Free testosterone calculator
Calculated free and bioavailable testosterone from total testosterone, SHBG and albumin, by the Vermeulen equation — in whatever units your lab printed. Free, no account.
Most of the testosterone in your blood is not available to your tissues. Roughly two-thirds is bound tightly to sex hormone binding globulin, most of the rest is bound loosely to albumin, and a couple of per cent circulates free. Because SHBG varies enormously between people — and moves with thyroid status, insulin, alcohol, oestrogen and age — two men with the same total testosterone can have substantially different free levels. That is the entire reason this calculation exists.
Worked through: 600 ng/dL total, SHBG 30 nmol/L, albumin 4.3 g/dL gives a calculated free testosterone of about 13.4 ng/dL — 134 pg/mL, or 2.2% of total, and a bioavailable testosterone of roughly 325 ng/dL. Hold the total at 600 and raise SHBG to 60 and the free figure falls by close to a third, without the total moving at all.
The equation, and where it comes from
This is the Vermeulen equation, published in 1999 and still the calculation most laboratories and clinical calculators use. It treats binding to SHBG and albumin as two equilibria and solves for the free concentration that satisfies both.
N·Kshbg·FT² + (N + Kshbg·([SHBG] − [T]))·FT − [T] = 0
FT = ( −b + √(b² − 4ac) ) ÷ 2a with a = N·Kshbg, b = N + Kshbg·([SHBG] − [T]), c = −[T]
bioavailable T = FT × N
Kalb = 3.6 × 10⁴ L/mol Kshbg = 1.0 × 10⁹ L/mol
albumin molar mass = 66,500 g/mol
unit factor: nmol/L × 28.84 = ng/dL; ng/dL × 10 = pg/mL
The constants are printed above rather than buried, because they are what a reader would need to check the arithmetic — and because different published implementations use slightly different association constants and albumin molar masses, which is one reason two online calculators can disagree by a few per cent on the same inputs. Source: 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):3666–72 (doi:10.1210/jcem.84.10.6079).
Why the calculated value depends on the SHBG assay
Established clinical use A calculated free testosterone is only as good as the three numbers going into it, and SHBG is the one that carries the most weight. The registry note the app uses for free testosterone puts it plainly: direct immunoassay free T is unreliable; equilibrium dialysis is the reference method; calculated values depend on the SHBG assay.
Three practical consequences. First, a calculated free testosterone from one lab and a calculated one from another are not interchangeable if their SHBG platforms differ — SHBG immunoassays are not harmonised the way many analytes are. Second, a direct immunoassay free testosterone (sometimes just labelled "free testosterone" with no method) is a different measurement again, and generally a worse one; it is not what this page computes. Third, equilibrium dialysis remains the reference method, and where a free level is genuinely decision-relevant — a total in range with symptoms that are not, or an unusually high or low SHBG — that is the test worth asking for rather than a calculation.
The total testosterone assay matters too. The app's registry note for it: LC/MS-MS and immunoassay diverge materially at low concentrations; do not trend across methods without flagging the switch. Feeding an immunoassay total into this equation and comparing the answer with a calculation from an LC/MS-MS total is comparing two different things.
Reading the result
- Generic reference range, free T
- 8–25 pg/mL — male default. The range printed on your own report belongs to the assay that produced your number and wins over this one.
- Optimal band, free T
- 15–25 pg/mL — non-diagnostic. Drawn from clinical literature and community practice for reading a trend over time; being outside it is not a finding.
- Generic reference range, total T
- 300–1000 ng/dL (male default)
- Generic reference range, SHBG
- 10–57 nmol/L
- What this does not do
- Diagnose anything. A calculated free testosterone is one input to a clinical picture that also includes symptoms, an examination, and the rest of the panel. Take the number to the clinician who ordered it.
Questions people actually ask
Which is better, calculated or direct free testosterone?
Calculated, in almost every case where equilibrium dialysis is not available. Direct (analog) immunoassay free testosterone has been criticised for decades for tracking total testosterone and SHBG poorly, and several professional bodies advise against it. Calculated free testosterone from a reliable total and SHBG agrees far better with equilibrium dialysis. Where the stakes are high, ask for the dialysis method.
What albumin should I enter if I do not have one?
4.3 g/dL (43 g/L) is the conventional default and the value most published calculators assume. Albumin varies much less between healthy people than SHBG does, so the error it introduces is small — but if you have a real albumin on the same panel, use it. Genuinely low albumin, as in liver or kidney disease, changes the answer materially and is a reason to use the measured value.
Why is my free testosterone low when my total is normal?
High SHBG is the usual explanation, and it is exactly what this calculation is for. SHBG rises with thyroid hormone excess, oestrogen, some anticonvulsants, liver disease, low insulin and age; it falls with insulin resistance, obesity, hypothyroidism and androgen exposure. A total in the middle of the range with a high SHBG can leave a free level near the bottom of it. What to do about that is a clinical question — the value of the calculation is that it makes the discrepancy visible instead of invisible.
Does this work for women?
The equation is not sex-specific and the arithmetic is the same, but the reference ranges quoted on this page are male defaults and would be badly misleading applied to a female result. The app applies sex-aware ranges with age banding; this page does not, and a female free testosterone should be read against a female interval from the lab that ran it.
Related: testosterone dose calculator, and the lab-marker pages for free versus total testosterone, SHBG and the assay questions behind both.
The app stores total testosterone, SHBG, albumin and the assay method for each together, flags each against your own lab’s printed range, and charts the trend.
Log this panel in the appThis calculator does the arithmetic you typed and nothing else. It does not know what is actually in your vial, whether the label is accurate, or anything about you. Confirm the vial strength and the diluent volume on your own label before you draw, and take dosing decisions to a qualified provider.