What is the Vermeulen formula for free testosterone?

The Vermeulen formula calculates free testosterone from total testosterone, SHBG, and albumin using established binding constants. It is considered the gold standard calculated method and is widely used in clinical practice when direct free testosterone measurement is unavailable.

What is a normal free testosterone level for men?

Free testosterone normal range for adult men is approximately 9–30 pg/mL (0.174–1.04 nmol/L). Optimal levels are generally 15–25 pg/mL. Below 9 pg/mL is considered low; above 30 pg/mL is elevated.

Free Testosterone Calculator (Advanced) — Vermeulen Formula

Enter Your Lab Values

Advanced tool: You need SHBG from your blood test for this calculator. If you only have Total Testosterone, use our simpler T levels tool instead.

Total Testosterone (ng/dL) *

Normal range: 300–1000 ng/dL

SHBG (nmol/L) *

Normal range: 10–57 nmol/L for adult men

Albumin (g/dL)

Default: 4.3 g/dL if not measured

Age (optional)

Used for age-adjusted interpretation

Your Results

Enter your values and click Calculate to see your free testosterone results.

Testosterone Reference Ranges for Men

Measurement	Low	Low-Normal	Normal	Optimal	High
Total T (ng/dL)	<300	300–400	400–700	500–900	>1000
Free T (pg/mL)	<9	9–12	12–20	15–25	>30
Free T (nmol/L)	<0.17	0.17–0.22	0.22–0.69	0.52–0.87	>1.04
Bioavailable T (ng/dL)	<70	70–130	130–250	150–300	>350
SHBG (nmol/L)	<10 (low)		10–57	>57 (high)

Age-Adjusted Total Testosterone Ranges (ng/dL)

Age Group	Low	Normal Range	Optimal
20–29	<400	400–1080	600–900
30–39	<350	350–1000	550–850
40–49	<300	300–950	500–800
50–59	<270	270–900	450–750
60–69	<250	250–850	400–700
70+	<200	200–800	350–650

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Frequently Asked Questions

The Vermeulen formula (published in the Journal of Clinical Endocrinology & Metabolism, 1999) is considered the most accurate calculated method for estimating free testosterone. It correlates well with equilibrium dialysis (the gold-standard direct measurement) in most patients. For routine clinical screening, it is widely accepted as sufficiently accurate.

Approximately 44–65% of circulating testosterone is tightly bound to SHBG and biologically inactive. Another 33–54% is weakly bound to albumin. Only 1–3% is "free" and immediately bioavailable. If your SHBG is high, you may have normal total T but low free T — and still experience symptoms of deficiency. Free T often tells the more complete story.

Bioavailable testosterone = free testosterone + albumin-bound testosterone. The albumin-bound fraction can be released relatively easily by tissues. Bioavailable T is sometimes preferred over free T alone as a measure of functional testosterone, particularly in older men whose albumin levels may decline.

Testosterone follows a circadian rhythm, peaking in the morning. Clinical guidelines recommend testing between 7–10 AM, fasting if possible. Diagnosis of hypogonadism typically requires at least two low morning readings. Single measurements can vary by 30% or more.