A letter sent from the United States to Japan via First-Class Mail requires $1.15 in postage, per United States Postal Service. Three First-Class Mail stamps or Forever stamps, which currently cost $0.49 each, cover the cost.
In order to qualify for the $1.15 postage rate, the letter to Japan must be in an envelope no more than 11 ½ inches long, 6 inches high and ¼ inch thick. The value of its contents must be less than $400.
If the letter is being sent via First-Class Mail in a large envelope – that is, no more than 15 inches long, 12 inches high and ¾ inch thick – the postage cost increases to $5.10.