Last updated: June 2026 · Reviewed June 2026 · Built by the InjectBuddy team
How does the free testosterone calculator work?
The free testosterone calculator works by taking three numbers from your blood panel — total testosterone, SHBG, and albumin — and solving the Vermeulen 1999 binding equation to estimate how much testosterone is unbound and biologically active. You enter total T (ng/dL or nmol/L), SHBG (nmol/L), and albumin (g/dL); it returns calculated free T in ng/dL and as a percentage of total. This guide explains the three inputs and their units, works through eight real examples, shows how SHBG drives the result, and answers the questions people ask most.
- Inputs: total testosterone, SHBG, and albumin (albumin defaults to 4.3 g/dL if unknown).
- It uses the Vermeulen equation, which models the equilibrium between testosterone and its two carrier proteins.
- Calculated free T tracks the gold-standard equilibrium dialysis far better than cheap direct immunoassays.
- Roughly 2–3% of total testosterone is free in a typical adult man; SHBG is the lever that moves it.
- Run your own numbers in the free testosterone index calculator.
Start with worked examples
The fastest way to understand the calculator is to watch the same total testosterone produce different free T as SHBG changes. Each example below uses the Vermeulen method with albumin held at the population default of 4.3 g/dL. Free T is reported in ng/dL and as a percentage of total — the calculator does the algebra, but the pattern is what matters.
Total T 600 ng/dL, SHBG 35 nmol/L, albumin 4.3 g/dL. Calculated free T is about 11.6 ng/dL — roughly 1.93% of total. This is a typical mid-range adult male result.
Total T 600 ng/dL, SHBG 70 nmol/L, albumin 4.3 g/dL. Free T drops to about 7.2 ng/dL (~1.2%). Total testosterone looks identical, yet far less is available because more is bound to SHBG.
Total T 600 ng/dL, SHBG 18 nmol/L, albumin 4.3 g/dL. Free T rises to about 17.0 ng/dL (~2.8%). Low SHBG, common in metabolic syndrome, frees up more testosterone for the same total.
Total T 300 ng/dL, SHBG 15 nmol/L, albumin 4.3 g/dL. Free T is about 9.4 ng/dL (~3.1%). A "low" total can still yield a near-normal free T when SHBG is low.
Total T 350 ng/dL, SHBG 60 nmol/L, albumin 4.3 g/dL. Free T is only about 4.7 ng/dL (~1.3%) — below most reference ranges despite a total that some labs would flag as "normal-ish."
Total T 600 ng/dL, SHBG 35 nmol/L, but albumin 3.5 g/dL instead of 4.3. Free T shifts only to about 11.0 ng/dL. Albumin moves the answer far less than SHBG does, which is why the calculator can default it safely.
Total T 20.8 nmol/L (≈600 ng/dL, dividing by 0.0347), SHBG 35 nmol/L. The calculator converts internally and returns the same ~11.6 ng/dL free T. Always check whether your lab reported nmol/L or ng/dL first.
If a result claims total T 500 ng/dL with free T 25 ng/dL, that is 5% free — well outside the ~1–3% Vermeulen range for a normal SHBG. A free percentage that high usually means a direct-immunoassay number, not a calculated one.
The three inputs and their units
Calculated free testosterone is only as good as the three values you feed it. Get the units right and the rest is arithmetic the calculator handles.
| Input | Typical unit | What it does in the equation |
|---|---|---|
| Total testosterone | ng/dL or nmol/L | The total pool the equation splits into bound and free |
| SHBG | nmol/L | High-affinity carrier; the main driver of how much T is free |
| Albumin | g/dL (default 4.3) | Low-affinity carrier; loosely bound T counts as "bioavailable" |
| Output: free T | ng/dL (and % of total) | The unbound fraction available to tissues |
Total testosterone and SHBG come from a standard panel. Albumin is often on the same comprehensive metabolic panel; if it is missing, the calculator substitutes 4.3 g/dL, which is close enough for most adults because albumin barely moves the result (see the albumin example above). For the difference between the carriers, see SHBG and albumin explained.
How this is calculated
The Vermeulen 1999 equation treats blood as a chemistry problem at equilibrium. Testosterone exists in three states: tightly bound to SHBG, loosely bound to albumin, and free. The free and albumin-bound fractions together make up bioavailable testosterone. The equation uses fixed association constants for SHBG (about 1.0 × 10⁹ L/mol) and albumin (about 3.6 × 10⁴ L/mol) and solves a quadratic for the free concentration that balances the binding reactions.
You do not need to solve the quadratic by hand — that is the entire point of the calculator. But the honest arithmetic underneath is: given total T, SHBG, and albumin, there is exactly one free-T value at which the amount bound to SHBG plus the amount bound to albumin plus the free amount equals the total. The calculator searches for that value. Because SHBG binds with roughly 30,000 times the affinity of albumin, SHBG dominates: doubling SHBG roughly halves free T, while a normal swing in albumin barely registers. Vermeulen and colleagues showed this calculated value agrees closely with equilibrium dialysis, the reference method, which is why it is preferred over direct analog immunoassays (Vermeulen 1999; Bhasin 2018).
Interpreting the result
Calculated free T is usually read in ng/dL against a lab reference range, commonly something like 5–21 ng/dL for adult men, though ranges vary by assay and population. The percentage (free T ÷ total T) is a useful sanity check: in a normal-SHBG man it sits around 1.5–2.5%, and the rest is bound. A free percentage above ~3% points to low SHBG; below ~1.3% points to high SHBG.
This number is a snapshot, not a diagnosis. The Endocrine Society recommends confirming low testosterone with repeat fasting morning samples and interpreting free T only when SHBG is abnormal or borderline (Bhasin 2018). The calculator is a maths tool — it converts three lab values into a calculated free T using a validated equation. It does not decide whether a result is "low for you" or whether treatment is warranted. For the concept behind the number rather than the maths, read free testosterone explained, and to interpret the wider panel see reading Testosterone (TRT) bloodwork.
So, how does the free testosterone calculator work?
The free testosterone calculator works by solving the Vermeulen 1999 binding equation with three inputs from your blood panel: total testosterone, SHBG, and albumin. It distributes the total pool across the SHBG-bound, albumin-bound, and free fractions using fixed association constants, returning the unbound fraction in ng/dL and as a percentage of total. Because SHBG binds roughly 30,000 times more tightly than albumin, SHBG is the dominant lever — doubling it roughly halves your free T even when your total T is unchanged. Run your own lab values through the free testosterone index calculator to see the result instantly.
FAQs
How does the free testosterone calculator work?
What inputs does the free testosterone calculator need?
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Why is calculated free T different from my lab's free T?
What units should I enter?
Sources
- 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. PMID 10523012.
- Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018. PMID 29562364.
- Sizar O, Schwartz J. Male Hypogonadism. StatPearls. NCBI Bookshelf NBK532933.
- CDC. Safe Injection Practices to Prevent Transmission of Infections to Patients. CDC injection safety guidance.
This guide is for general educational purposes only and does not constitute medical advice. The free testosterone calculator is a maths tool, not a diagnostic service. Always interpret lab results with your prescriber.