Free testosterone calculator

Where to find your numbers

Total testosterone, SHBG (sex hormone-binding globulin) and albumin are usually on the same lab report or can be added to one order. Check the units printed beside each result.

• Total testosterone is entered in ng/dL. If your report is in nmol/L, multiply by 28.84 to get ng/dL.
• SHBG is entered in nmol/L.
• Albumin is entered in g/dL. If your report is in g/L, divide by 10.

How it is calculated

The calculator uses the mass-action equations published by Vermeulen and colleagues in 1999. They treat testosterone as bound to two proteins, SHBG and albumin, with the rest left free. With every concentration in mol/L:

• N = 1 + K_ALB × [albumin]
• Free testosterone (FT) is the positive solution of N × K_SHBG × FT² + (N + K_SHBG × ([SHBG] − [total T])) × FT − [total T] = 0

It uses K_SHBG = 1 × 10⁹ L/mol and K_ALB = 3.6 × 10⁴ L/mol, an albumin molecular weight of 69,000 g/mol and a testosterone molecular weight of 288.42 g/mol. When albumin is left empty it assumes 4.3 g/dL.

Source: Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. Journal of Clinical Endocrinology & Metabolism. 1999;84(10):3666–3672.

What the result can’t tell you

• It is a calculated estimate. A lab that measures free testosterone directly may report a different number.
• It does not say whether your result is low, normal or high, and it does not diagnose anything. Ranges differ between labs, so compare with the range on your own report.
• It depends on the accuracy of the numbers you enter, and on the lab methods behind them.

Entry limits are there to catch typing and unit mistakes. Total testosterone must be 1 to 5,000 ng/dL, SHBG 0.1 to 1,000 nmol/L and albumin 1 to 8 g/dL.

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