How to Calculate Free Testosterone: The Vermeulen Equation, a Worked Example and What the Result Means

By Ares Men's Health · Updated October 10, 2026

Short answer: Free testosterone can be estimated from three blood results: total testosterone, SHBG and albumin. The Vermeulen equation converts them to mol/L, accounts for testosterone bound to albumin, and solves a quadratic equation for the unbound fraction. For 500 ng/dL total testosterone, SHBG of 30 nmol/L and albumin of 4.3 g/dL, free testosterone comes out near 0.38 nmol/L, about 2.2% of the total.

Key takeaways

• Free testosterone is the small unbound fraction of testosterone, usually around 1–3% of the total, that tissues can use directly.

• You need three lab values: total testosterone, SHBG and albumin, in matching units.

• The Vermeulen equation (1999) assumes testosterone binds SHBG tightly and albumin loosely, and solves for the free amount.

• Higher SHBG lowers free testosterone for the same total; lower SHBG raises it.

• A calculated value is an estimate, laboratory reference ranges differ, and no number should be used to diagnose or treat yourself.

What is free testosterone?

Most testosterone in your blood is attached to proteins. Roughly half or more is tightly bound to sex hormone-binding globulin (SHBG), a large share is loosely bound to albumin, and only a small slice floats free. That free slice, typically a percent or two of the total, is the part tissues can take up most readily.

Total testosterone counts everything: bound and free together. Bioavailable testosterone counts the free fraction plus the albumin-bound fraction, because albumin releases testosterone easily. Free testosterone counts only the unbound portion.

Two men can have identical total testosterone and very different free testosterone if their SHBG differs. That is the whole reason a calculation is useful.

Why calculate it instead of measuring it?

The reference method for measuring free testosterone is equilibrium dialysis, a specialised laboratory technique that is slower, more expensive and not available everywhere. Many standard assays that report "free testosterone" directly have been criticised for being unreliable, which is why guidelines favour either dialysis or a calculation from total testosterone, SHBG and albumin.

A calculation costs nothing once you have the lab values, and it correlates well with dialysis in typical adult men. It is not perfect, but it is far more informative than total testosterone alone when SHBG is unusually high or low.

What you need before you start

Gather three results from the same blood draw, ideally the same laboratory report:

• Total testosterone — reported in ng/dL, ng/mL or nmol/L

• SHBG — normally reported in nmol/L

• Albumin — reported in g/dL or g/L (4.3 g/dL equals 43 g/L)

Units are where most mistakes happen. To convert total testosterone to nmol/L, divide ng/dL by 28.84, or equivalently multiply ng/mL by 3.468. A total testosterone of 500 ng/dL is therefore 17.34 nmol/L. Make sure all three values use consistent units before calculating.

The Vermeulen equation in plain English

In 1999 Vermeulen, Verdonck and Kaufman published a method for estimating free testosterone from total testosterone, SHBG and albumin. The idea is straightforward even if the algebra looks intimidating.

1. Testosterone sits in three places: free, bound to SHBG, and bound to albumin.

2. SHBG grabs testosterone very tightly (association constant about 1 × 10⁹ L/mol). Albumin holds it loosely (about 3.6 × 10⁴ L/mol).

3. Because albumin is so abundant, its share depends on the albumin concentration. Define N = 1 + (3.6 × 10⁴ × albumin in mol/L).

4. Total testosterone must equal free plus albumin-bound plus SHBG-bound. Writing that out gives a quadratic equation in free testosterone.

5. Solve the quadratic and take the positive root. The result is free testosterone in mol/L, which you can convert to nmol/L, ng/dL or pg/mL.

In symbols, with all concentrations in mol/L, Ks for SHBG and Ka for albumin: N·Ks·FT² + (N + Ks(SHBG − TT))·FT − TT = 0. Once you have FT, bioavailable testosterone is simply FT × N.

A worked example, step by step

Take a man whose lab report shows total testosterone 500 ng/dL, SHBG 30 nmol/L and albumin 4.3 g/dL.

1. Convert total testosterone: 500 ÷ 28.84 = 17.34 nmol/L, which is 1.734 × 10⁻⁸ mol/L.

2. Convert SHBG: 30 nmol/L = 3.0 × 10⁻⁸ mol/L.

3. Convert albumin: 4.3 g/dL = 43 g/L. With albumin’s molecular weight of about 66,400 g/mol, that is roughly 6.5 × 10⁻⁴ mol/L.

4. Work out N: 1 + (3.6 × 10⁴ × 6.5 × 10⁻⁴) ≈ 24.3.

5. Solve the quadratic and take the positive root. Free testosterone ≈ 0.376 nmol/L.

6. Convert back: 0.376 nmol/L ≈ 10.8 ng/dL, or about 108 pg/mL. That is about 2.2% of the total.

7. Bioavailable testosterone: 0.376 × 24.3 ≈ 9.1 nmol/L, about 264 ng/dL, or roughly 53% of the total.

You do not need to do this by hand. Our free testosterone calculator performs the same Vermeulen calculation from your three values and returns the same answers shown here.

How SHBG changes the answer

Keeping albumin at 4.3 g/dL, here is how the same man looks with different SHBG or total testosterone values:

Total T	SHBG	Free T (approx.)	% free	Bioavailable T (approx.)	% bioavailable
500 ng/dL	30 nmol/L	10.8 ng/dL	2.2%	264 ng/dL	53%
500 ng/dL	15 nmol/L	14.6 ng/dL	2.9%	355 ng/dL	71%
350 ng/dL	60 nmol/L	4.6 ng/dL	1.3%	112 ng/dL	32%

Same total, different SHBG: 500 ng/dL with SHBG of 15 gives a much higher free fraction than the same total with SHBG of 30. And a lower total with high SHBG, like the last row, can leave surprisingly little testosterone available. This is why providers often order SHBG when total testosterone and symptoms do not seem to match.

What affects SHBG?

SHBG is not fixed. It tends to rise with age and with thyroid overactivity, liver conditions, certain medications and low body weight. It tends to fall with obesity, insulin resistance and type 2 diabetes, and some medications.

That matters because it means free testosterone can sit low when total testosterone looks acceptable, and the reverse. A provider will read your SHBG together with your history rather than treating it as a number in isolation.

Unit conversion cheat sheet

Most calculation errors come from mismatched units. These conversions cover nearly every lab report you are likely to see:

Measurement	From	To	How to convert
Total testosterone	ng/dL	nmol/L	Divide by 28.84
Total testosterone	ng/mL	nmol/L	Multiply by 3.468
Total testosterone	nmol/L	ng/dL	Multiply by 28.84
Free testosterone	nmol/L	pg/mL	Multiply by 288.4
Free testosterone	pg/mL	ng/dL	Divide by 10
Albumin	g/dL	g/L	Multiply by 10

If a result looks wildly out of line, say a free testosterone of several hundred ng/dL, check the units before worrying about the number.

Common mistakes when calculating free testosterone

• Mixing units, such as entering total testosterone in ng/dL where nmol/L is expected.

• Using values from different dates, even a few weeks apart, because SHBG and testosterone both shift over time.

• Typing albumin in the wrong scale, for example 43 instead of 4.3 when the tool expects g/dL.

• Leaving SHBG blank or guessing it, which makes the result meaningless.

• Comparing against the wrong range, since reference intervals for calculated free testosterone vary by method and laboratory.

• Treating one number as the verdict. Symptoms, repeat testing and overall health all count.

How free testosterone relates to symptoms

Researchers continue to debate how closely any single testosterone measure tracks how men feel. Some men with low-looking numbers feel fine, and some men with acceptable numbers feel poor, which is part of why guidelines ask for both symptoms and repeated, confirmed low results before treatment is considered.

Free testosterone is especially helpful in borderline cases, in men with unusual SHBG, and when total testosterone sits near a laboratory’s cut-off. It adds context; it does not replace judgement.

Limits and cautions

A calculated free testosterone is an estimate with built-in assumptions. Keep these points in mind:

• It assumes standard binding constants and a single binding site on SHBG, which is a simplification.

• Results depend on the accuracy of the underlying total testosterone and SHBG assays, which vary between laboratories.

• It was built for adult men and is less reliable in unusual situations such as severe illness, certain medications, or abnormal protein levels.

• Reference ranges for free and bioavailable testosterone differ between laboratories and populations, so a number that looks low on one report may be acceptable on another.

• A single result is one data point. Testosterone varies through the day, and a low result is usually confirmed with a repeat early-morning sample.

Most important: a calculator can inform a conversation, but it cannot diagnose low testosterone, and no result should be used to start, change or stop any treatment on your own.

What to do with your result

If your calculated free testosterone seems low, or if your symptoms do not match your numbers either way, the next step is a discussion with a provider rather than a search for the perfect threshold.

Bring your lab report, your symptom history and a list of medications. If you have not had bloodwork yet, our guide to the testosterone blood test explains what is measured and how to prepare. If you are new to the symptoms side, start with the free Low-T screening.

At Ares Men's Health, Brian Ruiz, APRN, FNP-BC reviews your results by secure video, explains what they mean, and recommends treatment only when it is clinically appropriate. You can see how the whole process works in our guide to telehealth TRT in Florida.

Frequently asked questions

What is the formula to calculate free testosterone?

The Vermeulen equation solves a quadratic: N·Ks·FT² + (N + Ks(SHBG − TT))·FT − TT = 0, where N = 1 + Ka × albumin, Ks is the SHBG binding constant and Ka the albumin constant. All values must be in mol/L.

Which lab values do I need to calculate free testosterone?

You need total testosterone, SHBG and albumin from the same blood draw, with their units. Without SHBG and albumin a reliable estimate is not possible.

How do I convert testosterone from ng/dL to nmol/L?

Divide the ng/dL value by 28.84. For example, 500 ng/dL equals about 17.3 nmol/L. To convert ng/mL to nmol/L, multiply by 3.468.

What percentage of total testosterone is free?

Usually a small fraction, often around 1% to 3%, though it varies with SHBG and albumin. Our worked example gave 2.2%.

Is bioavailable testosterone the same thing as free testosterone?

Free testosterone is only the unbound portion. Bioavailable testosterone adds the albumin-bound portion, because albumin releases testosterone easily.

Is calculated free testosterone as accurate as a lab measurement?

It correlates well with equilibrium dialysis in typical adult men, and guidelines accept it. It remains an estimate that depends on the quality of the underlying assays.

Why is my free testosterone low when my total testosterone looks fine?

A high SHBG binds more testosterone, leaving less free. Age, thyroid and liver conditions and some medications can raise SHBG. Your provider can interpret this with your history.

Can a calculator diagnose low testosterone?

No. Calculators are educational. Diagnosis requires a provider to read your symptoms, repeated early-morning lab results and your overall health.

Do reference ranges for free testosterone differ between labs?

Yes. Methods and populations differ, so each laboratory publishes its own ranges. Use the range printed on your report and let your provider interpret it.

Where can I use a free testosterone calculator online?

Ares Men's Health offers a free calculator that applies the Vermeulen equation to your total testosterone, SHBG and albumin and also shows bioavailable testosterone.

Talk to a provider

If you have lab results and want a clinician to explain them, or you have symptoms but no bloodwork yet, Ares Men's Health offers provider-led, labs-first care by secure telehealth anywhere in Florida.

Take the free 60-second Low-T screening · Book a consultation · Call 754-335-8868 (Mon–Fri 9am–5pm).

Sources and further reading

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 (PubMed)

2. Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018;103(5):1715–1744

3. Mulhall JP, et al. Evaluation and Management of Testosterone Deficiency: AUA Guideline. J Urol. 2018;200(2):423–432 (PubMed)

4. Testosterone Deficiency — Establishing a Biochemical Diagnosis (PMC)

This article is educational and is not medical advice. Calculated values are estimates and reference ranges vary by laboratory. Individual results vary.