Milliequivalents (mEq) and millimoles (mmol) are both units of measurement used to express the amount of a substance. For ions, such as HCO3-, the number of milliequivalents is equal to the number of millimoles multiplied by the valence of the ion. In the case of HCO3-, the valence is 1, so 1 mmol of HCO3 is equal to 1 mEq HCO3.