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Free testosterone calculator in UK units

Enter total testosterone and SHBG from your blood test. The calculator works out free and bioavailable testosterone with the Vermeulen equation, the method most UK laboratories use, and shows the result against the British Society for Sexual Medicine’s cut-off.

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Calculated free testosterone

Enter total testosterone and SHBG to see free and bioavailable testosterone.

Method
Vermeulen equation (1999), using SHBG and albumin binding constants of 1 × 10⁹ and 3.6 × 10⁴ L/mol.
Inputs
Total testosterone (nmol/L or ng/dL), SHBG (nmol/L), albumin (g/L or g/dL, 43 g/L if left blank).
Results
Free testosterone in nmol/L, pmol/L and pg/mL; percentage free; bioavailable testosterone.
UK cut-off
Calculated free testosterone below 0.225 nmol/L, with symptoms, supports treatment (BSSM 2023).
Privacy
Runs in your browser. Nothing is sent or stored.
Cost
Free, no account.

How the calculation works

Most testosterone in blood is bound to two proteins. About half is held loosely by albumin and around 40 to 45% tightly by sex hormone binding globulin (SHBG). Only 1 to 3% is free.[1] Free and albumin-bound testosterone together are called bioavailable testosterone, because both can reach tissues.

Vermeulen and colleagues showed that free testosterone can be worked out from total testosterone, SHBG and albumin using each protein’s binding strength, and that the result agrees closely with equilibrium dialysis, the slow reference method.[1] The calculator solves their equation for free testosterone:

The equationN = 1 + Ka × [albumin]; then Kt·N·FT² + (N + Kt·[SHBG] − Kt·[TT])·FT − [TT] = 0, solved for FT. Kt = 1 × 10⁹ L/mol (SHBG), Ka = 3.6 × 10⁴ L/mol (albumin), concentrations in mol/L.

Worked example: total testosterone 15 nmol/L, SHBG 40 nmol/L and albumin 43 g/L give a free testosterone of 0.274 nmol/L (274 pmol/L), 1.8% of the total.

Reading your result

The British Society for Sexual Medicine’s 2023 UK guideline says that, with symptoms, total testosterone below 12 nmol/L or calculated free testosterone below 0.225 nmol/L usually needs treatment, and that men between 8 and 14 nmol/L may be offered a trial. It recommends the Vermeulen calculation over direct free testosterone assays.[2] Blood should be taken between 7 and 11am, on at least two occasions, preferably four weeks apart.[2]

A number is not a diagnosis. Your laboratory’s own reference range and your GP’s reading of it come first, and results taken while using testosterone or other androgens describe that moment rather than your natural level.

Why SHBG matters, and why it runs in families

SHBG is the reason two people with the same total testosterone can have different free levels. Its level is partly inherited. Genome-wide studies of hundreds of thousands of people in UK Biobank found many genetic variants that shift SHBG and total testosterone, including several in and near the SHBG gene itself.[3]

One of those variants also changes how tightly SHBG holds testosterone. In a study of nearly 9,000 men, carriers of the SHBG variant rs6258 had SHBG that bound testosterone less tightly, and a measured free testosterone fraction 22% higher than non-carriers, a difference the standard calculation, which uses one binding constant for everyone, doesn’t see.[4] For most people the calculation is a good estimate; for carriers of variants like this it is an approximation. Read more in how genetics affects testosterone and steroid response, or see your own SHBG variants with the raw DNA hormone gene checker.

Questions

How do you calculate free testosterone?

Most UK laboratories and this calculator use the Vermeulen equation. It takes total testosterone, SHBG and albumin and works out how much testosterone is bound to SHBG, how much to albumin and how much is left free, using each protein's binding strength. Albumin is set to 43 g/L if you don't have it.

What is a normal free testosterone level in the UK?

The British Society for Sexual Medicine treats a calculated free testosterone below 0.225 nmol/L (225 pmol/L) as supporting treatment for testosterone deficiency when there are symptoms, alongside total testosterone below 12 nmol/L. Both should come from two morning samples. Each laboratory also prints its own reference range on the report, and that range is the one to compare against.

Why does SHBG change my free testosterone?

SHBG binds testosterone tightly, so the more SHBG you have, the less testosterone is free at the same total. Two people with the same total testosterone can have quite different free levels. SHBG levels are partly inherited: large genetic studies have found variants in and near the SHBG gene that shift SHBG and total testosterone.

Is calculated free testosterone accurate?

It agrees well with equilibrium dialysis, the reference method, across most of the normal range, which is why guidelines accept it. It is an estimate, and it is less reliable when SHBG or albumin are very unusual, in pregnancy, or with some medicines. Direct immunoassays for free testosterone are less reliable than the calculation.

Should I use nmol/L or ng/dL?

Either. UK reports use nmol/L for total testosterone, and US reports often use ng/dL. 1 nmol/L is 28.84 ng/dL. The calculator converts for you and shows free testosterone in nmol/L, pmol/L and pg/mL.

Can I use this if I take testosterone or anabolic steroids?

The arithmetic works on any result, but a result taken while using testosterone or other androgens describes that moment, not your natural level, and some anabolic steroids aren't measured by testosterone assays at all. Talk the result through with your GP or the clinic that ordered the test.

Is anything I type stored?

No. The calculation runs in your browser. The numbers are not sent to GeneticEdge or anyone else.

Sources

  1. Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab 1999;84:3666–72.
  2. Hackett G et al. British Society for Sexual Medicine guidelines on adult testosterone deficiency, with statements for practice. World J Mens Health 2023;41:508–37.
  3. Ruth KS et al. Using human genetics to understand the disease impacts of testosterone in men and women. Nat Med 2020;26:252–58.
  4. Ohlsson C et al. Genetic determinants of serum testosterone concentrations in men. PLoS Genet 2011;7:e1002313.

For information only, not medical advice. GeneticEdge does not recommend or endorse anabolic steroid use, and no genetic test can tell you whether steroids are safe for you. Talk to a GP or pharmacist about your health. Last reviewed 30 October 2026.