Free Androgen Index (FAI)
This free androgen index calculator lets you enter your total testosterone and SHBG to estimate the Free Androgen Index — a quick ratio of how much testosterone is free of its main binding protein. For men, a calculated free testosterone is more reliable; this is a fast screen.
Educational tool, not medical advice. FAI reference ranges are lab-dependent and the index is less validated in men than in women — a calculated (Vermeulen) free testosterone is the preferred estimate for men.
How to use the free androgen index calculator
- 1
Enter total testosterone
Use a morning blood draw when testosterone peaks. The FAI calculator converts common units, so ng/dL or nmol/L both work.
- 2
Enter SHBG
Take SHBG from the same sample. It sits in the denominator, so this input drives most of the result.
- 3
Read your FAI and zone
The tool returns the ratio and places it on the scale. In women this screens for androgen excess.
- 4
Men: cross-check free T
If SHBG is low, the FAI over-reads. Confirm with a calculated free testosterone before you trust a high number.
Low SHBG fakes a high FAI
Normal SHBG — the FAI and true free testosterone agree.
Low SHBG — the FAI spikes into “high,” but free T is unchanged.
The Free Androgen Index is a simple ratio — total testosterone ÷ SHBG × 100 — built to screen for androgen excess in women. This free androgen index calculator gives you that number instantly, but the honest headline is this: in adult men the index is not a valid measure of free testosterone. If you are a man tracking TRT or a cycle, treat the free androgen index calculator as context and lean on a calculated free-testosterone value instead.
What the free androgen index calculator actually measures
The Free Androgen Index (FAI) is one of the oldest shortcuts for estimating how much of your testosterone is unbound and biologically active. This free androgen index calculator takes two numbers from a blood panel — your total testosterone and your sex hormone-binding globulin (SHBG) — and expresses their relationship as a single percentage-style figure.
The logic is intuitive. Most testosterone in the bloodstream is stuck to SHBG and, to a lesser extent, albumin. Only a small unbound fraction can enter cells and act. Because SHBG is the main brake on that free fraction, dividing testosterone by SHBG should, in theory, tell you how “free” your androgens are. That is the whole premise the index is built on, and for one specific population it holds up reasonably well.
The problem is that this free androgen index calculator, like every version of the index, was validated on a group it was never meant to leave: women being screened for androgen excess. Ported into male physiology — where SHBG runs lower and testosterone runs an order of magnitude higher — the ratio quietly loses its meaning. Understanding that boundary is the single most useful thing this page can give you, so the rest of the article walks through the math, the failure mode in men, and the population where it genuinely earns its place.
How the free androgen index calculator does the math
The formula behind every free androgen index calculator is deliberately simple, which is both its appeal and its weakness.
FAI is calculated as total testosterone divided by SHBG, multiplied by 100, with both hormones expressed in the same molar units (nmol/L). Because it is a ratio of two concentrations in identical units, the result is dimensionless — it has no units of its own, even though people often read it like a percentage. This tool handles the unit conversion for you, but it is worth knowing that feeding in testosterone as ng/dL without converting to nmol/L is the most common way people get a nonsense number out of the index.
An FAI is only as good as its two inputs. Ask your lab for total testosterone and SHBG measured on the same sample, drawn in the morning when testosterone peaks. A ratio built from tests taken weeks apart, or from a low-quality testosterone immunoassay, will mislead no matter how precise the tool is.
FAI = (total testosterone ÷ SHBG) × 100, both in nmol/L. The free androgen index calculator does the unit math for you.
Two numbers from one morning blood draw: total testosterone and SHBG. Nothing else changes the output.
A unitless ratio. Higher means more testosterone relative to SHBG; lower means SHBG is binding more of it up.
Why the free androgen index calculator misleads in men
This is the part most tools skip. A landmark analysis put it bluntly: the Free Androgen Index is not valid for adult males. That is not an opinion — it is the title of the paper.
The reason is mathematical. In women, SHBG is relatively high and testosterone is low, so the ratio stays in a range where it tracks true free testosterone fairly well. In men the opposite is true: testosterone is high and SHBG is often modest, especially in younger or heavier men and in anyone on anabolic steroids, which suppress SHBG hard. When SHBG is low, the index divides a big number by a small one and the ratio balloons — overstating free testosterone far beyond what a proper calculated free-T value would show.
“A high FAI means high free testosterone.” In men with low SHBG this is exactly where the index breaks — the index runs high precisely because SHBG is low, not because usable free testosterone is genuinely elevated. The number inflates fastest in the men most likely to be checking it.
The clinical literature reinforces this from the other direction too: even in women, the index becomes inaccurate once SHBG falls into the low range, which is the same failure mode men live in by default. So when the index hands a man a reassuringly high figure, that figure is often an artifact of low SHBG rather than a real reading of androgen status.
“A high Free Androgen Index in a man with low SHBG is a math artifact, not a hormone result. Read it as a flag to calculate free testosterone properly — not as an answer.”
MuscleScience editorial standardWhere the free androgen index calculator genuinely helps
The index was born in gynecology, and that is still where it belongs. In women, it is a fast, cheap screen for androgen excess.
In the assessment of polycystic ovary syndrome (PCOS), hirsutism and other hyperandrogenic states, the Free Androgen Index remains a widely used marker. Female SHBG is high enough that the ratio behaves, and a raised FAI flags the excess androgen activity that drives symptoms like unwanted hair growth, acne and irregular cycles. In that setting the index is a reasonable first pass before more expensive equilibrium-dialysis free-testosterone testing.
The reference table below shows how FAI values are typically read in women. Use it the way it is meant to be used — as an orientation for female androgen screening, not as a male testosterone gauge.
| MedlinePlus (NIH) | Testosterone Levels Test — total, free and bioavailable testosterone explained. |
| Rosner W et al. | Utility, limitations and pitfalls in measuring testosterone: Endocrine Society position statement (JCEM 2007). |
If you are a woman investigating PCOS or hirsutism, an FAI paired with total testosterone and SHBG is a sensible starting point — then confirm with your clinician. If you are a man, skip straight to a calculated free testosterone.
What moves your FAI: SHBG is the lever
Because SHBG sits in the denominator, almost everything that changes your FAI is really a change in SHBG. Knowing what pushes SHBG around tells you why your FAI output drifts.
SHBG is not fixed. It responds to hormones, body composition, the liver and the drugs you take. When SHBG rises, the same testosterone produces a lower FAI; when SHBG falls, the index returns a higher number even if your testosterone has not moved at all.
- Raises SHBG (lowers FAI): aging, high thyroid hormone, estrogen, calorie restriction and low insulin, and some liver conditions.
- Lowers SHBG (raises FAI): obesity and high insulin, anabolic steroids and high androgen load, growth hormone, and hypothyroidism.
- The steroid angle: exogenous androgens crush SHBG, so a man on cycle can post a dramatically high FAI that reflects suppressed SHBG far more than usable free testosterone. This is the exact scenario where the index flatters the number most.
free androgen index calculator vs calculated free testosterone
If the Free Androgen Index is unreliable in men, what should a man use instead? The answer is calculated free testosterone.
Calculated free testosterone — most commonly the Vermeulen equation — uses total testosterone, SHBG and albumin together with the known binding constants to estimate the actual free fraction in pmol/L or ng/dL. Unlike the index, it does not collapse into nonsense when SHBG is low, because it models the binding chemistry rather than just dividing two numbers. Endocrine Society guidance has long favored equilibrium dialysis or a validated calculated free testosterone over crude indices for exactly this reason.
A ratio. Fast and useful for female androgen screening; unreliable in men, especially at low SHBG.
Models binding with albumin and SHBG. The preferred estimate of free testosterone in men.
Equilibrium dialysis or LC–MS/MS. Accurate but slow and costly; reserved for tricky cases.
Run this free androgen index calculator if you want the number, then open our free testosterone calculator and compare. In men the calculated free-T value is the one to act on; the FAI is context, not conclusion.
free androgen index calculator FAQ
Is a high FAI good or bad?
It depends entirely on who you are. In a woman being screened for PCOS, a high FAI flags androgen excess. In a man, a high FAI reading is often just a sign of low SHBG and does not reliably mean high usable testosterone. Neither direction is inherently “good” without the SHBG value and the clinical picture.
Can men use the free androgen index calculator at all?
You can calculate it, but you should not rely on it. The index has been shown to be invalid for adult males, so use this free androgen index calculator only as a rough orientation and base decisions on a calculated free testosterone instead.
Why does my FAI look so high on cycle?
Anabolic steroids suppress SHBG sharply. With SHBG low in the denominator, the index divides by a small number and the ratio climbs — reflecting suppressed SHBG far more than a true rise in free testosterone.
What units does the free androgen index calculator need?
Both testosterone and SHBG must be in the same molar units (nmol/L). This free androgen index calculator converts common units for you, but a raw ng/dL testosterone value entered without conversion will produce a meaningless result.
Does the FAI diagnose anything?
No. The free androgen index calculator is a screening and orientation tool, not a diagnosis. Any abnormal result should be confirmed with proper testing and interpreted by a clinician alongside symptoms and the full panel.
Key takeaways
The free androgen index calculator is a good tool used in the wrong place more often than not. Keep these points in mind and it becomes useful again.
Using the free androgen index calculator in practice
A short checklist for getting something meaningful out of this free androgen index calculator without over-trusting it.
- Enter total testosterone and SHBG from the same morning blood draw; mismatched samples corrupt the ratio.
- Always read the free androgen index calculator result next to the raw SHBG value — a high FAI on low SHBG is the classic trap.
- If you are male, follow up with a calculated free testosterone and act on that instead.
- If you are female investigating PCOS or hirsutism, use the free androgen index calculator as a first screen and confirm with your clinician.
- Track the trend across panels rather than reacting to one number, since SHBG shifts with weight, thyroid status and drugs.
References
Final educational note
This free androgen index calculator and the article around it are educational tools, not medical advice. The Free Androgen Index is a screening ratio with well-documented limits — particularly in men — and no single number from any free androgen index calculator can diagnose a hormonal condition.
Always interpret results with a qualified clinician who can see your full panel, symptoms and history. If a value concerns you, confirm it with appropriate testing before acting. See our full Disclaimer for details.
MuscleScience.org does not sell hormones or prescribe treatment. Our goal is to help you read your own bloodwork more intelligently and ask better questions of your doctor.
