Health calculators

Free Androgen Index Calculator

Updated Sep 16, 2026 By Infinity Calculator
Rate Formulas Medical disclaimer
Lab Values
Biological Sex
Determines which reference range is applied to your result.
Values are converted to nmol/L before the index is computed (ng/dL ÷ 28.818).
Most labs report SHBG in nmol/L; mass units are converted using SHBG MW ≈ 90 kDa.
Your FAI
57.81
(dimensionless index)
Interpretation
Normal
Normal range (male): 30 – 150
Total Testosterone used
18.50
nmol/L  |  533.13 ng/dL
SHBG used
32.00
nmol/L  |  288.00 µg/dL
Where Your Result Falls
Normal
Your FAI falls within the normal reference range for males.
Step-by-Step Solution
FAI vs. SHBG at Your Testosterone Level
FAI Reference Ranges by Sex
Sex Low Normal High

Introduction

The Free Androgen Index (FAI) is a simple number that shows how much of your testosterone is free to work in your body. Most testosterone in your blood is stuck to a protein called SHBG (sex hormone-binding globulin). Only the testosterone that is not stuck can act on your cells. The FAI gives you a quick estimate of that active part.

Enter your total testosterone and your SHBG from your lab report, pick your units, and choose male or female. The calculator converts your values to nmol/L, then uses this formula:

FAI = (Total Testosterone ÷ SHBG) × 100

You get your FAI score, a plain-word reading (Low, Normal, or High), a step-by-step solution, and a chart that shows how your FAI changes as SHBG changes. Normal FAI is about 30 to 150 for males and about 7 to 10 for females.

Doctors often use FAI to help look into low testosterone in men and into PCOS, extra hair growth, or acne in women. This calculator is a learning tool, not a diagnosis. Always go over your lab results with your doctor.

How to use our Free Androgen Index Calculator

Enter your sex, your total testosterone, and your SHBG lab result. The calculator gives you your Free Androgen Index (FAI), tells you if it is low, normal, or high for your sex, and shows the math step by step.

Biological Sex: Pick Male or Female. This sets the reference range used to read your FAI score.

Total Testosterone: Type the total testosterone number from your blood test. Then pick the unit your lab used: nmol/L, ng/dL, ng/mL, or pg/mL. The calculator converts it to nmol/L.

SHBG (Sex Hormone-Binding Globulin): Type your SHBG result and pick its unit: nmol/L, µg/dL, or mg/L. Most labs use nmol/L.

Click Calculate FAI to see your result, or click Reset to start over with the sample values.

What Is the Free Androgen Index (FAI)?

The Free Androgen Index, or FAI, is a simple number that estimates how much of your testosterone is "free" and able to work in your body. It compares your total testosterone to your SHBG level. Doctors use it as a quick screen for hormone problems in both men and women.

The FAI Formula

FAI = (Total Testosterone ÷ SHBG) × 100

Both lab values must be in nmol/L. Because the units cancel out, FAI has no unit. It is just an index number.

Why SHBG Matters

SHBG stands for sex hormone-binding globulin. It is a protein made by your liver that grabs onto testosterone and carries it in your blood. Testosterone stuck to SHBG cannot enter cells and do its job. Only the free and loosely bound part is active.

This is why total testosterone alone can fool you. If your SHBG is high, your total testosterone may look normal while your body still acts low. If your SHBG is low, more testosterone stays active. FAI helps show that difference.

What Changes SHBG Levels

  • Raises SHBG: aging, high estrogen, an overactive thyroid, liver disease, birth control pills
  • Lowers SHBG: obesity, insulin resistance, type 2 diabetes, an underactive thyroid, steroid use

Typical FAI Ranges

GroupLowNormalHigh
MaleBelow 3030 to 150Above 150
FemaleBelow 77 to 10Above 10

Ranges can differ a little from lab to lab. Always check the range printed on your own report.

What Your Result Can Mean

Low FAI in men may go along with low sex drive, tiredness, weak erections, or loss of muscle. It can point to low testosterone or high SHBG.

High FAI in women is often linked to PCOS (polycystic ovary syndrome), extra body hair, acne, or missed periods. It usually happens when SHBG drops, testosterone rises, or both.

Things to Keep in Mind

Testosterone changes through the day, so blood is best drawn in the morning. FAI is an estimate, not a direct measure of free testosterone. Equilibrium dialysis is the gold-standard test, and calculated free testosterone using albumin is often more accurate than FAI, especially in men with very high or very low SHBG. Use FAI as one clue, not a diagnosis, and talk with a doctor about your full results and symptoms.

Formulas used

Free Androgen Index (FAI)
\text{FAI} = \frac{T_{\text{total}}\ (\text{nmol/L}) \times 100}{\text{SHBG}\ (\text{nmol/L})}
Testosterone unit conversion to nmol/L
T_{\text{nmol/L}} = \frac{T_{\text{ng/dL}}}{28.818} = T_{\text{ng/mL}} \times \frac{100}{28.818} = T_{\text{pg/mL}} \times \frac{0.1}{28.818}
SHBG unit conversion to nmol/L
\text{SHBG}_{\text{nmol/L}} = \frac{\text{SHBG}_{\mu\text{g/dL}}}{9} = \text{SHBG}_{\text{mg/L}} \times \frac{100}{9}
Interpretation zones (male: 30–150, female: 7–10)
\text{Zone} = \begin{cases} \text{Low}, & \text{FAI} < \text{L} \\ \text{Normal}, & \text{L} \le \text{FAI} \le \text{H} \\ \text{High}, & \text{FAI} > \text{H} \end{cases}
Gauge scale maximum and marker position
S_{\max} = 1.5 \times \text{H}, \qquad \text{pos}\% = \frac{\min(\text{FAI},\, S_{\max})}{S_{\max}} \times 100
FAI vs. SHBG curve (chart)
\text{FAI}(x) = \frac{T_{\text{nmol/L}} \times 100}{x}, \quad x \in \left[\max(0.25\,\text{SHBG},\, 0.5),\ 2.5\,\text{SHBG}\right]

Frequently asked questions

How do you convert total testosterone from ng/dL to nmol/L?

Divide the ng/dL number by 28.818, or multiply it by 0.0347.

  • 500 ng/dL ÷ 28.818 = 17.35 nmol/L
  • 300 ng/dL ÷ 28.818 = 10.41 nmol/L

You need nmol/L for both testosterone and SHBG before you can work out FAI. If your lab reports ng/mL, multiply by 100 first, then divide by 28.818.

Is the Free Androgen Index the same as free testosterone?

No. FAI is only an estimate. Free testosterone is a real amount measured in pmol/L or pg/mL.

FAI compares testosterone to SHBG and gives a plain number with no unit. Free testosterone tells you how much hormone is truly unbound. FAI tracks free testosterone well in most women, but it can miss the mark in men with very high or very low SHBG.

What FAI level points to PCOS?

An FAI above 5 is the cutoff used most often for too much androgen in women. Some labs set the line higher, near 7 to 10.

A high FAI alone does not prove PCOS. Doctors also look for missed periods, extra hair growth, acne, and ovary scans before making that call.

Does birth control lower the Free Androgen Index?

Yes. Pills with estrogen push the liver to make more SHBG. More SHBG grabs more testosterone, so less is free and FAI drops.

FAI can fall by half or more on the pill. That is why doctors often prescribe it for acne and extra hair growth. If you want a true reading of your own hormones, testing is usually done off hormonal birth control.

How can I lower a high FAI?

Most high FAI results come from low SHBG, so raising SHBG helps. Things that often work:

  • Lose extra body fat, even 5% to 10% helps
  • Cut back on sugar and refined carbs to improve insulin
  • Move daily and add strength or walking workouts
  • Sleep 7 to 9 hours
  • Treat thyroid problems or insulin resistance with your doctor

Doctors may also use birth control pills, metformin, or anti-androgen drugs when needed.

Can you have normal total testosterone but a high FAI?

Yes. If SHBG is low, more testosterone stays free even when the total looks fine.

Example: total testosterone 2.5 nmol/L with SHBG 20 gives FAI 12.5. The same 2.5 nmol/L with SHBG 60 gives FAI 4.2. Same testosterone, very different active amount. This is a common pattern in PCOS and in people with insulin resistance.

What is a normal SHBG level?

Typical adult ranges are about:

  • Men: 10 to 57 nmol/L
  • Women: 18 to 144 nmol/L

Ranges shift with age, pregnancy, and the testing method, so compare your number to the range printed on your own lab report.

Does FAI go down with age in men?

Usually yes. Total testosterone slowly falls after about age 30, while SHBG slowly rises. Both changes push FAI down.

That means an older man can have a total testosterone that still looks normal but a much lower share of active hormone. FAI or calculated free testosterone shows this better than the total alone.

When should blood be drawn for a testosterone and SHBG test?

Get the blood draw in the morning, usually between 7 and 10 a.m., when testosterone peaks. Many labs ask you to fast.

Avoid the test during an illness or right after very hard exercise. Women who still have periods are often tested in the first week of the cycle. One low result is not enough; doctors often repeat the test on a second morning.

What causes a high FAI in men?

Most often it is low SHBG rather than high testosterone. Common causes:

  • Obesity and insulin resistance or type 2 diabetes
  • An underactive thyroid
  • Anabolic steroid use or testosterone therapy
  • Fatty liver disease

A high FAI with a normal total testosterone is a hint to check blood sugar, weight, and thyroid.

Does testosterone therapy change your FAI?

Yes, and in two ways. It raises total testosterone and it often lowers SHBG. Both push FAI up, sometimes far above the normal range.

Because of this, FAI is not a good way to fine-tune a dose. Doctors track total testosterone, and often free testosterone, hematocrit, and estradiol instead.

Why does FAI use a multiplier of 100?

Testosterone in nmol/L is a much smaller number than SHBG in nmol/L. Dividing them gives a tiny decimal like 0.58, which is hard to read.

Multiplying by 100 turns it into a friendlier number like 58. The 100 is just a scaling step, so FAI has no unit at all.

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