Type your total testosterone and SHBG from the same blood test (and albumin, if it is on the report). You get free and bioavailable testosterone by the Vermeulen equation — the one labs use — and where your total sits in the harmonized table for your age. Your lab’s ranges, and your doctor, have the last word.
Free testosterone
8.98 ng/dL free 89.8 pg/mL · 311 pmol/L · 1.99% of the total
- Bioavailable
- 210 ng/dL bioavailable (7.29 nmol/L)
- Total
- 450 ng/dL (15.6 nmol/L)
| Centile | ng/dL |
|---|
This is not medical advice and it does not diagnose anything. It computes a published equation from your own results and shows where they sit in published tables; your lab prints its own ranges, and what your numbers mean for you is a conversation with your doctor.
Free testosterone is calculated from total testosterone, SHBG and albumin with the Vermeulen equation (1999), which its authors found “a rapid, simple, and reliable index” of bioavailable testosterone, except in pregnancy. At 450 ng/dL total and 35 nmol/L SHBG it gives 8.98 ng/dL free, 1.99% of the total. For age, the harmonized range in healthy nonobese men 19 to 39 is 264 to 916 ng/dL.
How to use it
Take the total testosterone and the SHBG from the same blood test — both are needed, because SHBG holds most of the testosterone and decides how much is left free. Pick the unit printed beside the testosterone (ng/dL in the US, nmol/L in most other countries); switching the unit converts the number you typed. Type albumin if it is on the report, in g/dL or g/L; leave it empty and the equation uses 4.3 g/dL. Pick an age band to see where the total sits in the harmonized table. It does not tell you what to take: that is the conversation with your doctor.
The equation, and where its numbers come from
Testosterone in blood is free, loosely bound to albumin, or tightly bound to SHBG. Vermeulen, Verdonck and Kaufman (1999) solve those bindings for the free part, with an association constant of 109 L/mol for SHBG and 3.6 × 104 L/mol for albumin, as the same group’s lab at Ghent University Hospital publishes them. Bioavailable testosterone is the free part plus the albumin-bound part. The Ghent lab’s own worked example — SHBG 40 nmol/L, testosterone 288.4 ng/dL, albumin 4.3 g/dL — gives 1.74% free, 5.02 ng/dL, and 118 ng/dL bioavailable; this calculator gives 1.74%, 5.01 and 118 (the lab rounds its molar mass). To convert, 1 nmol/L of testosterone is 28.84 ng/dL.
| SHBG | Free testosterone | % free | Bioavailable |
|---|---|---|---|
| 15 nmol/L | 15.02 ng/dL | 3.00% | 352 ng/dL |
| 25 nmol/L | 12.19 ng/dL | 2.44% | 286 ng/dL |
| 35 nmol/L | 10.14 ng/dL | 2.03% | 238 ng/dL |
| 50 nmol/L | 7.99 ng/dL | 1.60% | 187 ng/dL |
| 70 nmol/L | 6.16 ng/dL | 1.23% | 144 ng/dL |
| 100 nmol/L | 4.55 ng/dL | 0.91% | 107 ng/dL |
Testosterone levels by age
Travison et al. (2017) measured 9,054 men in four cohorts against the CDC’s reference method, so their numbers hold across labs that are calibrated to it: “Harmonized normal range in a healthy nonobese population of European and American men, 19 to 39 years, is 264 to 916 ng/dL.” Their Table 4 gives the model-based centiles for nonobese men by age:
| Age | 2.5th | 5th | 50th | 95th | 97.5th |
|---|---|---|---|---|---|
| 19–39 | 267 | 304 | 531 | 850 | 929 |
| 40–49 | 235 | 273 | 481 | 839 | 929 |
| 50–59 | 219 | 256 | 477 | 839 | 929 |
| 60–69 | 218 | 254 | 477 | 839 | 929 |
| 70–79 | 218 | 252 | 477 | 839 | 926 |
| 80–99 | 157 | 218 | 476 | 839 | 913 |
The paper deemed values under the 2.5th centile low. The American Urological Association’s guideline (2018, reaffirmed 2024) uses one line instead: “Clinicians should use a total testosterone level below 300 ng/dL as a reasonable cut-off in support of the diagnosis of low testosterone” — and only “after two total testosterone measurements are taken on separate occasions with both conducted in an early morning fashion”.
Worked examples
- 450 ng/dL, SHBG 35, age 40–49 (the page’s example): 8.98 ng/dL free (1.99%), 210 ng/dL bioavailable; the total is between the 25th and 50th centiles for nonobese men 40–49.
- 16 nmol/L, SHBG 30, albumin 4.5, age 50–59: 16 nmol/L is 461 ng/dL; 9.85 ng/dL free (2.13%), 241 ng/dL bioavailable.
- 280 ng/dL, SHBG 22, age 60–69: 6.79 ng/dL free (2.42%). The total is under the AUA’s 300 ng/dL line and between the 5th and 10th centiles for the age — a result for a second morning test and a doctor.
Common mistakes
- Typing nmol/L as ng/dL. 15 nmol/L is 433 ng/dL. The calculator flags a “ng/dL” total under 60.
- Mixing two blood tests. The equation needs the total and the SHBG from the same sample.
- Using it in pregnancy, or on DHT or oral testosterone. The authors and their lab say the calculated value is not reliable then.
- Reading one result as a diagnosis. The AUA makes the diagnosis only with two early-morning tests and symptoms.
Frequently asked questions
How do you calculate free testosterone?
From total testosterone, SHBG and albumin, with the Vermeulen equation, which solves how much testosterone SHBG and albumin hold and returns the free part. Its authors found it “a rapid, simple, and reliable index” of bioavailable testosterone, except in pregnancy.
What is a normal testosterone level by age?
In Travison et al.’s harmonized table, the middle (50th centile) for nonobese men is 531 ng/dL at 19 to 39 and about 477 ng/dL from 50 on; the 2.5th centile falls from 267 ng/dL at 19 to 39 to 157 ng/dL at 80 and over. Your lab prints its own range.
What is a normal free testosterone level?
There is no harmonized free testosterone range the way there is for total testosterone, so labs print their own. The Ghent lab that published the equation puts the free share at “approximately 2 – 3%” of the total.
What does SHBG do to free testosterone?
SHBG binds testosterone tightly, so at the same total, more SHBG leaves less free: at 500 ng/dL total, free testosterone is 15.02 ng/dL with SHBG at 15 nmol/L and 4.55 ng/dL at 100 nmol/L. That is why the Endocrine Society recommends a free testosterone result when SHBG may be altered.
What is bioavailable testosterone?
The free testosterone plus the part loosely bound to albumin — everything not held by SHBG. The equation gives it as the free amount times (1 + the albumin binding), 118 ng/dL in the Ghent lab’s example.
What testosterone level is considered low?
The AUA guideline uses a total testosterone below 300 ng/dL “as a reasonable cut-off in support of the diagnosis of low testosterone”, confirmed by two early-morning tests and combined with symptoms. Travison et al. deemed values under the 2.5th centile low. Either way, it is your doctor’s call.
How do I convert testosterone from nmol/L to ng/dL?
Multiply nmol/L by 28.84 to get ng/dL, or divide ng/dL by 28.84 for nmol/L: 15 nmol/L is 433 ng/dL, and 300 ng/dL is 10.4 nmol/L.
Read next
- Nugenix Total-T vs Nugenix Ultimate: how to choose
- What are testosterone boosters, and do they work?
- Is high testosterone good for males?
- Testosterone replacement therapy: science, myths and facts
- Does Adderall affect testosterone?
This is not medical advice, and nothing here diagnoses or treats anything. It computes a published equation from your own results; what they mean for you is a conversation with your doctor, who knows your lab’s ranges and your history. Some links on this site are affiliate links: I buy what I review at full price, and a commission never changes a verdict.