The charge will generate more than $100 million in revenue and cost savings each year, UAL Corp. said. The change takes effect May 5 and applies to tickets purchased on or after Monday.

Travelers would have to log at least 25,000 miles in a year on United to ensure they can check their second bag for free.

Airlines want to charge more for not only checked baggage but assigned seats and other services. Investors have urged airlines to pass on the higher costs of fuel to passengers through ticket-price increases or similar surcharges.

United's customer research showed a quarter of its customers check a second bag, Chief Revenue Officer John Tague said in a statement. The new policy will allow customers with many bags to continue bringing them for a fee and "enables us to offer competitive fares to everyone," Tague said.

Customers who have "Premier" status or higher within United's Mileage Plus program, or "Silver" status or higher within its Star Alliance program still will be able to check in a second bag for free. The new charge applies to those who purchase nonrefundable domestic economy tickets.

United will charge all customers $100 per bag for up to four additional bags. Previous charges ranged from $85 to $125 per bag. The cost to check items that require special handling because they are large, overweight or fragile will now be either $100 or $200, depending on the item.

New fees apply to trips within the U.S. and or those that include Canada, San Juan, Puerto Rico and St. Thomas. Customers whose itineraries include other international flights still will be able to check the second bag for free. The cost to check more than two bags, or items that are overweight or require special handling, on such trips will vary by destination.