guideline groups treat as a reason to look further; it is not a diagnosis on its own. The age intervals in the section above come from Jasuja and colleagues (2023) and the decline rates from Feldman and colleagues (2002).

Medical review: Medical review pending. Last updated .

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;84(10):3666-3672. doi:10.1210/jcem.84.10.6079
  2. Bhasin S, Brito JP, Cunningham GR, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2018;103(5):1715-1744. doi:10.1210/jc.2018-00229
  3. Salonia A, Bettocchi C, Boeri L, et al. European Association of Urology guidelines on sexual and reproductive health, 2021 update: male sexual dysfunction. Eur Urol. 2021;80(3):333-357. doi:10.1016/j.eururo.2021.06.007
  4. Jasuja R, Pencina KM, Spencer DJ, et al. Reference intervals for free testosterone in adult men measured using a standardized equilibrium dialysis procedure. Andrology. 2023;11(1):125-133. doi:10.1111/andr.13310
  5. Feldman HA, Longcope C, Derby CA, et al. Age trends in the level of serum testosterone and other hormones in middle-aged men: longitudinal results from the Massachusetts Male Aging Study. J Clin Endocrinol Metab. 2002;87(2):589-598. doi:10.1210/jcem.87.2.8201

Free Testosterone Calculator (Vermeulen) FAQ

Most testosterone in the blood is tightly bound to SHBG and cannot act on tissues. Two men with the same total can have very different free levels if their SHBG differs, which is common with age, obesity, thyroid disease and some medications.

Total testosterone in ng/dL (US labs), SHBG in nmol/L and albumin in g/dL. If your lab reports total testosterone in nmol/L, multiply by 28.8 to get ng/dL. If albumin is in g/L, divide by 10. The result is shown in ng/dL and pg/mL; 1 ng/dL is 10 pg/mL.

Reference ranges differ between laboratories. Several guideline groups use a calculated free testosterone below about 6.5 ng/dL (65 pg/mL, 225 pmol/L), with symptoms, as a reason to investigate further. By equilibrium dialysis, healthy nonobese men of all ages ran 66 to 309 pg/mL and men aged 19 to 39 ran 120 to 368 pg/mL in the 2023 reference study. One result is not enough: guidelines ask for two morning samples before any diagnosis.

The equation itself applies to anyone, but the interpretation band and the age ranges on this page are written for men. Women's free testosterone is far lower and is judged against different ranges.

Equilibrium dialysis is the reference method. The Vermeulen calculation tracks it closely in the published comparison and is what most laboratories report. Direct analog immunoassays for free T are less reliable and are not recommended by the Endocrine Society.

Yes. Your answers and the numbers you enter are scored in your browser and are never sent to us. They stay in this browser tab so you can return to your result, and closing the tab clears them. The only thing the page sends us is a counts-only event: which test was started or completed, the result band and the headline number, with no answers, no entered values and no account. There is nothing to sign up for.

Talk to a clinic that evaluates testosterone properly

Browse US clinics offering testosterone replacement therapy. Look for ones that repeat the labs, check the cause, monitor hematocrit and PSA, and explain the risks before prescribing. A listing on Regenerated.com is not an endorsement.

Browse TRT clinics

A listing is not a treatment endorsement. Read each clinic's published checks and talk to your clinician before starting any treatment.

Important: This calculator is educational arithmetic on numbers you enter. It is not a diagnosis of low testosterone and not a recommendation for or against testosterone therapy. Only a clinician who knows your history and sees your full laboratory results can interpret them. If your symptoms are severe or sudden, seek urgent medical care.