What a free testosterone calculator does
A free testosterone calculator estimates how much of your testosterone is circulating unbound, the portion tissues can use most readily. MedlinePlus explains that most testosterone travels attached to proteins, and that a total testosterone test counts both the bound and the free forms.
Measuring the free portion directly by equilibrium dialysis is accurate but slow and specialized. In a 1999 study, Vermeulen and colleagues showed that free testosterone calculated from total testosterone and SHBG matched the dialysis method almost exactly, except during pregnancy. That equation, which also accounts for albumin, is what most calculators use today.
The three inputs
The calculation needs three numbers, ideally from the same morning blood draw.
| Input | Usual units | Role in the equation |
|---|---|---|
| Total testosterone | ng/dL or nmol/L | The starting pool. Multiply nmol/L by 28.84 to get ng/dL |
| SHBG | nmol/L | Binds testosterone tightly; more SHBG means less free |
| Albumin | g/dL | Binds testosterone loosely; small effect within normal limits |
Albumin usually runs about 3.4 to 5.4 g/dL according to MedlinePlus’s metabolic panel reference. If albumin was not measured, some calculators assume a typical value, which costs little accuracy. SHBG is the input that moves the answer most.
Worked examples: SHBG of 20 versus 70 nmol/L
The examples below were calculated with the Vermeulen equation and its published binding constants, holding albumin at 4.3 g/dL. Other calculators and labs can return somewhat different numbers.
| Total testosterone | SHBG 20 nmol/L | SHBG 40 nmol/L | SHBG 70 nmol/L |
|---|---|---|---|
| 300 ng/dL | about 77 pg/mL | about 52 pg/mL | about 35 pg/mL |
| 400 ng/dL | about 105 pg/mL | about 72 pg/mL | about 48 pg/mL |
| 600 ng/dL | about 166 pg/mL | about 116 pg/mL | about 76 pg/mL |
Read across the 400 ng/dL row. The total is the same, yet free testosterone more than halves as SHBG rises from 20 to 70. A man with a “normal” total of 400 ng/dL and a high SHBG can have less free testosterone than a man at 300 ng/dL with a low SHBG.
For context, the Endocrine Society guideline cites a large European study in which sexual symptoms tracked with free testosterone below 64 pg/mL. Albumin matters far less: at 400 ng/dL and SHBG 40, moving albumin from 3.5 to 4.8 g/dL shifts the estimate only from about 79 to about 69 pg/mL.
What pushes SHBG up or down
SHBG is the swing factor, so it helps to know what moves it. The Endocrine Society lists aging, liver disease, an overactive thyroid, HIV, some seizure medicines and estrogen use as causes of higher SHBG. It lists obesity, diabetes, steroid medicines and anabolic steroids, an underactive thyroid and kidney protein loss among the causes of lower SHBG.
Those are exactly the situations in which the guideline recommends checking free testosterone instead of relying on the total. Our pages on high SHBG with normal testosterone and low SHBG in men explore each direction, and the SHBG blood test page explains the marker itself.
Limits of any calculator
A calculated free testosterone inherits every weakness of the numbers fed into it. A non-fasting or afternoon total testosterone will give a misleadingly low result, and mixing values from different days adds error.
The Endocrine Society also notes that no harmonized reference range exists for free testosterone, so the lower limit depends on the lab and method, and it advises against direct “analog” free testosterone immunoassays because they are inaccurate. A calculator output is therefore one piece of the picture. Our comparison of free versus total testosterone and the free testosterone test page explain when each is the better guide.
When free testosterone changes the decision
Free testosterone matters most when the total sits near the line or when SHBG is likely abnormal. A total in the gray zone around 264 to 350 ng/dL, combined with symptoms, is a common reason to add SHBG and calculate free testosterone; our page on borderline testosterone levels covers that situation.
It works in the other direction too. A low total with a low SHBG, common with excess weight or diabetes, may come with a normal free testosterone, which argues against treatment and toward the underlying cause.
How Ultimate Male uses calculated free testosterone
Our TRT pre-screening panel starts with total testosterone, estradiol, PSA and a complete blood count. When the total is borderline or something suggests SHBG is off, your provider adds SHBG and albumin so free testosterone can be calculated from a fasting morning draw taken on site at San Gabriel or Downey.
You then go through the numbers with a PA-C or MD, including how the calculated value compares with your lab’s range and with your symptoms. The visit is there to evaluate, and treatment is never assumed in advance. Explore all of our panels on the preventative blood testing page.
