Peritonitis is the most important therapy-related complication of peritoneal dialysis (PD). Monthly or quarterly PD peritonitis rate statistics are used to identify special cause variation within or between individual PD centres, to highlight any need for quality improvement. Unfortunately, many PD centres do not accurately “patient flow” (i.e., when patients start and finish on PD), and therefore cannot measure PD peritonitis rate. In this study, we validate an estimating formula for month-on-month annualised PD peritonitis rate, that calculates time-at-risk from “patient stock” (i.e., the number of prevalent patients on PD at the beginning and end of the month). We compared centers’ estimated peritonitis rates with gold-standard measurements in the Australia and New Zealand Dialysis and Transplant Registry / New Zealand PD Registry, and Le Registre de Dialyse Péritonéale de Langue Française et hémodialyse à domicile. A total of 268 centers from 9 countries with 1,020,260 patient-months of follow-up and 19,669 episodes of peritonitis were modeled. Overall agreement was excellent between estimates and gold-standard measurements with a concordance correlation coefficient (CCC) of 0.998 (95% confidence interval [CI] 0.998-0.998) in both registries. There was statistically significant lower agreement for smaller centers, although the CCC was still greater than 0.995. There were no instances of clinically significant misclassification of centers as being compliant or non-compliant with PD peritonitis standards with the use of the estimating formula. The simplified method of calculating the PD peritonitis rate is accurate and will allow more centers around the world to measure, report, and work on reducing PD peritonitis rates.