Methods and systems for accurately measuring and monitoring accumulated volume of hydraulic fluid in a blowout preventer (BOP) system, specifically for a function of interest, are disclosed. One method includes initializing a state machine algorithm, the state machine algorithm responsive to a BOP function of interest being activated; measuring an initial hydraulic flow rate baseline and an initial pressure baseline to create a hydraulic impedance variable for use in the state machine algorithm; monitoring an aggregate hydraulic flow rate and pressure of the BOP system over time; applying the hydraulic impedance variable to negate BOP system hydraulic flows not related to the BOP function of interest; and applying the state machine algorithm to determine when the BOP function of interest has been completed responsive to a total accumulated volume of hydraulic fluid.