It depends mainly on the weight. Take it to the post office and they will happily calculate this for you. (I can't actually guarantee they'll be happy about it, though they certainly should be happy that someone is still using the US mail.)
how many stamps for a 10.4 ounce envelope
4
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.
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.
If you envelope weighs less than one ounce, it will take one forever stamp or one current first class stamp. If the letter and envelope weigh more than one ounce, the number of stamps will depend on the weight and the denomination of the stamps.
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.
A 4-ounce large envelope currently costs $1.57.
The number of stamps needed for an 8x10 envelope with 4 sheets of paper depends on the weight of the envelope after adding the sheets. Generally, if the total weight is under 1 ounce, you would typically need one first-class stamp. If the weight exceeds 1 ounce, you may need additional stamps, usually adding one for each additional ounce. It's best to weigh the envelope or check with the postal service for exact postage requirements.
The number of stamps needed to send a 9x12 envelope depends on its weight and the mailing service you choose. For a standard 9x12 envelope weighing up to 1 ounce, you typically need two First-Class stamps. However, if the envelope exceeds 1 ounce, you'll need additional postage based on the weight. It's best to check the current USPS rates or use their online postage calculator for accurate pricing.
2
Currently in the US up to one ounce is 45 cents. So however many stamps it takes to make that amount.
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.