Seven of the nine Masters tournaments will now be held across 12 days each. The Italian Open, Madrid Open and Shanghai Masters were expanded to 12 days last year.
The number of ATP 500 events will also be increased from 13 after tournaments in Dallas, Doha and Munich were upgraded from ATP 250 events. In all, the ATP said there would be 60 tournaments across 29 countries.
"Next season will see yet more enhancements to the ATP Tour's premium product across a streamlined calendar," ATP chairman Andrea Gaudenzi said in a statement.
"This is central to everything we've been building under OneVision, which ultimately aims to create the best possible experience for our fans.
"Last season we broke new ground, welcoming a record five million fans on site across our events. This is just one indication of how positively the sport is trending, and we believe there's incredible potential to still be unlocked."
Other changes included the rescheduling of the ATP 500 event in Hamburg from July to May while the ATP 250 event in Cordoba Open will be moved to Mallorca in the lead up to Wimbledon.
The ATP added that the ATP 250 event in Estoril, which does not hold an ATP membership but has operated under an ATP licence since 2015, is not on the calendar next year.
The ATP Finals will be hosted in Turin while the Next Gen ATP Finals will be played in Jeddah for a third year.
