Testosterone is an important hormone involved in reproductive function, muscle and bone health, energy, sexual function, and many other physiological processes. However, looking at total testosterone alone does not always provide the complete picture of how much testosterone is available in the bloodstream.

A significant portion of circulating testosterone is attached to proteins, particularly sex hormone-binding globulin (SHBG) and albumin. Only a relatively small fraction circulates unbound, commonly referred to as free testosterone. Testosterone that is free or loosely bound to albumin is often grouped together as bioavailable testosterone. NCBI

The Bioavailable Testosterone Calculator estimates these fractions using three laboratory values:

• Total testosterone
• SHBG
• Albumin

The calculator uses a Vermeulen mass-action model to estimate free testosterone and then calculates albumin-bound and bioavailable testosterone. It also provides results in both nmol/L and ng/dL and calculates the percentage of total testosterone that is considered bioavailable.

This article explains how the calculator works, what each input means, how the formula is derived, how to interpret the results, and why SHBG and albumin can make testosterone interpretation more complicated.

Important: This calculator provides an estimate and is not a diagnostic test. Testosterone results should be interpreted using the laboratory's reference ranges and, when clinically relevant, by a qualified healthcare professional. Different laboratory methods and calculation equations can produce different results. NCBI

What Is Bioavailable Testosterone?

Bioavailable testosterone generally refers to testosterone that is not tightly bound to SHBG. It commonly includes:

1. Free testosterone
2. Albumin-bound testosterone

Testosterone bound strongly to SHBG is generally treated separately from this fraction when calculating bioavailable testosterone.

According to Endotext, free testosterone represents a small fraction of circulating testosterone, while a larger portion is loosely bound to albumin; together, free and non-SHBG-bound testosterone are often described as bioavailable testosterone. NCBI

The basic relationship is:

Bioavailable Testosterone = Free Testosterone + Albumin-Bound Testosterone

This is the central relationship used by the calculator.

What Is Free Testosterone?

Free testosterone is testosterone that is not attached to circulating proteins.

Only a relatively small percentage of testosterone in the blood is free. The exact percentage varies between individuals and depends partly on SHBG and albumin concentrations. Endotext describes free testosterone as roughly 1–4% of total testosterone, although the exact proportion varies with physiological and laboratory conditions. NCBI

Because free testosterone is only a small fraction of total testosterone, even relatively small changes in protein binding can affect its calculated concentration.

This is why the calculator does not simply multiply total testosterone by a fixed percentage. Instead, it considers:

• Total testosterone
• SHBG
• Albumin
• Binding constants between testosterone and these proteins

What Is SHBG?

SHBG stands for sex hormone-binding globulin.

It is a protein produced primarily by the liver that binds testosterone and other sex steroids. Testosterone has a relatively high affinity for SHBG, meaning SHBG can substantially influence how much testosterone remains free or loosely bound. NCBI

When SHBG changes, total testosterone and free testosterone may not change in exactly the same way.

For example, two people could have similar total testosterone concentrations but different SHBG levels and therefore different calculated free testosterone concentrations.

This is one reason SHBG is included as an input in the Bioavailable Testosterone Calculator.

What Is Albumin?

Albumin is the most abundant protein in human blood plasma.

Unlike SHBG, testosterone binds to albumin relatively weakly. This weaker binding means albumin-bound testosterone is commonly included with free testosterone when estimating bioavailable testosterone. NCBI

The calculator asks for albumin in g/dL.

A default value of 4.3 g/dL is provided, but using an actual laboratory albumin result can be preferable when one is available, particularly when albumin is substantially different from typical concentrations.

Research examining calculated free testosterone has found that a fixed albumin value can be adequate for many calculations, but differences become more important in certain combinations of albumin and SHBG concentrations. PubMed

What Does the Bioavailable Testosterone Calculator Calculate?

The calculator provides six main results:

1. Free Testosterone in nmol/L

This is the estimated concentration of testosterone that is not protein-bound.

2. Free Testosterone in ng/dL

The same estimated free testosterone value converted into another commonly used unit.

3. Albumin-Bound Testosterone

This represents the estimated amount of testosterone bound to albumin.

4. Bioavailable Testosterone in nmol/L

This is calculated as:

Free Testosterone + Albumin-Bound Testosterone

5. Bioavailable Testosterone in ng/dL

The bioavailable testosterone result converted into ng/dL.

6. Bioavailable Percentage

This represents the calculated bioavailable testosterone as a percentage of total testosterone.

The calculator also displays the calculation method used.

How to Use the Bioavailable Testosterone Calculator

Using the calculator requires three laboratory values.

Step 1: Enter Total Testosterone

Enter your total testosterone result in:

nmol/L

For example:

18 nmol/L

Total testosterone is the amount of testosterone measured in the blood, including free and protein-bound testosterone.

When using laboratory results, use the value from the relevant blood test and make sure you are entering the correct units.

Step 2: Enter SHBG

Enter your SHBG concentration in:

nmol/L

For example:

35 nmol/L

SHBG is essential for estimating how testosterone is distributed between free, albumin-bound, and SHBG-bound forms.

Step 3: Enter Albumin

Enter your albumin concentration in:

g/dL

For example:

4.3 g/dL

The calculator provides 4.3 g/dL as the default value.

If your laboratory report provides albumin in another unit, make sure it is converted appropriately before entering it.

Step 4: Click Calculate

After entering all three values, click Calculate.

The calculator will estimate:

• Free testosterone
• Albumin-bound testosterone
• Bioavailable testosterone
• Bioavailable percentage

It also provides free and bioavailable testosterone in both nmol/L and ng/dL.

Bioavailable Testosterone Formula Explained

The calculator uses a form of the Vermeulen mass-action model.

The model treats testosterone as binding to SHBG and albumin according to binding equilibria.

The calculation involves several steps.

Step 1: Convert Albumin to the Required Concentration

The calculator accepts albumin in g/dL.

It first converts the value to grams per liter:

Albumin (g/L) = Albumin (g/dL) × 10

For example:

4.3 g/dL × 10 = 43 g/L

The calculator then converts albumin to mol/L using an approximate molecular weight of 66,500 g/mol:

Albumin (mol/L) = Albumin (g/L) ÷ 66,500

For 43 g/L:

43 ÷ 66,500 ≈ 0.0006466 mol/L

Step 2: Convert Testosterone to mol/L

The calculator accepts total testosterone in nmol/L.

Because the mass-action equation requires molar concentration, total testosterone is converted:

Total Testosterone (mol/L) = Total Testosterone (nmol/L) × 10⁻⁹

For example, if total testosterone is 18 nmol/L:

18 × 10⁻⁹ = 1.8 × 10⁻⁸ mol/L

Step 3: Convert SHBG to mol/L

SHBG is also entered in nmol/L.

The calculator converts it using:

SHBG (mol/L) = SHBG (nmol/L) × 10⁻⁹

For example, 35 nmol/L becomes:

35 × 10⁻⁹ mol/L

Step 4: Apply Testosterone Binding Constants

The calculator uses two association constants:

• Ka = 3.6 × 10⁴ L/mol for testosterone binding to albumin
• Ksh = 1.0 × 10⁹ L/mol for testosterone binding to SHBG

These constants represent the assumed strength of the interaction between testosterone and the corresponding binding proteins.

The literature contains several published equations and binding constants for calculating free and bioavailable testosterone. Different algorithms can produce meaningfully different estimates, which is an important limitation when interpreting calculated results. PubMed

Step 5: Calculate Free Testosterone

The calculator uses the quadratic mass-action relationship.

It defines:

A = Ka × Albumin

and:

B = 1 + A + Ksh × SHBG

The free testosterone concentration is then estimated using:

Free T = [−B + √(B² + 4 × A × Total T)] ÷ (2 × A)

The concentrations in this equation are expressed in compatible molar units.

The resulting free testosterone concentration is then converted back to nmol/L.

This approach allows the calculator to account for the interaction between testosterone, SHBG, and albumin rather than treating each binding process independently.

Step 6: Calculate Albumin-Bound Testosterone

Once free testosterone has been calculated, the calculator estimates albumin-bound testosterone using:

Albumin-Bound T = Free T × Ka × Albumin

This estimates the testosterone associated with albumin under the assumptions of the model.

Step 7: Calculate Bioavailable Testosterone

The calculator then adds free testosterone and albumin-bound testosterone:

Bioavailable T = Free T + Albumin-Bound T

This is the key formula behind the bioavailable testosterone result.

Step 8: Calculate Bioavailable Percentage

The calculator also determines how much of total testosterone is bioavailable:

Bioavailable % = (Bioavailable T ÷ Total T) × 100

For example, if total testosterone is 20 nmol/L and bioavailable testosterone is 6 nmol/L:

(6 ÷ 20) × 100 = 30%

Therefore, the calculated bioavailable testosterone would represent 30% of total testosterone.

Unit Conversion: nmol/L to ng/dL

The calculator uses:

1 nmol/L testosterone ≈ 28.84 ng/dL

Therefore:

ng/dL = nmol/L × 28.84

For example, if free testosterone is 0.50 nmol/L:

0.50 × 28.84 = 14.42 ng/dL

The reverse conversion is approximately:

nmol/L = ng/dL ÷ 28.84

Unit conversion is particularly important because laboratory reports can use different units depending on the country and testing laboratory.

Worked Bioavailable Testosterone Example

Consider the following hypothetical laboratory results:

The calculator uses these three values to estimate the free testosterone concentration.

Albumin is first converted:

4.3 g/dL × 10 = 43 g/L

Then:

43 ÷ 66,500 ≈ 0.0006466 mol/L

Total testosterone is converted:

18 nmol/L = 18 × 10⁻⁹ mol/L

SHBG is converted:

35 nmol/L = 35 × 10⁻⁹ mol/L

The calculator then applies the Vermeulen-style mass-action calculation using the specified binding constants.

After calculating free testosterone, it determines albumin-bound testosterone and adds the two fractions:

Bioavailable Testosterone = Free T + Albumin-Bound T

Finally, the result is converted to ng/dL and expressed as a percentage of total testosterone.

The important point is that the calculator is performing a binding model calculation, not simply assuming that a fixed percentage of total testosterone is bioavailable.

Example of How SHBG Can Change the Result

Imagine two hypothetical people have the same total testosterone:

Total testosterone = 18 nmol/L

But suppose their SHBG concentrations are different.

Although total testosterone is identical, the estimated free testosterone can differ because SHBG affects testosterone binding.

Generally, higher SHBG tends to mean a greater proportion of testosterone is tightly protein-bound, while lower SHBG can alter the relationship between total and free testosterone. The exact calculated result depends on the mathematical model and the other inputs.

This illustrates why total testosterone should not always be interpreted without considering SHBG, particularly when total testosterone is borderline or SHBG is altered. NCBI

Why Total Testosterone May Not Tell the Whole Story

Total testosterone is an important laboratory measurement, but it includes several fractions.

Broadly, testosterone circulates as:

• Free testosterone
• Albumin-bound testosterone
• SHBG-bound testosterone
• Other protein-associated fractions

SHBG has high-affinity binding for testosterone, while albumin binds testosterone more weakly. NCBI

Consequently, two people with similar total testosterone can potentially have different calculated free or bioavailable testosterone concentrations.

This is especially relevant when SHBG concentrations are significantly altered or when total testosterone is near a clinical decision threshold.

The Endocrine Society recommends diagnosing hypogonadism in men based on symptoms and signs together with consistently low testosterone concentrations, and its guidance includes free testosterone when indicated. Endocrine

When Calculated Free Testosterone May Be Helpful

Calculated free testosterone can be particularly informative when total testosterone does not clearly match the clinical situation.

For example, a clinician may consider additional assessment when:

• Total testosterone is borderline
• SHBG is unusually high or low
• Symptoms and total testosterone do not appear to agree
• Conditions are present that can alter SHBG
• Additional information is needed during endocrine evaluation

Endotext notes that free or non-SHBG testosterone assessment may be useful when total testosterone is borderline or when conditions that alter SHBG are present. NCBI

However, a calculated value should not be treated as an independent diagnosis.

Factors That Can Affect SHBG

SHBG is not a fixed number.

It can vary with physiological and medical factors, and this can affect the relationship between total and free testosterone.

Some factors associated with changes in SHBG include:

• Age
• Body composition
• Liver-related conditions
• Thyroid status
• Certain medications
• Metabolic conditions
• Hormonal states

Because SHBG can change, a total testosterone value should sometimes be interpreted together with SHBG rather than in isolation.

The exact significance of an abnormal SHBG result depends on the individual's circumstances and laboratory findings.

Why Albumin Matters in the Calculation

Albumin may have a smaller effect on calculated testosterone than SHBG in many circumstances, but it is still part of the binding model.

The calculator uses a default albumin concentration of:

4.3 g/dL

That value is commonly used as a representative albumin concentration in some calculated testosterone approaches.

However, when a person's actual albumin concentration differs substantially from this value, using the measured albumin may provide a more individualized calculation.

Research has found that using fixed albumin can be acceptable for many calculations, although discrepancies become more relevant in some people with lower albumin and lower SHBG. PubMed

Calculated vs. Measured Free Testosterone

There are different ways to determine free testosterone.

One important laboratory technique is equilibrium dialysis, which Endotext describes as the current benchmark reference method for free testosterone measurement. NCBI

Calculated free testosterone uses mathematical equations based on total testosterone, SHBG, albumin, and assumed binding constants.

This makes calculation much more accessible, but it also introduces assumptions.

Different equations can produce different results. Research comparing published algorithms has found substantial differences between calculated free and bioavailable testosterone estimates. PubMed

Therefore, a calculated result should always be interpreted according to the method used and the laboratory or clinical context.

Why Different Calculators Can Give Different Results

If you enter the same testosterone, SHBG, and albumin values into two different online calculators and receive different answers, that does not necessarily mean one calculator is malfunctioning.

Different tools may use:

• Different binding constants
• Different mathematical equations
• Different assumptions about protein binding
• Different unit conversions
• Different definitions of bioavailable testosterone
• Different numerical methods

Studies have demonstrated substantial differences among published testosterone calculation algorithms. PubMed

For this reason, calculated testosterone results should be interpreted with awareness of the underlying calculation method.

Bioavailable Testosterone vs. Free Testosterone

These two terms are related but not identical.

Free Testosterone

Free testosterone is the portion that is not bound to proteins.

Bioavailable Testosterone

Bioavailable testosterone is generally calculated as:

Free Testosterone + Albumin-Bound Testosterone

The distinction matters because albumin-bound testosterone is weakly bound and is often included in the bioavailable fraction.

A person's free testosterone may therefore be relatively small while the calculated bioavailable fraction is considerably larger.

Bioavailable Testosterone Percentage

The calculator reports a bioavailable percentage.

This is calculated as:

Bioavailable Percentage = Bioavailable Testosterone ÷ Total Testosterone × 100

This number provides a convenient way to understand the calculated bioavailable fraction relative to total testosterone.

For example:

These are simplified examples demonstrating the percentage calculation rather than clinical reference ranges.

Practical Tips for Using the Calculator

Use the Correct Laboratory Units

Make sure total testosterone and SHBG are entered in nmol/L and albumin in g/dL.

Entering a value in the wrong unit can produce a dramatically incorrect result.

Use the Same Blood Test Where Possible

Total testosterone, SHBG, and albumin should ideally correspond to the same blood sample or relevant testing period.

Use Measured Albumin When Appropriate

The calculator provides a default albumin value of 4.3 g/dL. If you have a laboratory-measured albumin value, you can enter that instead.

Do Not Change Units Without Converting Them

For example, albumin reported in g/L cannot be entered directly into a field expecting g/dL.

The conversion is:

g/dL = g/L ÷ 10

Check the Laboratory Reference Range

A numerical result by itself does not establish whether testosterone status is normal, low, or high.

Reference ranges vary between laboratories, testing methods, populations, and circumstances.

Limitations of the Bioavailable Testosterone Calculator

This calculator is an estimation tool and has important limitations.

It Is Not a Diagnostic Tool

A calculated free or bioavailable testosterone result cannot independently diagnose testosterone deficiency, excess androgen states, or another medical condition.

Calculations Depend on Assumptions

The Vermeulen mass-action approach depends on assumed testosterone-protein binding constants.

Laboratory Methods Differ

Total testosterone and SHBG measurements can vary according to assay methods.

Clinical Interpretation Requires Context

Symptoms, medical history, medications, age, timing of testing, and other laboratory results can all matter.

Different Equations Can Produce Different Results

Research has demonstrated meaningful variation between different testosterone calculation algorithms. PubMed

The Free Hormone Concept Is Not Completely Settled

The biological significance of circulating free and protein-bound fractions is more complicated than simply dividing testosterone into "active" and "inactive" categories. Endotext notes ongoing debate concerning the free hormone hypothesis and the biological significance of these fractions. NCBI

Frequently Asked Questions

1. What is a Bioavailable Testosterone Calculator?

A Bioavailable Testosterone Calculator estimates free testosterone, albumin-bound testosterone, and total bioavailable testosterone from total testosterone, SHBG, and albumin values.

2. What inputs does the calculator require?

The calculator requires three values: total testosterone in nmol/L, SHBG in nmol/L, and albumin in g/dL.

3. What formula does this calculator use?

The calculator uses a form of the Vermeulen mass-action model to estimate free testosterone from total testosterone, SHBG, and albumin. Bioavailable testosterone is then calculated as free testosterone plus albumin-bound testosterone.

4. What is the difference between free and bioavailable testosterone?

Free testosterone is testosterone that is not protein-bound. Bioavailable testosterone generally includes free testosterone plus testosterone loosely bound to albumin.

5. Why is SHBG important?

SHBG binds testosterone strongly and influences the amount of testosterone that remains free or loosely bound. Therefore, SHBG can affect the relationship between total and free testosterone.

6. Why does the calculator use albumin?

Albumin binds testosterone more weakly than SHBG. The calculator uses albumin to estimate the albumin-bound testosterone fraction and therefore the overall bioavailable testosterone concentration.

7. What does 4.3 g/dL mean for albumin?

The calculator provides 4.3 g/dL as its default albumin input. If an actual laboratory albumin result is available, it can be entered instead, provided it is expressed in the required units.

8. Can calculated free testosterone diagnose low testosterone?

No. A calculated result should not be used by itself to diagnose testosterone deficiency. Clinical diagnosis generally combines symptoms and signs with appropriately obtained laboratory measurements and clinical interpretation. Endocrine

9. Why might another testosterone calculator give a different result?

Different calculators may use different equations, binding constants, assumptions, and methods. Studies have shown that published equations can produce substantially different calculated free and bioavailable testosterone results. PubMed

10. Is the Bioavailable Testosterone Calculator suitable for medical decisions?

It can be useful for educational and preliminary estimation purposes, but it should not replace laboratory testing or professional medical advice. If you are concerned about testosterone levels or symptoms, discuss your results with a qualified healthcare professional.

Final Thoughts

The Bioavailable Testosterone Calculator provides a practical way to estimate how total testosterone may be distributed between free, albumin-bound, and SHBG-bound fractions.

By entering total testosterone, SHBG, and albumin, the calculator uses a Vermeulen-style mass-action model to estimate free testosterone. It then calculates albumin-bound testosterone and combines the two to estimate bioavailable testosterone.

The key relationship is:

Bioavailable Testosterone = Free Testosterone + Albumin-Bound Testosterone

The calculator also converts the results between nmol/L and ng/dL and calculates the percentage of total testosterone represented by the estimated bioavailable fraction.

Understanding these calculations can be especially useful when total testosterone alone does not provide enough information, such as when SHBG is altered or total testosterone is near a clinical decision threshold. Clinical guidelines and endocrine references recognize that free testosterone assessment can be useful in selected situations. NCBI

At the same time, calculated testosterone values have limitations. Different equations and laboratory methods can produce different results, and the biological interpretation of protein-bound testosterone is more nuanced than a simple "available versus unavailable" model. NCBI

For the most meaningful interpretation, use accurate laboratory values, pay close attention to units, consider the laboratory's reference ranges, and interpret testosterone results alongside symptoms and other relevant clinical information. A calculator can help explain the numbers, but it should complement—not replace—professional medical evaluation.