Free Testosterone Estimator
Calculate your free and bioavailable testosterone using the Vermeulen equation — the same method used by endocrinologists. Enter your lab values to get a clinically meaningful estimate of your biologically active testosterone, not just the total.
Enter your lab values
Free testosterone estimates are based on the Vermeulen et al. (1999) equation. For educational use only. Not a substitute for medical advice.
Why free testosterone matters more than your total
When most men get a testosterone blood test, the result they see is total testosterone — a number that includes all forms of testosterone circulating in the blood, whether biologically active or not. The problem is that the vast majority of total testosterone is bound to carrier proteins and unavailable for the body to use. Only the small unbound fraction — free testosterone — can enter cells, activate androgen receptors, and produce the effects men associate with healthy testosterone levels: energy, muscle, libido, mood, and cognitive sharpness.
This distinction explains one of the most frustrating experiences in men's hormonal health: a man with a normal total testosterone result who still feels every symptom of low T. If his SHBG is elevated, a disproportionate share of his testosterone is locked in an inactive bound state — and his free testosterone may be genuinely low even while his total looks fine on paper.
The three forms of testosterone in blood
Testosterone circulates in three states, each with a different biological role:
- SHBG-bound testosterone (roughly 60–70%): Tightly bound to sex hormone-binding globulin. This fraction is biologically inert — it cannot enter cells, cannot be converted to DHT, and cannot be aromatized to estrogen. It functions essentially as a reservoir.
- Albumin-bound testosterone (roughly 28–38%): Weakly bound to albumin. Because the binding is reversible, this fraction can dissociate and become available to tissues. It is considered part of the bioavailable pool.
- Free testosterone (roughly 1–3%): Completely unbound. This is the biologically active fraction that enters cells directly and exerts testosterone's effects on androgen receptors throughout the body.
The Vermeulen equation — how this calculator works
This calculator uses the Vermeulen equation, published by Alex Vermeulen, Lieve Verdonck, and Jean-Marc Kaufman in the Journal of Clinical Endocrinology and Metabolism in 1999. The method models the binding equilibrium between testosterone, SHBG, and albumin using established association constants: approximately 1.0 × 10¹⁰ L/mol for SHBG binding and 3.6 × 10⁴ L/mol for albumin binding. By solving the mass-action equations for these equilibria, the model calculates the free fraction of testosterone with high precision.
A validation study comparing multiple free testosterone estimation methods found the Vermeulen calculation correlated with the gold-standard equilibrium dialysis method at r = 0.918 — substantially better than the direct analog immunoassay used by most commercial labs, particularly in men with abnormal SHBG levels. Many endocrinologists prefer the calculated value over the direct assay for this reason.
When to use this calculator: It is most useful when your total testosterone is borderline or low-normal and you want to know whether your free T is contributing to your symptoms; when your SHBG is known to be high or low; or when your direct free T assay result seems inconsistent with how you feel. It is less informative when SHBG has not been measured — the albumin default of 4.3 g/dL is reasonable but cannot substitute for a measured SHBG value.
Free testosterone reference ranges by age
Free testosterone declines with age faster than total testosterone, because SHBG rises with age — binding progressively more testosterone and reducing the free fraction. The following ranges are from LabCorp's direct assay reference data, the most widely used US clinical laboratory. Note that calculated free T using the Vermeulen equation is typically expressed in ng/dL; divide by 10 to convert pg/mL to ng/dL.
| Age group | Normal range (pg/mL) | Normal range (ng/dL) | Average |
|---|---|---|---|
| 20–29 | 9.3–26.5 pg/mL | 0.93–2.65 ng/dL | ~15–18 pg/mL |
| 30–39 | 8.7–25.1 pg/mL | 0.87–2.51 ng/dL | ~13–16 pg/mL |
| 40–49 | 6.8–21.5 pg/mL | 0.68–2.15 ng/dL | ~11–14 pg/mL |
| 50–59 | 7.2–24.0 pg/mL | 0.72–2.40 ng/dL | ~10–13 pg/mL |
| 60+ | 6.6–18.1 pg/mL | 0.66–1.81 ng/dL | ~8–11 pg/mL |
The Framingham Heart Study established a lower limit for free testosterone in healthy young men of 70 pg/mL (7.0 ng/dL) using calculated free T — values below this threshold were associated with physical dysfunction, frailty, and metabolic disease in large population samples.
What SHBG does to your free testosterone
SHBG is the single biggest determinant of free testosterone in most men. When SHBG is high — whether due to age, liver conditions, hyperthyroidism, or other causes — it binds a larger share of total testosterone and dramatically reduces the free fraction. A man with total testosterone of 550 ng/dL and SHBG of 70 nmol/L may have a calculated free testosterone below 8 pg/mL — well below the reference range — while a man with the same total testosterone and SHBG of 20 nmol/L might have a free T above 18 pg/mL. Same total T number, entirely different hormonal picture.
Conversely, men with naturally low SHBG may have adequate free testosterone even at a relatively low total. This is why total testosterone alone is an incomplete picture, and why measuring SHBG alongside total testosterone should be standard practice in any evaluation for low T.
Frequently asked questions
About free testosterone and this calculator