Assuming that each of those fleeing will need to stop for the bathroom every two hours, the report said 810,000 people will be desperately looking for a toilet within hours of the quake, even if half of the public facilities remained intact. Some might wait 17 hours before finding a toilet, the simulation said.
Japan is one of the world's most earthquake-prone countries, and experts believe Tokyo has a 90 percent chance of being hit by a major quake over the next 50 years.
The report, issued Monday, also noted about 7 million people will be unable to return home, and 4.6 million of them will seek public shelter if the event of a quake of that size.
Although officials tend to focus on providing food and water in the aftermath of a disaster, toilet needs should get more attention, said Masaya Takahashi, a Cabinet official in charge of disaster prevention. He noted that the search for toilets might cause unnecessary panic.
The city has said it plans to make toilets available at schools, convenience stores, gas stations and other public facilities, but the report urged further effort.
Local events | Contests | Headlines at a glance