As of 2001[update] India census, Nirmali had a population of 16,144. Males constitute 53% of the population and females 47%. Nirmali has an average literacy rate of 45%, lower than the national average of 59.5%: male literacy is 57%, and female literacy is 31%. In Nirmali, 18% of the population is under 6 years of age.
Nirmali, a small town in the arms of two well known rivers, Koshi & Balaan. The famous leader Jayaprakash Narayan took refuge here after breaking away from Hanuman nagar Jail (Nepal) during the freedom struggle.The town has been famous for its extensive rice production.