Disclosed are analyte monitoring systems and methods for calibrating an analyte sensor using one or more reference measurements. These systems and methods may include using a conversion function and first sensor data to calculate a first sensor analyte level, weighting a first reference analyte measurement (RM1) and one or more previous reference analyte measurements according to a weighted average cost function, updating the conversion function using the weighted RM1 and the one or more weighted previous reference analyte measurements as calibration points, and using the updated conversion function and second sensor data to calculate a second sensor analyte level. In some aspects, the systems and methods may include updating one or more of lag parameters used to calculate the sensor analyte levels.