---
title: Free Testosterone Calculator (Vermeulen Method)
type: tool
url: "https://www.test-well.com/tools/free-testosterone-calculator"
mcp_tool: "calculate_free_testosterone"
date_published: 2026-09-05
free: true
---

# Free Testosterone Calculator (Vermeulen Method)

> Enter total testosterone, SHBG and (optionally) albumin, and the calculator returns free testosterone in pg/mL and ng/dL, the free fraction as a percent, and bioavailable testosterone, using the Vermeulen mass-action equation. It's the same math behind most lab-reported 'calculated free testosterone' values, and it explains why a normal total can hide a low free level.

## Key facts

| Fact | Value |
| --- | --- |
| Method | Vermeulen 1999 |
| Inputs | Total T · SHBG · albumin |
| Outputs | Free · bioavailable · % |
| Cost | Free |

## Method

Vermeulen et al. (1999) mass-action equation: free testosterone is solved from total testosterone, SHBG and albumin using association constants Ka(SHBG) = 1 × 10⁹ L/mol and Ka(albumin) = 3.6 × 10⁴ L/mol; bioavailable testosterone = free + albumin-bound. Albumin defaults to 4.3 g/dL when not measured.

## For agents

The same computation is exposed as the MCP tool `calculate_free_testosterone` at https://www.test-well.com/mcp and documented in https://www.test-well.com/openapi.json.

## Frequently asked questions

### Why calculate free testosterone instead of measuring it?

Direct free-T immunoassays are notoriously unreliable, so the Endocrine Society recommends either equilibrium dialysis (the reference method, expensive and slow) or a calculation from accurately measured total testosterone and SHBG. The Vermeulen calculation tracks equilibrium dialysis closely for most people and is what appears on most lab reports as 'calculated free testosterone'.

### What SHBG and albumin values should I use?

Use the SHBG from the same blood draw as your total testosterone — it varies with age, weight, thyroid status and insulin resistance. If albumin wasn't measured, 4.3 g/dL is the standard assumed value and changes the result very little in healthy adults.

### Why is my total testosterone normal but my free testosterone low?

Because most testosterone is bound to SHBG and unavailable to tissues. When SHBG is high (common with aging, thyroid excess, liver disease or some medications), more of the total is locked up, so the free fraction falls even while the total looks fine. That is exactly the situation this calculator makes visible — and why testing SHBG alongside total T matters.

### Does this calculator diagnose low testosterone?

No. Diagnosis needs symptoms plus a confirmed low morning total testosterone on at least two separate days, interpreted by a clinician. The calculation helps explain a result; it doesn't replace that evaluation.

## Related

- [free testosterone](https://www.test-well.com/biomarkers/free-testosterone)
- [total testosterone](https://www.test-well.com/biomarkers/total-testosterone)
- [shbg](https://www.test-well.com/biomarkers/shbg)
- [albumin](https://www.test-well.com/biomarkers/albumin)
- [testosterone free total shbg](https://www.test-well.com/tests/testosterone-free-total-shbg)
- [testosterone free and total](https://www.test-well.com/tests/testosterone-free-and-total)
- [testosterone total](https://www.test-well.com/tests/testosterone-total)

---

Canonical page: https://www.test-well.com/tools/free-testosterone-calculator
Machine-readable catalog: https://www.test-well.com/api/catalog.json · MCP server: https://www.test-well.com/mcp · Overview for agents: https://www.test-well.com/llms.txt

Prices are cash-pay, in USD, and include physician authorization; every order adds one flat $6 lab processing fee. Educational content is not a diagnosis — discuss results with a clinician.