Take your envelope to the Post Office and they will weight it, and give you your answer as the cost of mailing anything is done by weight.
The first ounce will cost 44 cents. Each additional ounce, or fraction, is another 17 ounces. That would be 78 cents for that envelope at the start of 2011.
I think you mean 9x12 envelope.
A 4-ounce large envelope currently costs $1.57.
That depends on where you are sending it from (you country of origin) and to (the country of destination).
As of June 10, one ounce of first class mail is 44¢. Every additional ounce is 20¢. So, a six ounce envelope would cost $1.44 to mail, or four forever stamps.
For an envelope that weighs 5 ounces, you would need 5 Forever stamps. Forever stamps are typically for 1-ounce letters, so you would need to use multiple stamps to cover the additional weight.
how many stamps for a 10.4 ounce envelope
how many stamps for a 3.4oz envelope
As of 2016, a 2.5 ounce envelope it would cost 93 cents (one Forever stamp plus two extra ounces).
Postage is determined by weight and by size. An envelope with three pieces of normal paper can normally be sent with one first class postage stamp. Additional stamps may be required based on additional ounces and on the size of the envelope.
After 3.5 ounces, the item moves up to large envelope rates. The current rate as of 2016 for an 11 ounce large envelope is $3.18.
The amount of stamps needed for an envelope is determined by weight. Weigh your a4 envelope and you will know how much postage is needed.