Methods and Systems for calibrating a Single Ramp Multiple Slope Analog to Digital Converter (SRMS ADC), the ADC including a counter and a plurality N of charge and discharge elements of different time constant i.e. slope, wherein the relationships between slopes is defined as a function of the shallowest slope SN such that S1=K1·SN, S2=K2·SN, . . . SN-1=KN-1·SN-1 where the K values are integers, and the code count for conversion is C=K1·C1+K2·C2+ . . . KN-1·CN-1+CN where each Ci represents an observed counts per each slope for a conversion, including; sampling for a first calibration pass a voltage with the ADC, discharging the voltage on the steepest slope for a number of counter counts C11, charging and discharging on the remaining slopes up to K2 to KN-1 for a number of counts per slope, Ci1 e.g. C21 to CN-1,1, discharging the remaining voltage residue on the shallowest slope and note the count, CN,1, sampling the same voltage on the ADC for a second calibration pass, discharging the voltage on the steepest slope for a modified number of counter counts C12=C11+/−X, modifying the number of charge/discharge counts time Ci2 for the slopes K2 to KN-1 to adjust for the change expected from the modified steep slope discharge to reach the shallowest slope with the same expected residue as for the first calibration pass, discharging the remaining voltage residue on the shallowest slope and note the actual count, CN,2, adjusting K1 to K1a based on the difference between CN,1 and CN,2, and; using C=K1a·C1+K2·C2+ . . . KN-1·CN-1+CN as the count code for conversion.