Free Testosterone
Calculator
Calculate your free testosterone and bioavailable testosterone from Total T, SHBG, and albumin using the Vermeulen formula — the gold-standard method validated in the Journal of Clinical Endocrinology and Metabolism. Results in pg/mL, nmol/L, percentage of total, and bioavailable T in ng/dL.
Most testosterone in your blood is bound and inactive. Only the free fraction (plus loosely albumin-bound) reaches your tissues. Drag any slider or type a value — everything recalculates instantly.
Typical adult male: 300–1000 ng/dL
Typical adult male: 18–54 nmol/L
Default 4.3 g/dL if unknown
Enter your values above to see how your free testosterone compares to the reference range.
Educational estimate using the Vermeulen equation (Ka SHBG 5.97×10⁸, Ka albumin 4.06×10⁴). Reference ranges vary by lab and age — always interpret results with your physician, not in isolation.
How to use the free testosterone calculator
- 1
Enter your total testosterone
From a morning blood draw, in ng/dL or nmol/L — every testosterone molecule, bound or free.
- 2
Enter your SHBG
From the same draw. SHBG sets how much of your testosterone stays locked away and unavailable.
- 3
Add albumin (optional)
Leave the default 43 g/L if you do not have it — it barely changes the result.
- 4
Read free and bioavailable T
The tool solves the Vermeulen equation and shows the active fraction your cells actually use.
Free testosterone is the ~1–2% of your total testosterone that is unbound and biologically active. This calculator estimates it from total T, SHBG and albumin using the Vermeulen equation — the method labs trust far more than a direct “free T” immunoassay. It matters most when total testosterone looks normal but SHBG is high, quietly starving you of the free hormone that actually does the work.
What this free testosterone calculator covers
Total testosterone is only half the story. Most of it is bound to proteins and unavailable to your cells; what actually drives libido, energy, mood and muscle is the small free and bioavailable fraction. This free testosterone calculator turns three lab values into that active number and, unlike generic tools, explains what it means for someone on TRT or a steroid protocol — where SHBG shifts, assay choice and interpretation change everything.
The unbound, active slice of your testosterone — roughly 1–2% of the total.
The Vermeulen equation from total T, SHBG and albumin — the lab-standard calculated method.
A normal total T with high SHBG can hide a genuinely low free T and real symptoms.
Total T, free T, and bioavailable T — three different numbers
Testosterone travels in your blood in three states. Most of it, roughly 44–65%, is locked tightly to sex hormone-binding globulin (SHBG) and is effectively unavailable. Another large share is bound loosely to albumin, and because that grip is weak, this portion can still be used by tissues. Only a tiny 1–2% floats completely free.
Two terms matter. Free testosterone is that unbound 1–2%. Bioavailable testosterone is the free fraction plus the albumin-bound fraction — everything that is realistically available to your cells. The SHBG-bound portion is the part your body cannot easily touch, which is exactly why a high SHBG can leave you symptomatic on a “normal” total.
The Vermeulen equation — how the calculator works
Enter your total testosterone, your SHBG and (optionally) your albumin, and the calculator solves the Vermeulen equation — the mass-action model published in 1999 that has become the reference standard for calculated free testosterone. It models how testosterone distributes between SHBG, albumin and the free state using established binding constants, and returns both your free T and, from it, your bioavailable T.
If you leave albumin blank, the tool uses the standard 43 g/L, which is close enough for almost everyone. Accuracy hinges on your SHBG and total T being measured on the same blood draw: because free T is calculated from the interplay of the two, a mismatched or old SHBG value throws the whole result off.
Normal free testosterone ranges
Reference ranges vary by lab and method, but calculated free testosterone in healthy adult men sits roughly in the 5–21 ng/dL band (about 50–210 pg/mL), which works out to around 1.5–3% of total. As with total testosterone, the number falls gradually with age, and the threshold that matters is less a hard line than the point where your free T stops matching how you feel.
Free T is only meaningful alongside the numbers it comes from. A low free T with a high SHBG points to a very different fix than a low free T with a low total — always interpret the three together, not the free value alone.
What drives SHBG — and why it changes your free T
SHBG is the dial that quietly sets how much of your testosterone is free. Anything that raises SHBG pulls more testosterone into the tightly-bound state and lowers your free T even if total testosterone has not moved. SHBG rises with ageing, an overactive thyroid, liver disease, higher estrogen, and calorie restriction or very low insulin.
The opposite is just as important. SHBG is pushed down by obesity and insulin resistance, an underactive thyroid, high insulin, growth hormone, and androgens themselves. This is why men on testosterone or anabolic steroids often show a suppressed SHBG and a disproportionately high free T — and why a lean, older man with high SHBG can feel low on a total that looks perfectly normal.
Normal total T with low free T: the most missed picture
The single most common reason to run this calculator is a man whose total testosterone comes back mid-range and “normal,” yet who has every symptom of low testosterone — flat libido, fatigue, low mood, poor recovery. Very often the answer is a high SHBG: it binds so much of the total that the free, usable fraction is genuinely low. The total reassures; the free explains.
“A total testosterone of 550 with an SHBG of 80 can leave less free testosterone than a total of 350 with an SHBG of 20. Treat the free number, and the symptoms — not the total alone.”
MuscleScience editorial standardThe reverse trap exists too: a man with low SHBG (often from carrying extra weight) can show a reassuring free T on a low-ish total. In both directions, the free number is the one that lines up with how you actually feel, which is the whole point of calculating it.
Calculated free T versus the direct “free testosterone” test
Many lab panels offer a direct free testosterone (analog immunoassay) result. It is cheap and convenient — and, at the low concentrations seen in men, widely regarded as unreliable. The Endocrine Society has cautioned against the direct analog method precisely because it tracks poorly against the gold-standard equilibrium dialysis.
“My lab printed a free testosterone number, so that is my free T.” If it was a direct analog assay, treat it with caution. A calculated free T (Vermeulen, from total T + SHBG) or equilibrium dialysis is far more dependable — which is exactly what this calculator gives you.
The practical takeaway: get an accurate total testosterone and an SHBG on the same draw, and calculate. That path is cheaper than dialysis, more reliable than the analog test, and reproducible enough to track over time.
A worked free testosterone example
Take a 45-year-old man with a total testosterone of 550 ng/dL — comfortably mid-range — and an SHBG of 80 nmol/L, which is high. Run through the Vermeulen equation with albumin at the default 43 g/L and his calculated free testosterone lands near the bottom of the range, well below where his symptoms would predict. The total said “fine”; the free number explained the fatigue and flat libido.
Now compare a second man: total 350 ng/dL, SHBG 20 nmol/L. His total is lower, yet with so little SHBG binding it up, his free testosterone comes out higher than the first man’s. Same calculator, opposite conclusion — and a clear illustration of why free T, not total, is the number that maps onto how each of them feels.
When and how often to recheck your free testosterone
Always draw total testosterone and SHBG from the same morning sample — testosterone peaks early and the calculation only holds if both values reflect the same moment. Get a baseline before starting or changing TRT, then recheck around six to eight weeks after any dose change once levels have settled, and periodically thereafter on a stable protocol.
Because free T is a calculated value, consistency in the inputs is everything: the same lab, the same assay method, and a fasting-neutral, consistent time of day. Watch the trend across several calculations rather than reacting to a single figure, and re-run the numbers whenever your SHBG shifts — from weight change, thyroid, or a new medication.
Common mistakes with free testosterone
- Trusting a direct analog free T. At male concentrations it is unreliable — calculate from total T and SHBG instead.
- Using a mismatched SHBG. An old or separate-draw SHBG breaks the calculation; measure both together.
- Reading free T in isolation. It only makes sense next to your total and SHBG, not on its own.
- Ignoring SHBG drivers. Thyroid, insulin resistance, weight and medications move SHBG and, with it, your free T.
- Chasing the total. If symptoms persist on a normal total, the free number is where the answer usually hides.
Free testosterone calculator FAQ
The questions people ask most before trusting a calculated free testosterone.
What is a normal free testosterone level?
In healthy adult men, calculated free testosterone is roughly 5–21 ng/dL (about 1.5–3% of total), though ranges vary by lab and drift down with age. Judge it against your symptoms and your total and SHBG, not a single cut-off.
Is calculated free T better than the direct free T test?
For most men, yes. The direct analog immunoassay is unreliable at male concentrations. A calculated free T from the Vermeulen equation, or equilibrium dialysis, is far more dependable — and calculating is cheap and reproducible.
Why is my total testosterone normal but my free testosterone low?
Usually high SHBG. It binds a large share of your total, leaving little free and active testosterone — which is why you can feel low despite a “normal” total. That gap is exactly what this calculator surfaces.
What albumin value should I enter?
If you do not have it, leave it at the default 43 g/L — it is close enough for almost everyone and barely moves the result. Enter your own only if you have a recent measured value.
Does TRT raise free testosterone more than total?
Often, yes. Androgens tend to suppress SHBG, so free testosterone can rise proportionally more than total on TRT. That is why monitoring the free (and estradiol) matters, not just the total.
Free testosterone — key takeaways
Free testosterone is the small, unbound fraction that actually reaches your cells, and it tracks symptoms far better than the total. Calculate it from total T and SHBG on the same draw using the Vermeulen equation rather than trusting a direct analog test, and always read it beside the two numbers it comes from.
SHBG is the lever: raise it and free T falls even on an unchanged total, which is why a “normal” total can still mean genuinely low free testosterone and real symptoms. Track the trend, not one reading, and pair it with the rest of your hormone panel. Resources below for the full picture.
How to use your free testosterone in practice
Turn the number into a decision with a simple routine:
- Draw total testosterone and SHBG from the same early-morning sample.
- Calculate free (and bioavailable) T here, leaving albumin at 43 g/L unless you have a measured value.
- Read the free number beside your total and SHBG — and beside your symptoms, which outrank any single figure.
- If your total is normal but free is low, look at SHBG and its drivers (thyroid, insulin, weight, medications) before anything else.
- Re-test six to eight weeks after any change and compare the trend, not one reading.
References
Every PMID on this page is verified against the PubMed record (title, author, journal and year) before it is published.
Final educational note
This free testosterone calculator and the content on this page are provided for educational purposes only. Calculated free testosterone is an estimate derived from a mathematical model (the Vermeulen equation) and depends on accurate, same-draw total testosterone and SHBG values, assay methodology and the assumed albumin — it is not a direct measurement and is not a diagnosis.
A single free testosterone result outside any reference range is not a diagnosis of hypogonadism. Interpretation requires repeat testing, the full hormone picture, and evaluation by a qualified physician alongside your symptoms and medical history. Decisions about testosterone therapy should be made with your doctor. Nothing on this page constitutes medical advice. See our full Disclaimer.
MuscleScience.org does not sell supplements, therapy programmes, or medical services. This is an educational publication only.
