Free Testosterone Calculator (SHBG-Based)

Total testosterone tells you how much is in your blood. Free testosterone tells you how much your body can actually use. The difference between those two numbers — shaped entirely by SHBG — can be the difference between a man who feels fine and a man who is symptomatic despite an apparently normal total T.

The Vermeulen Formula

The most widely used method for calculating free testosterone from total T and SHBG is the Vermeulen equation (1999). It accounts for SHBG-binding affinity, albumin-binding, and a fixed albumin concentration of 4.3 g/dL. Most men's health platforms and endocrinology tools use this calculation.

What is free testosterone and why does it matter more than total?

Free testosterone is the fraction not bound to SHBG or albumin, making it immediately available for cellular use. A man with high total T but very high SHBG may have lower bioavailable testosterone than a man with moderate total T. Free T drives the hormonal effects men actually feel.

Free T (nmol/L) = (Total T − (SHBG × Bound T)) / (1 + Ka × Albumin), where Ka is albumin's association constant for testosterone (3.6 × 10⁴ L/mol).

In practical terms: a man with Total T of 550 ng/dL and SHBG of 60 nmol/L will have a significantly lower free T than a man with the same Total T and SHBG of 25 nmol/L. The SHBG difference alone can cut bioavailable testosterone by 30–40%.

What is SHBG and how does it affect free testosterone?

SHBG (sex hormone-binding globulin) tightly binds testosterone, rendering it inactive. High SHBG—common in older men, those with low insulin, or high thyroid function—reduces free testosterone significantly even when total T appears normal. Lowering SHBG or calculating free T is essential for accurate assessment.

How to Calculate Yours

You need three values from your blood test: Total Testosterone (ng/dL), SHBG (nmol/L), and Albumin (g/dL) — albumin is typically assumed to be 4.3 g/dL if not measured. Input these into any Vermeulen calculator (available online via Issam, ADAM, or similar tools) to get your estimated free T.

Reference Ranges for Free Testosterone	Age Group
Free T (pg/mL) — Typical Range	Optimal Target
20–29	15–25
18–25	30–39
13–22	15–22
40–49	10–18
12–18	50–59
8–14	10–15
60+	5–11
8–12	What Raises or Lowers SHBG

Can you calculate free testosterone without a blood test?

The Vermeulen formula calculates free testosterone from total T and SHBG levels—both of which are standard blood test values. This calculated free T correlates well with direct measurement. A basic blood panel provides all the inputs needed for an accurate estimate.

SHBG determines how much of your testosterone is free. Understanding what moves it is as important as knowing the number itself.

• Raises SHBG: aging, thyroid hormone, estrogen, alcohol, liver disease, low body weight, caloric restriction

• Lowers SHBG: insulin/IGF-1, androgens, obesity, hypothyroidism, growth hormone

Men with chronically elevated SHBG (above 50 nmol/L) may have total T in the normal range but genuinely insufficient free T. This is a clinical situation that deserves attention, even if their lab report shows no flags.

What conditions cause high SHBG in men?

Liver disease, hyperthyroidism, aging, very low insulin levels, and certain medications (particularly anticonvulsants) elevate SHBG. Insulin resistance and obesity typically lower SHBG. Understanding why SHBG is high guides treatment—addressing the underlying cause is often more effective than ignoring it.

Should You Treat High SHBG?

SHBG is not straightforwardly "good" or "bad" — elevated SHBG is associated with reduced cardiovascular risk in some studies. The question is whether the reduced free T is producing symptoms. If it is, options include: addressing the underlying cause (e.g., treating hypothyroidism), adding exogenous testosterone, or using enclomiphene to drive up total T high enough that free T reaches adequate levels despite elevated SHBG.