Last updated: May 2026

Free Testosterone Index Calculator

Quick answer: With a total testosterone of 600 ng/dL (about 20.8 nmol/L) and SHBG of 40 nmol/L, FAI = (20.8 ÷ 40) × 100 ≈ 52 (broadly normal for adult men). Formula: FAI = (Total Testosterone ÷ SHBG) × 100, with both values in nmol/L.

About this calculator

The Free Androgen Index (FAI) calculator applies the formula FAI = (Total Testosterone ÷ SHBG) × 100, where both values are in nmol/L. It is intended for people interpreting their own blood test results alongside clinical guidance — not a standalone diagnostic tool. If you are on Testosterone (TRT) and need to calculate your injection draw volume, use the Testosterone (TRT) dosage calculator. Free, instant, no login.

The formula

FAI = (Total Testosterone ÷ SHBG) × 100

Both values go into the formula in the same molar units (nmol/L). US labs report total testosterone in ng/dL — the calculator converts it for you; SHBG is reported in nmol/L everywhere.

How to use this calculator

You need two values from a blood test: your total testosterone and your SHBG. US labs report total testosterone in ng/dL (select ng/dL in the calculator) and SHBG in nmol/L. Both are standard markers on a hormone blood panel — enter them and the FAI calculates immediately.

The calculator accepts either unit for total testosterone. To convert manually, divide ng/dL by 28.84 to get nmol/L (e.g. 600 ng/dL ÷ 28.84 = 20.8 nmol/L). US labs typically report ng/dL, while UK, Australian and New Zealand labs report nmol/L.

Worked example

Example from a blood panel

Total testosterone: 600 ng/dL (20.8 nmol/L)

SHBG: 40 nmol/L

FAI = (20.8 ÷ 40) × 100 = 52

High SHBG scenario

Total testosterone: 650 ng/dL (22.5 nmol/L, appears normal)

SHBG: 80 nmol/L (elevated)

FAI = (22.5 ÷ 80) × 100 = 28 (low — suggests high SHBG binding)

Interpreting your result

Reference ranges vary between laboratories. Broadly, for adult men:

FAI range	General interpretation
Below 30	Low — may indicate reduced bioavailable testosterone
30 – 150	Broadly normal range for adult men
Above 150	Elevated — discuss with your doctor

Reference ranges vary between laboratories. The 30–150 range is a broad population estimate for adult men — always interpret your result alongside your specific laboratory's reference range and in consultation with your doctor.

Reference range per Vermeulen A, et al. J Clin Endocrinol Metab. 1999;84(10):3666–72. Individual lab ranges may vary — verify with your prescribing clinician.

These are approximate population guidelines. Your individual reference range may differ. A low FAI alongside a normal total testosterone result often indicates elevated SHBG, which is binding and inactivating a significant portion of your testosterone. This can be clinically relevant even if total testosterone appears adequate.

Always discuss your results in context with your doctor or endocrinologist — the FAI is one data point, not a standalone diagnosis.

Frequently asked questions

Why does SHBG matter?

SHBG is a protein that binds strongly to testosterone, preventing it from interacting with androgen receptors in your tissues. High SHBG means more of your total testosterone is bound and inactive. You can have a total testosterone reading within the normal range and still experience symptoms of low T if SHBG is elevated.

Is FAI the same as free testosterone?

No. Free testosterone is a direct measurement of the testosterone not bound to any protein. The FAI is a calculated estimate that uses SHBG as a proxy. Direct free testosterone measurement (usually by equilibrium dialysis) is more accurate but more expensive and less commonly ordered. FAI correlates reasonably well with free T in most clinical contexts.

What can raise SHBG?

SHBG tends to rise with age, low body weight, elevated thyroid hormones, liver disease, certain medications (including some anticonvulsants), and high estrogen levels. It tends to fall with obesity, insulin resistance, high androgen levels, and hypothyroidism.

What is a normal FAI for adult men?

Reference ranges vary between laboratories. Broadly, an FAI of 30 to 150 is considered within the normal range for adult men. An FAI below 30 alongside a normal total testosterone result may indicate that elevated SHBG is binding and inactivating a significant portion of your testosterone — which can be clinically relevant even when total T appears adequate. Always interpret your result with your doctor.

How does Testosterone (TRT) affect SHBG and the Free Androgen Index?

Testosterone replacement therapy typically causes SHBG to fall over weeks to months as exogenous testosterone suppresses natural SHBG production. Lower SHBG means a higher FAI — more of your total testosterone is bioavailable. Many clinicians recheck SHBG and FAI at 3 and 6 months after starting Testosterone (TRT) to assess the full effect on bioavailable testosterone.

Other calculators

Free Testosterone (Vermeulen) Testosterone (TRT) Dosage Peptide Reconstitution Peptide Dosage BMI Semaglutide Tirzepatide Retatrutide BPC-157 BPC-157 + TB-500 Blend

Related guides

How this is calculated

The Free Androgen Index is calculated as (total testosterone ÷ SHBG) × 100, with both values in the same molar units (nmol/L). It runs entirely in your browser — nothing is stored — and is an interpretive aid, not medical advice. Discuss results with your prescriber.