Free Estradiol Estimator

Note: Results may be lower if you're taking Boron or if T is a significant proportion of SHBG.

To use, enter your E2 and T in either SI or US units, then click the button to calculate.

Free E2 pmol/L		Free E2 pg/mL
Free T pmol/L		Free T ng/dL

Free E2 calculation:

$$f_{E2} = \frac{E2 - N_2 \cdot f_{E2}}{K_{sE2} \cdot (SHBG - E2 + N_2 \cdot f_{E2})}$$
$$\text{where } N_2 = (K_{aE2} \times C_a) + 1$$

Free T calculation:

$$f_T = \frac{T - N_1 \cdot f_T}{K_{sT} \cdot (SHBG - T + N_1 \cdot f_T)}$$
$$\text{where } N_1 = (K_{aT} \times C_a) + 1$$

Constants:

$K_{aE2} = 4.21 \times 10^4 \text{ L/mol}$ (E2-Albumin)

$K_{sE2} = 3.14 \times 10^8 \text{ L/mol}$ (E2-SHBG)

$K_{aT} = 4.06 \times 10^4 \text{ L/mol}$ (T-Albumin)

$K_{sT} = 1.0 \times 10^9 \text{ L/mol}$ (T-SHBG)

$C_a = \frac{\text{Albumin}}{66500} \text{ mol/L}$ (from input)

Solved iteratively to $1 \times 10^{-15}$ precision