Most testosterone blood tests report your total testosterone. That number includes testosterone bound to proteins in your blood — which your body cannot use. The fraction that matters is free testosterone: the small, unbound portion that actually enters your cells and drives the effects you care about: muscle, libido, mood, energy, cognitive sharpness.
A 1999 paper in the Journal of Clinical Endocrinology and Metabolism by Vermeulen, Verdonck, and Kaufman published a mass-action binding formula that calculates free testosterone from three routine lab values. Endocrinologists and clinical labs worldwide still use it. The calculation below runs the same math.
Research Calculator · Vermeulen et al., JCEM 1999
Free Testosterone Calculator
Enter your blood test results. The formula calculates the biologically active fraction of your testosterone — the number that matters most.
Why Total Testosterone Misleads
Total testosterone measures everything in your blood: testosterone bound to sex hormone-binding globulin (SHBG), testosterone loosely attached to albumin, and the free fraction. Only free testosterone and albumin-bound testosterone (sometimes grouped as "bioavailable testosterone") can cross into cells.
SHBG is the key variable. It binds testosterone with high affinity and renders that fraction biologically inactive. As men age, SHBG tends to rise — which means two men with identical total testosterone readings can have very different free testosterone levels depending on their SHBG.
Consider a 47-year-old man with a total testosterone of 480 ng/dL. His lab flags this as normal. But if his SHBG is 65 nmol/L (elevated, common after 40), his free testosterone calculates to roughly 50 pg/mL — the low-normal zone where symptoms are documented. His friend with the same total T but an SHBG of 30 nmol/L would calculate to around 100 pg/mL — solidly normal.
Same total. Completely different clinical picture.
The Vermeulen Formula
The formula models the binding equilibria between testosterone, SHBG, and albumin using association constants established through mass spectrometry:
- SHBG–testosterone association constant (Ka): 5.97 × 10⁸ L/mol
- Albumin–testosterone association constant (Ka): 3.6 × 10⁴ L/mol
From these constants and the concentrations of each binding protein, the formula solves a quadratic equation to isolate the free fraction. The full derivation is in the original paper (JCEM, DOI: 10.1210/jcem.84.10.6019).
The calculator above uses exactly this formula. If you enter your albumin from your lab report, it uses your actual value. If you leave it at the default 4.3 g/dL, it uses the population mean — accurate for most men.
What you need from your blood test:
- Total testosterone (in ng/dL if you're in the US, nmol/L if you're in the UK or EU)
- SHBG in nmol/L
- Albumin in g/dL (optional — most standard metabolic panels include this)
All three are routine. If your doctor ordered a testosterone panel and a basic metabolic panel, you have these numbers.
What SHBG Does and Why It Rises
SHBG is a protein produced primarily by the liver. Its job is to carry sex hormones through the bloodstream and regulate how much reaches tissue. High SHBG means less free testosterone. Low SHBG means more.
SHBG rises with:
- Age — the dominant driver after 40; levels increase roughly 1–2% per year
- Liver disease — cirrhosis dramatically elevates SHBG
- Hyperthyroidism
- Certain medications — anticonvulsants, some antidepressants, statins in some men
- Low body fat — counterintuitively, very lean men tend toward higher SHBG
SHBG falls with:
- Obesity and insulin resistance — excess body fat suppresses SHBG, which is why obese men can have low total T but normal free T, and vice versa
- Hypothyroidism
- Anabolic steroids
- High insulin levels
If your free testosterone is low but your total testosterone is normal, elevated SHBG is the usual explanation. That matters clinically — the treatment for high-SHBG-driven low free T is different from the treatment for low total T across the board.
Reference Ranges: What the Numbers Mean
Reference ranges for free testosterone vary between labs and methodologies. The zones used in the calculator above are based on mass-action calculations (not direct measurement by equilibrium dialysis or analog immunoassay, which have their own reference ranges).
For Vermeulen-calculated free testosterone in men:
| Range (pg/mL) | Zone | What it suggests |
|---|---|---|
| ≥ 120 | Optimal | Androgen receptor saturation adequate for most functions |
| 70–119 | Normal | Within normal range; some men at the lower end report mild symptoms |
| 50–69 | Low-Normal | Borderline; symptomatic men have reasonable basis for clinical evaluation |
| < 50 | Low | Below threshold where symptoms are well-documented in research |
These are starting points, not diagnoses. Symptoms matter as much as lab values. A man at 68 pg/mL with no symptoms needs no intervention. A man at 72 pg/mL who is exhausted, losing muscle, and experiencing low libido has a legitimate clinical concern worth exploring.
When to Get Your SHBG Measured
Standard testosterone panels often include only total testosterone. If your total is normal but you have symptoms of low testosterone — fatigue, reduced libido, difficulty building muscle, poor recovery, mood changes — ask your doctor to add SHBG to the next draw. The delta between your total T and free T is where the clinical insight lives.
Most labs can run SHBG for under $30 as an add-on. Direct-to-consumer lab services (LabCorp, Quest, Ulta Lab Tests, Let's Get Checked) offer full hormone panels that include SHBG and albumin without a prescription in most US states.
Symptoms of Low Free Testosterone
Low free testosterone produces the same symptoms as low total testosterone, because the mechanism is the same — insufficient androgen reaching target cells:
- Persistent fatigue not explained by sleep quality or duration
- Reduced libido and sexual function
- Slower recovery from exercise, disproportionate muscle soreness
- Difficulty maintaining or building muscle despite consistent training
- Increased body fat, particularly around the abdomen
- Mood changes: irritability, low motivation, depression
- Brain fog: reduced focus and working memory
- Reduced bone density (longer-term consequence)
None of these symptoms are specific to low testosterone. They overlap with thyroid dysfunction, sleep apnea, depression, anemia, and a dozen other conditions. That's why the lab value matters — it gives context to the symptoms.
What to Do If Your Free Testosterone Is Low
Low-normal (50–69 pg/mL) with symptoms:
Start with the lifestyle levers that directly affect both SHBG and testosterone production:
- Sleep 7–9 hours — testosterone peaks during sleep; one week of sleep below 5 hours reduces levels by 10–15% (JAMA, 2011)
- Resistance training 3×/week — compound movements (squat, deadlift, press) stimulate luteinizing hormone, the pituitary signal that drives testosterone production
- Reduce alcohol — alcohol suppresses testosterone and raises SHBG through liver effects
- Manage body fat — if you're carrying excess weight, fat loss improves both total T and SHBG
- Vitamin D and zinc — deficiency in either is associated with reduced testosterone; get your levels tested before supplementing
Retest in 90 days. Lifestyle changes take time to show up in labs.
Low (< 50 pg/mL) with symptoms:
Request a full hormonal panel: total T, free T (Vermeulen or equilibrium dialysis), LH, FSH, estradiol, prolactin, thyroid panel. This establishes whether the issue is primary (testicular) or secondary (pituitary/hypothalamic) and rules out other causes.
Bring your calculated free testosterone to the appointment. Not all clinicians run it routinely, and presenting the Vermeulen result — with the source study — gives you a more productive conversation.
Testosterone replacement therapy (TRT) is a medically established option when low free T is symptomatic and confirmed. It carries real benefits and real risks. The decision requires a physician who understands men's hormonal health, not a telehealth provider who prescribes to anyone over 35.
FAQ
Can I use this calculator without SHBG on my lab report?
No — SHBG is required for the Vermeulen formula. Without it, you can only look at total testosterone, which may mislead. Ask your doctor to add SHBG to your next testosterone draw, or order it through a direct-to-consumer lab.
My total testosterone is normal but my free T calculated low. Is that possible?
Yes, and it's common in men over 40. Rising SHBG with age binds more testosterone, reducing the free fraction even when total levels are adequate. This is sometimes called "functional hypogonadism" — the production side is fine, but bioavailability is reduced.
How accurate is the Vermeulen formula compared to direct measurement?
Equilibrium dialysis is the gold standard for free testosterone measurement, but it's expensive and not widely available. The Vermeulen formula correlates well with dialysis results and is validated in multiple clinical studies. It's the method used by major clinical lab systems including Epic Beaker (PMC5545775). It's not perfect — it assumes standard albumin binding and may be less accurate in men with albumin disorders — but it's the best accessible approximation.
What's the normal SHBG range for men over 40?
SHBG in adult men typically ranges from 10 to 57 nmol/L, with levels trending higher with age. Men over 40 often see levels in the 30–60 nmol/L range. Above 60 nmol/L is elevated and worth discussing with your doctor if you have low T symptoms.
Can I lower my SHBG to raise free testosterone?
Lifestyle factors that reduce SHBG include fat loss (if overweight), reducing alcohol, and managing insulin resistance through diet and exercise. Some physicians prescribe medications that lower SHBG in specific clinical contexts. There's no reliable supplement-based approach. Boron has weak evidence for modest effects; the studies are small and the effect sizes are not clinically meaningful for most men.
Should I get total or free testosterone tested first?
Start with total testosterone. If it's clearly low (below 300 ng/dL), free T will likely be low too and the clinical picture is straightforward. If total T is in the normal range (300–600 ng/dL) and you have symptoms, add SHBG to calculate free T — that's where the insight lives.
Medical Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult your healthcare provider before starting any new exercise, nutrition, or supplement program.