Commuters can save hundreds of dollars next year on their MetroCards when a new law goes into effect, officials said Sunday.

Businesses and nonprofit employers with 20 or more full-time staffers will be required on Jan. 1 to let workers use their pretax dollars to cover their transit costs.

The average rider can save up to $443 a year in taxes if they buy an unlimited MetroCard every month, which costs $116.50.

It had been optional for employers to give workers that benefit.

The Riders Alliance launched an online calculator on Sunday at the Union Square station that helps straphangers figure out how much money they can save. Other groups such as the Straphangers Campaign and elected officials were there as well to explain the new law to riders.

"Everyone should know that the new law puts real money back in the pockets of riders for the subway and bus service they're already using," said Zachary Arcidiacono, a Riders Alliance member who works for the TWU Local 100, which represents many MTA workers.

A subway rider who makes between $47,000 and $101,000 a year would save almost $40 a month in taxes by using the benefit. The employer also sees its taxes reduced for each worker who signs up and would save almost $9 a month in payroll taxes.

The Department of Consumer Affairs is giving employers a six-month grace period before the agency starts enforcing the law on July 1. Companies are exempt if their workers are unionized because their benefits are covered by a collective bargaining agreement.

Chain businesses will be covered by the new law. The DCA will calculate whether a company has 20 workers by counting all full-time workers in all its business locations in New York City.