Free clinical tool
Bioavailable Testosterone Calculator
Bioavailable testosterone — the free fraction plus the albumin-bound fraction — from total T, SHBG and albumin, by the Vermeulen equation, with the whole partition shown.
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What bioavailable testosterone is, and what the calculation is not
Testosterone in blood sits in three places at once. A small part is unbound and circulating free. A much larger part is held loosely by albumin, which lets go readily enough in the capillary bed that the hormone is still usable. The rest is locked to sex hormone-binding globulin, which binds it tightly and keeps it out of play. Bioavailable testosterone is the first two added together — free plus albumin-bound — and the SHBG-bound remainder is what it deliberately leaves out.
Goldman and colleagues, reviewing the whole field of testosterone binding for Endocrine Reviews in 2017, put it this way: "The term bioavailable testosterone refers to the fraction of circulating testosterone that is not bound to SHBG and largely represents the sum of free testosterone plus HSA-bound testosterone." That "largely" is doing real work, and the next paragraph explains why.
Arithmetically this is not a second equation. It is the free testosterone calculation read at a different point. Vermeulen's model lumps albumin binding into a single term N = 1 + K(alb)·[albumin], solves a quadratic for the free concentration, and bioavailable testosterone is then simply N × free. At an albumin of 4.3 g/dL, N is 24.30 — so bioavailable testosterone is about 24.3 times whatever the free figure is, always. This page and the free testosterone calculator call one function and print two views of the same solve; they cannot give different answers.
What that buys you is a number less hostage to SHBG than total testosterone is. At a total of 500 ng/dL, a man with SHBG of 15 nmol/L has 355 ng/dL bioavailable and a man with SHBG of 70 has 148 ng/dL. Same total, same report heading, and a difference of more than twofold in the fraction that can reach tissue.
What it does not buy you is a measurement. The measured version of this quantity is made by dropping SHBG out of the serum with saturated ammonium sulphate and assaying what stays in the supernatant — which is why Vermeulen labels it non-SHBG-T rather than bioavailable T, a quantity "often referred to as bioavailable T" but not defined as free plus albumin-bound. A calculation is not that assay, and it is not equivalent to it: the precipitation methods "quantitate it as the non−SHBG-bound fraction of circulating testosterone, which approximates but is not equivalent to its original conceptualization as the sum of HSA-bound plus unbound testosterone levels". The arrow does not point cleanly in the other direction either. Goldman and colleagues, having reviewed the direct assays, wrote: "Because of the high levels of imprecision and inaccuracy in bioavailable testosterone assays, we do not favor the direct measurement of bioavailable testosterone measurement in clinical practice." So neither side of this comparison is a gold standard. Equilibrium dialysis is the reference method, and it measures free testosterone, not this.
The formula
N = 1 + K(alb) · [albumin]
(N · K(SHBG)) · FT² + (N + K(SHBG) · ([SHBG] − [T])) · FT − [T] = 0
Bioavailable = free + albumin-bound
= FT + (N − 1) · FT
= N · FT
SHBG-bound = total − bioavailable
Solved for the positive root, with K(SHBG) = 1.0 × 10⁹ L/mol and K(alb) = 3.6 × 10⁴ L/mol as published by Vermeulen. The albumin term N is 24.30 at 4.3 g/dL of albumin, so bioavailable testosterone is always the same multiple of free testosterone for a given albumin — the SHBG value changes how much free there is, not the ratio between the two outputs.
Worked example
Total testosterone 500 ng/dL (17.3 nmol/L), SHBG 30 nmol/L, albumin 4.3 g/dL — the same panel the free testosterone calculator works through, so the two can be checked against each other:
- N = 1 + 3.6×10⁴ × 6.47×10⁻⁴ = 24.30
- Free T = 10.8 ng/dL (2.17% of total) — the identical figure the free testosterone calculator prints for this panel
- Albumin-bound = (N − 1) × free T = 252.7 ng/dL (50.5% of total)
- Bioavailable = free + albumin-bound = N × free T = 263.6 ng/dL (9.14 nmol/L, 52.7% of total)
- Locked to SHBG = total − bioavailable = 236.4 ng/dL (47.3% of total) — the part this number deliberately excludes
- Check: 10.8 + 252.7 + 236.4 reconstructs the 500 ng/dL total exactly before the display rounding, which is the test that this is one equilibrium read three ways rather than three calculations.
Bioavailable testosterone chart — by total T and SHBG
Calculated bioavailable testosterone in ng/dL at albumin 4.3 g/dL, with the percentage of total beneath. Read down for your total testosterone, across for your SHBG.
| Total T | SHBG 15nmol/L | SHBG 25nmol/L | SHBG 35nmol/L | SHBG 50nmol/L | SHBG 70nmol/L |
|---|---|---|---|---|---|
| 200 ng/dL6.9 nmol/L | 13265.8% | 10652.8% | 8843.9% | 7034.8% | 5427.2% |
| 300 ng/dL10.4 nmol/L | 20367.6% | 16454.5% | 13645.3% | 10835.9% | 8428.0% |
| 400 ng/dL13.9 nmol/L | 27769.3% | 22556.2% | 18746.8% | 14837.1% | 11528.8% |
| 500 ng/dL17.3 nmol/L | 35570.9% | 28957.9% | 24148.3% | 19138.2% | 14829.6% |
| 600 ng/dL20.8 nmol/L | 43472.4% | 35759.5% | 29949.8% | 23639.4% | 18330.4% |
| 700 ng/dL24.3 nmol/L | 51673.8% | 42761.0% | 35851.2% | 28440.6% | 21931.3% |
| 800 ng/dL27.7 nmol/L | 60075.0% | 50062.5% | 42152.6% | 33441.8% | 25832.2% |
| 900 ng/dL31.2 nmol/L | 68676.2% | 57563.9% | 48654.0% | 38743.0% | 29833.1% |
| 1000 ng/dL34.7 nmol/L | 77377.3% | 65265.2% | 55455.4% | 44244.2% | 34034.0% |
Every figure is computed by the same function the calculator above calls, and by the same function the free testosterone calculator calls — the two pages are two readings of one solve. NOTHING IN THIS TABLE IS MARKED NORMAL OR LOW, because no harmonized reference range for calculated bioavailable testosterone exists to mark it against. Across this grid the bioavailable fraction runs from 27.2% to 77.3% of total, and the variation is driven almost entirely by SHBG rather than by the total.
Where the testosterone in one sample actually is
The full partition of a total testosterone of 500 ng/dL at albumin 4.3 g/dL, as SHBG rises. The first two columns added together are the third; the fourth is what is left.
| SHBG | Freeunbound | Albumin-boundloosely held | Bioavailablefree + albumin | SHBG-boundnot available |
|---|---|---|---|---|
| 10 nmol/L | 16.33.26% | 38075.9% | 39679.2% | 10420.8% |
| 15 nmol/L | 14.62.92% | 34068.0% | 35570.9% | 14529.1% |
| 20 nmol/L | 13.12.63% | 30661.3% | 31963.9% | 18136.1% |
| 25 nmol/L | 11.92.38% | 27755.5% | 28957.9% | 21142.1% |
| 30 nmol/L | 10.82.17% | 25350.5% | 26452.7% | 23647.3% |
| 40 nmol/L | 9.11.83% | 21342.6% | 22244.5% | 27855.5% |
| 50 nmol/L | 7.91.57% | 18336.6% | 19138.2% | 30961.8% |
| 70 nmol/L | 6.11.22% | 14228.4% | 14829.6% | 35270.4% |
| 100 nmol/L | 4.50.90% | 10521.0% | 11021.9% | 39078.1% |
| 150 nmol/L | 3.10.63% | 7314.6% | 7615.2% | 42484.8% |
All four columns are ng/dL, with the share of the 500 ng/dL total beneath, and the three components sum back to 500 on every row before the display rounding. Two things are worth reading off it. The albumin-bound column is far larger than the free column throughout — that is why bioavailable and free move together but sit an order of magnitude apart. And the SHBG-bound column climbs from 20.8% to 84.8% across these rows on an unchanged total, which is the entire reason a total-only result can mislead.
What the calculation returns across the harmonized total-T range
The published percentiles of TOTAL testosterone, put through the equation at three SHBG values. This is arithmetic on a total-T reference range, not a bioavailable reference range — see the footnote before reading anything into it.
| Percentile of total T | Total Tng/dL | SHBG 20bioavailable, ng/dL | SHBG 30bioavailable, ng/dL | SHBG 50bioavailable, ng/dL |
|---|---|---|---|---|
| 2.5th9.2 nmol/L | 264 | 1585.5 nmol/L | 1294.5 nmol/L | 943.3 nmol/L |
| 5th10.5 nmol/L | 303 | 1836.4 nmol/L | 1505.2 nmol/L | 1093.8 nmol/L |
| 50th (median)18.4 nmol/L | 531 | 34211.9 nmol/L | 2829.8 nmol/L | 2057.1 nmol/L |
| 95th29.5 nmol/L | 852 | 58920.4 nmol/L | 49417.1 nmol/L | 36112.5 nmol/L |
| 97.5th31.8 nmol/L | 916 | 64122.2 nmol/L | 54018.7 nmol/L | 39613.7 nmol/L |
THIS TABLE IS NOT A REFERENCE RANGE AND NO ROW IN IT IS NORMAL. There is no harmonized reference range for calculated bioavailable testosterone, exactly as there is none for calculated free testosterone, and this site says the same thing on /tools/testosterone-unit-converter. What Travison 2017 harmonized was TOTAL testosterone: Travison TG, et al. J Clin Endocrinol Metab. 2017;102(4):1161–1173 (PMID 28324103), healthy nonobese men aged 19–39. The left-hand column is that published range; every other column is this equation applied to it at an SHBG the study never reported, so the spread across the three SHBG columns is a property of the assumption, not of any population. Read your own figure against your own laboratory's stated interval, which will be specific to its assay.
What the albumin molar mass does to the answer
Two defensible numbers exist for the molar mass of human serum albumin, and every bioavailable calculator silently picks one. Both are solved here at a total of 500 ng/dL and albumin 4.3 g/dL.
| SHBG | MW 66,430this page, N = 24.30 | MW 69,000Vermeulen's own, N = 23.43 | Differenceng/dL and % |
|---|---|---|---|
| 15 nmol/L | 354.6ng/dL | 351.9ng/dL | 2.80.78% |
| 25 nmol/L | 289.3ng/dL | 285.7ng/dL | 3.61.24% |
| 35 nmol/L | 241.4ng/dL | 237.5ng/dL | 3.91.61% |
| 50 nmol/L | 191.1ng/dL | 187.2ng/dL | 3.92.03% |
| 70 nmol/L | 148.0ng/dL | 144.5ng/dL | 3.52.39% |
| 100 nmol/L | 109.7ng/dL | 106.7ng/dL | 3.02.72% |
The worked derivation published by Vermeulen's own laboratory divides the albumin concentration by a figure it annotates "69000 =(molecular weight alb.)" and reaches "[S] + [SA] = (1 + 22.43)[S] = 23.43 [S]", having defined "[Bio T] = [S] + [SA]". 69,000 is the older value for albumin; 66,430 is the modern one, and it is what this site uses everywhere. Fed their own published example — SHBG 40 nmol/L, testosterone 288.4 ng/dL — under their molar mass, this calculator returns free 5.01 ng/dL and bioavailable 118 ng/dL against the "FT = 5.02 ng/dl" and "Bio T = 118 ng/dL" they print, which is how you can tell this is their equation rather than a lookalike. Under the modern molar mass the same panel gives 120 ng/dL. The gap runs 0.78% at SHBG 15 to 2.72% at SHBG 100 and never exceeds about 3% anywhere — smaller than it looks, because a larger albumin term also leaves less free hormone and the two moves partly cancel. It is also far smaller than the disagreement between two laboratories measuring the same total testosterone.
Frequently asked questions
- What is bioavailable testosterone?
- It is the testosterone in your blood that is not locked to SHBG — the free, unbound fraction plus the fraction loosely held by albumin, which releases it readily enough to be usable. Goldman and colleagues define it as "the fraction of circulating testosterone that is not bound to SHBG and largely represents the sum of free testosterone plus HSA-bound testosterone". On a total testosterone of 500 ng/dL with SHBG 30 nmol/L and albumin 4.3 g/dL the calculation returns 264 ng/dL bioavailable — 10.8 ng/dL free plus 253 ng/dL on albumin — leaving 236 ng/dL tied up on SHBG.
- How is bioavailable testosterone calculated?
- By the Vermeulen equation, from total testosterone, SHBG and albumin. The albumin binding is collapsed into a single term N = 1 + K(alb) × [albumin], a quadratic is solved for the free concentration, and bioavailable testosterone is N × free. At an albumin of 4.3 g/dL, N is 24.30, so bioavailable testosterone comes out at roughly 24.3 times the free figure. That is why this calculator and the free testosterone calculator on this site always agree: they are the same solve, printed twice.
- What is a normal bioavailable testosterone level?
- This page will not tell you, because nobody has published a figure it could honestly quote. There is no harmonized reference range for calculated bioavailable testosterone, in the way that Travison 2017 provides one for total testosterone (264–916 ng/dL in healthy nonobese men aged 19–39). The same is true of calculated free testosterone, and this site says so on its unit converter and its reference-range-by-age pages too. What exists instead is a range printed by whichever laboratory ran your panel, specific to its assays and its population. Read your result against that one, with the clinician who ordered it. Any site quoting a universal "normal bioavailable testosterone" range is quoting something it cannot source.
- What is the difference between free and bioavailable testosterone?
- Free testosterone is the unbound fraction alone. Bioavailable is that plus the albumin-bound fraction. They are not independent numbers — bioavailable is exactly N times free, where N is 24.30 at a typical albumin of 4.3 g/dL — so one never tells you anything the other does not. On the 500 ng/dL panel above, free is 10.8 ng/dL (2.17% of total) and bioavailable is 264 ng/dL (52.7%). Which one your laboratory reports is a matter of local convention.
- Is a calculated bioavailable testosterone the same as a measured one?
- No, and neither one is a gold standard. The measured version drops SHBG out of the serum with saturated ammonium sulphate and assays what remains in the supernatant; Vermeulen describes it as "the fraction of serum T not precipitated by 50% ammonium sulfate concentration", labels it non-SHBG-T, and notes that it is "often referred to as bioavailable T". He found the calculated figure "correlated highly significantly with and was almost identical to the values of non-SHBG-T obtained by ammonium sulfate precipitation", which is the validation this calculation rests on. But the precipitation assay is itself imperfect: it measures non-SHBG-bound testosterone, which "approximates but is not equivalent to" free plus albumin-bound, and Goldman and colleagues concluded that "because of the high levels of imprecision and inaccuracy in bioavailable testosterone assays, we do not favor the direct measurement of bioavailable testosterone measurement in clinical practice". The reference method in this field is equilibrium dialysis, and it measures free testosterone, not this.
- Why does bioavailable testosterone rise with albumin when free testosterone falls?
- Because they are counting different things and both are behaving correctly. More albumin means more binding sites, so more hormone is pulled out of the free pool — free testosterone falls. But albumin-bound testosterone is counted as bioavailable, and it rises by more than the free fraction loses. On the 500 ng/dL panel, dropping albumin from 4.3 to 3.0 g/dL raises free testosterone from 10.8 to 13.2 ng/dL and lowers bioavailable from 264 to 228 ng/dL. If you have liver disease, nephrotic syndrome or significant malnutrition, enter your measured albumin rather than the default.
- Why does another bioavailable testosterone calculator give a different answer?
- Most often the albumin molar mass. Vermeulen's own laboratory used 69,000 g/mol, giving an albumin term of 22.43 and N of 23.43; the modern value for human serum albumin is 66,430, giving 23.30 and N of 24.30. Calculators pick one without saying which. The effect on bioavailable testosterone is real but modest — between about 0.8% and 2.7% over ordinary SHBG values, and never more than about 3% anywhere in the plausible range. The other common cause is a different binding model entirely: Södergard used association constants of 4.06 × 10⁴ and 5.97 × 10⁸ L/mol where Vermeulen used 3.6 × 10⁴ and 1 × 10⁹. This page prints both albumin molar masses in the chart above rather than choosing silently.
- How do I convert bioavailable testosterone from ng/dL to nmol/L?
- Multiply by 0.0347, the same factor used for total testosterone, because it is the same molecule — the factor comes from testosterone's molar mass of 288.42 g/mol. A bioavailable testosterone of 264 ng/dL is 9.1 nmol/L. Going the other way, multiply nmol/L by 28.84. The calculator above accepts total testosterone in either unit and reports bioavailable in both.
- Should I ask for a bioavailable testosterone test?
- The Endocrine Society guideline does not route you there. It recommends measuring fasting morning total testosterone with an accurate assay as the initial test, confirming it by repeating it, and then — in men whose total is near the lower limit of normal or who have a condition that alters SHBG — "obtaining a free T concentration using either equilibrium dialysis or estimating it using an accurate formula". Bioavailable testosterone is not what that recommendation asks for, and since it is a fixed multiple of calculated free testosterone it adds no information to it. The practical value of this page is the same as the free calculation: it tells you whether a normal-looking total is being held hostage by SHBG.
Sources
- [1]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) — the source of the equation, the association constants and the ammonium sulfate comparison
- [2]Free & Bioavailable Testosterone calculator, explanation and worked example — Hormonology department, University Hospital Ghent (Fiers T, Kaufman JM), the laboratory of the 1999 paper. Source of "[Bio T] = [S] + [SA]" and the 69,000 g/mol albumin molar mass
- [3]Goldman AL, Bhasin S, Wu FCW, Krishna M, Matsumoto AM, Jasuja R. A Reappraisal of Testosterone’s Binding in Circulation: Physiological and Clinical Implications. Endocr Rev. 2017;38(4):302–324 (PMID 28673039) — the definition of bioavailable testosterone and the critique of the direct assays
- [4]Travison TG, Vesper HW, Orwoll E, et al. Harmonized Reference Ranges for Circulating Testosterone Levels in Men of Four Cohort Studies in the United States and Europe. J Clin Endocrinol Metab. 2017;102(4):1161–1173 (PMID 28324103) — the total testosterone range, which is the only harmonized range this page uses
- [5]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) — what the guideline actually recommends measuring
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Educational reference, not medical advice. A calculated estimate from three lab values is not a diagnosis, and calculated bioavailable testosterone is not interchangeable with an ammonium sulphate precipitation assay. No harmonized reference range exists for it, so nothing on this page marks a result normal or low. Interpretation belongs to your clinician.