Testosterone Level Calculator: Free vs Bound, SHBG and What Your Numbers Mean
A normal total testosterone result doesn't tell the full story. What matters is free testosterone — the biologically active fraction that actually reaches your muscles, libido and energy. With high SHBG, the free fraction can be deficient even if total T looks "normal."
This article explains the Vermeulen equation, reference ranges, and what low values mean for your health.
How testosterone circulates in blood
Testosterone exists in three fractions in the bloodstream:
| Fraction | Share | Activity |
|---|---|---|
| Free | 1–3 % | Fully active |
| Albumin-bound | ~54 % | Weakly active (bioavailable) |
| SHBG-bound | ~44 % | Inactive |
SHBG (sex hormone-binding globulin) binds testosterone with high affinity. When SHBG rises, free testosterone falls — even if production is unchanged.
The Vermeulen equation
Published in 1999 by Alex Vermeulen, this equation is today the gold standard for estimating free testosterone from routine lab values — and it correlates strongly with direct measurement by mass spectrometry.
N = KA × (Albumin / 69 000) + 1
a × FT² + b × FT + c = 0
a = N × KT
b = N + KT × (SHBG − Total)
c = −Total
KT = 1·10⁹ L/mol (SHBG binding), KA = 3.6·10⁴ L/mol (albumin binding). Solving the quadratic yields free testosterone in nmol/L.
Reference ranges (adult men)
| Marker | Normal | Low |
|---|---|---|
| Total T | 8.6 – 29 nmol/L | < 8 nmol/L |
| Free T | 200 – 620 pmol/L | < 200 pmol/L |
| SHBG | 10 – 57 nmol/L | < 10 or > 70 |
The Endocrine Society defines hypogonadism as total testosterone < 8 nmol/L (≈ 230 ng/dL) plus clinical symptoms. The 8–12 nmol/L range is a gray zone.
Symptoms of low testosterone
Sexual
Low libido, erectile dysfunction, absent morning erections.
Physical
Loss of muscle despite training, central body fat gain, reduced bone density, hot flashes.
Mental
Low motivation, depressed mood, poor concentration, sleep problems.
What affects your numbers
Testosterone has a strong diurnal rhythm. Peak is between 7 and 10am, trough in late afternoon. A single afternoon measurement can run 20–40 % below the morning value — falsely suggesting deficiency.
SHBG rises with hyperthyroidism, estrogen therapy, age, and liver disease. It drops with obesity, insulin resistance and Cushing's syndrome — the latter can artificially raise free T even though the hormonal axis is disturbed.
Calculate your free testosterone now
Instant result with Vermeulen formula, reference-range comparison and hypogonadism flag — free, no sign-up.
Calculate now →Natural ways to raise testosterone
For mild deficiency, studies show 10–20 % higher levels through:
- Resistance training with heavy compound lifts (squats, deadlifts, bench press) 3× per week
- Sleep: 7–9 hours per night — even one week of 5 hours drops T by 10–15 %
- Vitamin D at 40–60 ng/mL plus adequate zinc intake
- Weight loss if obese — every 10 kg lost adds about 2 nmol/L to total testosterone
- Reduce alcohol — chronic intake lowers T by 15–25 %
Related calculators
Low testosterone often clusters with other risk factors. Check your prostate cancer risk after age 45, your biological age as a whole-body marker, and cover your protein requirements to support muscle maintenance.
Bottom line
Total testosterone alone isn't enough — free testosterone is the better marker for actual symptoms. The Vermeulen equation lets you derive it from routine lab values. If your numbers are low and you have symptoms, see an endocrinologist or urologist. Use our testosterone calculator for a quick first read.