Not medical advice. This calculator does arithmetic on numbers you enter, for general education. It does not diagnose, treat, or recommend a dose or treatment for any condition, and it does not replace an evaluation by a licensed physician. It returns a calculated number only. It does not tell you whether a result is low, normal, or high, and it is not a diagnosis of low testosterone or a reason to start, stop, or change any treatment. Talk to your own clinician before acting on any result.
Free testosterone can be calculated from three values on a standard lab report: total testosterone, sex hormone-binding globulin (SHBG), and albumin. The calculator below uses the Vermeulen equation, a law-of-mass-action formula published in 1999 that tracked free testosterone measured by equilibrium dialysis closely in its validation study.1
Every lab we order at Opt Health is read by a physician before anyone is offered a treatment. The calculator runs the Vermeulen equation with its constants printed so you can check it.
Free Testosterone Calculator
Enter total testosterone and SHBG from the same blood draw. Albumin is optional: if you leave it at 4.3 g/dL, the result says the value was assumed.
Calculated free testosterone
…
Same result, other units
…
Percent free
…
Calculated bioavailable testosterone
…
Enter total testosterone and SHBG to calculate.
Method: Vermeulen et al., 1999. Association constants: SHBG 1.0 × 109 L/mol, albumin 3.6 × 104 L/mol; albumin molar mass 69,000 g/mol; testosterone molar mass 288.42 g/mol.1 Calculated results depend on the equation used and do not interchange with a free testosterone your lab measured directly or calculated another way.
How the Vermeulen Calculation Works
The Vermeulen equation treats testosterone binding to SHBG and to albumin as two competing reactions with fixed association constants, and solves for the free fraction that satisfies both. Our guide to testosterone levels through the years explains the free, albumin-bound, and SHBG-bound pools.
The constants are printed under the calculator: 1.0 × 109 L/mol for SHBG and 3.6 × 104 L/mol for albumin.1 The working line under each result shows the quadratic the calculator solves and the inputs it used, so the figure can be reproduced on paper or in a spreadsheet.
Bioavailable testosterone is the free fraction plus the albumin-bound fraction. Albumin releases testosterone easily, so some laboratories report this combined figure alongside, or in place of, free testosterone. The calculator reports both figures.
The Three Values From Your Lab Report
- Total testosterone. Reported in ng/dL in the United States and nmol/L in most other countries. One nmol/L equals 28.84 ng/dL, and the unit buttons convert between them. The Endocrine Society recommends a fasting, early-morning sample, confirmed with a repeat measurement.2
- SHBG. Reported in nmol/L. Use the value from the same blood draw as the total testosterone, because the calculation needs both values from one sample.
- Albumin. Usually part of a standard metabolic panel, in g/dL in the United States or g/L elsewhere. The calculator assumes 4.3 g/dL if you do not have one, and albumin moves the result much less than SHBG does.
How SHBG Moves Free Testosterone
Two men with identical total testosterone can carry very different amounts of free testosterone, and SHBG accounts for most of the difference. The table holds total testosterone at 500 ng/dL and albumin at 4.3 g/dL, and changes only SHBG.
| SHBG | Calculated free testosterone | Percent free | Bioavailable testosterone |
|---|---|---|---|
| 20 nmol/L | 134.9 pg/mL | 2.70% | 316 ng/dL |
| 40 nmol/L | 93.2 pg/mL | 1.86% | 218 ng/dL |
| 60 nmol/L | 69.7 pg/mL | 1.39% | 163 ng/dL |
SHBG rises with age, liver disease, hyperthyroidism, some anticonvulsants, and estrogens, and it falls with obesity, type 2 diabetes, glucocorticoids, and hypothyroidism.2 That is why the Endocrine Society advises checking free testosterone when total testosterone sits near the lower limit of normal or when a condition that alters SHBG is present.2 A total testosterone result read alongside its SHBG gives a physician the fuller picture.
Calculated and Measured Free Testosterone
Equilibrium dialysis measures free testosterone directly and serves as the reference method. The Endocrine Society accepts either equilibrium dialysis or a calculation from accurate total testosterone and SHBG measurements, and advises against the direct analog immunoassays some labs still offer.2
Laboratories that calculate free testosterone do not all use the same equation, so the number here can differ from the free testosterone printed on your report even with identical inputs. Compare a result with the reference range from the same laboratory and the same method, and track it over time with one method.
Reading the Result Beside Your Lab's Range
The calculator gives a number and its units, and the reference range that number is compared against belongs to the laboratory that ran your sample. Free testosterone ranges also shift with age. Our guides to free testosterone levels by age and how total testosterone changes through the decades cover the published ranges, and the testosterone calculator by age compares a total testosterone result with them.
A diagnosis of low testosterone takes consistent symptoms and unequivocally low results on repeat morning testing, according to both the Endocrine Society and the American Urological Association.2,3 If you are deciding whether to get tested, start with the signs that justify a testosterone test.
The Opt Take on Free Testosterone
A calculated free testosterone is one number from one draw, and a physician reads it as part of a pattern. At Opt Health, a member starts with a blood panel of 55+ biomarkers spanning hormones, inflammation, and cardiovascular risk. An Opt Health physician then reviews the results 1:1 and sets a plan.
Reassessment with a full blood panel and a physician consultation follows every 3 to 4 months, so a free testosterone figure is compared with your own earlier results rather than read alone. Existing conditions and current medications, two things that move SHBG, are reviewed at the first consultation.
Get started: Begin with Opt Health, where a physician reads your labs, builds your plan, and adjusts it over the loop.
Frequently Asked Questions
Free testosterone is calculated from total testosterone, SHBG, and albumin with a binding equation such as the Vermeulen formula. Enter the three values from one lab report into the calculator and it solves the equation, prints the constants it used, and shows the result in pg/mL, ng/dL, pmol/L, and nmol/L.
Total testosterone is the first test in both the Endocrine Society and American Urological Association guidelines. Free testosterone adds information when total testosterone is near the lower limit of normal or when SHBG is likely to be abnormal, because SHBG changes how much of the total is available.
Yes. A high SHBG binds more testosterone, so a man can have a total testosterone in range and a low free fraction. SHBG tends to rise with age and with conditions such as hyperthyroidism and liver disease, which is why a physician reads both values together.
Equilibrium dialysis is the reference method for measuring free testosterone. The Endocrine Society also accepts a free testosterone calculated from accurate total testosterone and SHBG measurements, and advises against direct analog immunoassays.
Reference ranges differ by laboratory and by method, so the range printed on your own report is the one to compare against. Our guide to free testosterone levels by age walks through the published ranges and how they change across the decades.
A free testosterone result means more beside SHBG, symptoms, and earlier labs. An Opt Health membership puts your full panel in front of a physician at each follow-up, 2 to 4 times a year depending on plan.
References
- 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. PMID: 10523012.
- 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. PMID: 29562364.
- Mulhall JP, Trost LW, Brannigan RE, et al. Evaluation and management of testosterone deficiency: AUA guideline. J Urol. 2018;200(2):423-432. PMID: 29601923.
This content is for informational purposes and does not replace evaluation, diagnosis, or treatment by a qualified medical professional.
This article is part of our guide to testosterone. See how physician-managed TRT works at Opt Health.
Start Today
Your health, your terms. Discover how personalized care can transform not just the way you feel, but how you live.