Free Testosterone Calculator - Vermeulen Equation Method

A free testosterone calculator is a clinical decision tool that estimates the unbound, biologically active fraction of testosterone in your blood from a standard hormone panel. Most testosterone in circulation is bound to sex hormone-binding globulin (SHBG) or albumin, and only a small free fraction can actually enter cells and act on tissues. It uses the Vermeulen equation to take your total testosterone, SHBG, and serum albumin and solve for the unbound, albumin-bound, and SHBG-bound portions.

• • Low-T Symptom Investigation: Men with fatigue, low libido, reduced muscle mass, or mood changes can use the calculator to translate a borderline total testosterone into a clearer free testosterone reading.
• • Re-evaluating Total T With Normal Lab Ranges: Total testosterone can look 'normal' while SHBG is high, leaving very little free testosterone; the tool surfaces that gap so a normal total result is not over-interpreted.
• • Tracking Therapy Progress: Patients on testosterone replacement or SHBG-lowering interventions can monitor free and bioavailable testosterone over time to see whether treatment is moving the fraction that matters.
• • Comparing Two Lab Reports: When the same total testosterone is reported in ng/dL on one report and nmol/L on another, the tool gives a consistent free testosterone number to compare directly.

Free testosterone is often a more informative number than total testosterone because it represents the fraction that can bind androgen receptors in muscle, bone, brain, and reproductive tissues. Two men with identical total testosterone can have different free testosterone values depending on SHBG and albumin, which is why the Endocrine Society recommends calculating free testosterone whenever SHBG is measured.

If you already have a cholesterol panel handy, the Cholesterol Ratio Calculator pairs naturally with this output, since lipid and sex hormone balance tend to move together. For a related body composition view, the Body Fat Percentage Calculator gives a comparable number that often moves alongside SHBG in men.

The calculator uses the Vermeulen binding model to solve a small system of mass-action equations. It assumes testosterone binds to two carrier proteins in serum, SHBG with high affinity and albumin with low affinity, and finds the equilibrium free testosterone concentration that makes the total add back up to your measured value.

• T_total: Your measured total testosterone, converted to mol/L using molecular weight 288.42 g/mol (the standard testosterone MW in clinical chemistry).
• SHBG_total: Your measured sex hormone-binding globulin in nmol/L, converted to mol/L for the binding equilibrium.
• A: Your serum albumin concentration in g/dL, converted to mol/L using albumin molecular weight 66,500 g/mol. Albumin binds testosterone loosely but contributes meaningfully to the bioavailable fraction.
• Ka: High-affinity binding constant between SHBG and testosterone at 37 degrees C, equal to 1.0 x 10^9 L/mol in the Vermeulen model.
• KaA: Low-affinity binding constant between albumin and testosterone at 37 degrees C, equal to 3.6 x 10^4 L/mol.

The same SHBG and albumin binding constants are used by Mayo Clinic Laboratories and the ISSM / University of Michigan online calculator. Iterating until the free and bound fractions add back up to your total testosterone keeps the result consistent with your lab values.

According to Vermeulen, Verdonck, and Kaufman (1999), free and bioavailable testosterone can be estimated from total testosterone, SHBG, and albumin using the SHBG-testosterone binding constant of 1.0 x 10^9 L/mol and the albumin-testosterone constant of 3.6 x 10^4 L/mol, both at 37 degrees C. The model agrees well with equilibrium dialysis across the clinically relevant range and is the reference implementation behind the Mayo Clinic Laboratories report.

The result is reported in the same units your lab uses, so you can drop a free testosterone reading in pg/mL or ng/dL directly into a clinic conversation. According to Mayo Clinic Laboratories, the result is reported alongside total testosterone and SHBG and uses the Vermeulen equation. A doctor often pairs the result with a follow-up check after any change in weight, sleep, or medication.

Four short definitions help you read the output with confidence.

These four ideas work together: total testosterone is the headline number, SHBG is the throttle on the free fraction, bioavailable testosterone is what the body can actually use, and the Vermeulen model is the engine that connects them. If you want to see how a related lab value, your LDL cholesterol, fits into a similar interpretation model, the LDL Calculator walks through a comparable calculation.

Follow these four steps to estimate your free testosterone.

1. 1 Enter Total Testosterone: Type the total testosterone value from your blood test in ng/dL. If your report uses nmol/L, multiply by 28.85 to convert (1 nmol/L equals 28.85 ng/dL).
2. 2 Enter SHBG: Enter your sex hormone-binding globulin in nmol/L. SHBG is almost always included on a male hormone panel alongside total testosterone.
3. 3 Confirm or Override Serum Albumin: Leave the serum albumin at the default of 4.3 g/dL if you do not have a recent value. Change it to your measured value if you do, because the albumin-bound fraction depends on it.
4. 4 Read the Free, Bioavailable, and SHBG-Bound Values: Review free testosterone in pg/mL and ng/dL, bioavailable testosterone in ng/dL, percent free testosterone, and the SHBG-bound amount. Compare with reference ranges or your prior results.

A 47-year-old man enters total testosterone 420 ng/dL, SHBG 55 nmol/L, and the default albumin 4.3 g/dL. The result is free T about 47 pg/mL (4.7 ng/dL), bioavailable T around 113 ng/dL, and percent free near 1.1%. Compared with the typical adult male reference range of 50 to 210 pg/mL, he sits in the lower third, a useful starting point for a clinician conversation. Short-term changes in sleep and alcohol can each shift SHBG by 5 to 10 percent.

This tool is most useful when you want to go beyond a single number on a lab report. It also helps when a borderline total reading and a normal SHBG have to be reconciled before a clinic appointment.

• • Reveals the Active Fraction: Translates one total testosterone reading into the free and bioavailable fractions that drive androgen-related symptoms.
• • Surfaces High-SHBG Edge Cases: Catches situations where total testosterone looks normal but elevated SHBG has reduced the free fraction, a pattern common in older men and in thyroid or liver conditions.
• • Tracks Therapy Changes Meaningfully: Lets you re-check free and bioavailable testosterone as you adjust medications, weight, or training, rather than relying on total testosterone alone.
• • Works With Existing Lab Results: Uses the total testosterone and SHBG you already get from a routine male hormone panel, so no extra testing is required for most users.
• • Complements Other Health Calculators: Pairs naturally with body composition and lipid calculators, since sex hormone balance interacts with body fat, lean mass, and cardiovascular markers.

These advantages make this tool a good fit for any user who already has a hormone panel and wants a clearer read on what their numbers mean. It is also a practical checkpoint before a clinic appointment, especially when you want to come prepared with a free testosterone number rather than a single total testosterone value. For the broader metabolic picture, the BMI Calculator translates your body mass index into a comparable health category for the same visit.

A few clinical factors drive the output more than anything else. The result also assumes a healthy albumin level, a routine hormone panel, and no recent anabolic steroid exposure, so it is most reliable when those assumptions hold.

• • The calculator estimates free testosterone using the Vermeulen binding model; it is not a substitute for a directly measured free testosterone by equilibrium dialysis, rarely available in routine labs.
• • Results assume a normal albumin level. Severe hypoalbuminemia, very high SHBG, or recent anabolic steroid or estrogen therapy can shift the actual free testosterone away from the calculated value.
• • This tool is for informational use and does not diagnose low testosterone, hypogonadism, or any other medical condition. Always review results with a qualified clinician.

These factors explain why two men with the same total testosterone can sit on opposite sides of the free testosterone reference range, and why repeat measurements are usually more meaningful than a single number. Clinicians therefore track the trend over weeks.

According to the Endocrine Society clinical practice guideline on testosterone therapy, calculating free testosterone from total testosterone and SHBG is the preferred approach when direct free testosterone measurement by equilibrium dialysis is not available, and the same guideline notes that calculated free testosterone should be used alongside symptoms, not in place of clinical judgment. The free testosterone calculator reflects that same recommendation.

Because SHBG and body fat both change with age in men, the BMI Men Calculator provides a useful companion readout when reviewing your free testosterone trend over time.