Postal rates will go up in April, but the cost of sending the basic letter will remain the same.

The Postal Service said Thursday that most rates will go up April 17 under a formula that allows the agency to increase prices within the rate of inflation.

The post office said the 44-cent price of a first-class stamp will remain unchanged, but heavier letters will cost more. The basic rate is for the first ounce, and the price for each extra ounce will rise to 20 cents from 17 cents.

“While changing prices is always a difficult decision, we have made every effort to keep the impact minimal for consumers and customers doing business with us at retail lobbies,” Postmaster General Patrick R. Donahoe said in a statement.

Under the law, the post office can raise rates as long as it doesn’t exceed inflation, which would permit an increase of 1.7 percent averaged over all mail.

The Postal Service lost $8.5 billion last year despite cuts of more than 100,000 jobs and other reductions in recent years.

The agency has lost considerable business to the Internet in recent years, and the post office last summer sought an increase that would have exceeded inflation, citing the unusual business conditions of the recession. That was rejected by the independent Postal Regulatory Commission.

The post office has appealed that, but in the meantime is moving to increase rates within the legal limit.

In addition to the 3-cent boost for additional ounces of first-class mail the new rates are as follows:

–  Post cards will rise to 29 cents from 28 cents.

–  Letters to Canada or Mexico increase to 80 cents, from 75 cents to Canada and 79 cents to Mexico.

–  Letters to other international destinations will remain at 98 cents.

– Express Mail and Priority Mail prices are not affected.

– There will be a variety of price changes for other services, including advertising mail, periodicals and packages.