Since the 1970s, C-peptide has been used as a surrogate marker for monitoring the progression of type 1 and type 2 diabetes and to determine the effects of interventions designed to preserve or improve residual beta cell function. C-peptide measurement is a well-established surrogate of residual beta cell activity and of clinical significance as it is associated with HbA1c, risk for microvascular complications and the incidence of hyperglycaemia in longitudinal studies. Measurement of C-peptide after a mixed meal tolerance test is considered the gold standard of measuring beta cell function in type 1 diabetes, but the method is laborious and inconvenient. In this issue of Diabetologia, Wentworth et al ( https://doi.org/10.1007/s00125-018-4722-z ) report an algorithm for estimating C-peptide (CPEST) based on six routine clinical measures. These do not include stimulated C-peptide measurement and outperform other prevailing algorithms for estimating residual beta cell function. Going forward it is very likely that this new algorithm will serve as a simple measure of beta cell function in routine practice and as a more acceptable primary outcome measure in future trials of disease-modifying therapies.