Onset of monsoon is an important rainfall characteristic for drought management and crop planning. Morris and Zandstra method for onset was compared with observed values for a period of 20 years, which revealed a mismatch in the Bhilwara region in 40 per cent of years. The criteria for rainfall accumulation for both onset and withdrawal was modified in such a way that the determined and observed onset of monsoon showed a significant match. For early and late onset of monsoon and intermittent low /nil rainfall weeks. Withdrawals were adjusted and length of growing period for Bhilwara region was determined.