A standard #10 envelope 4.5″ x 9.75″ weighs 0.238 ounces (6.75 grams), on average.
There are a bunch of different types of envelopes the #10 is the most common commercial envelope that is used. You can assume an envelope of any shape and paper weight will weigh 0.1 to 0.5 ounces.
The Weight of Different Envelopes
This measurement was done in house with a scale, and not to reflect all paper weights and manufacturers.
0.238 ounces - #10 envelope 4.5″ x 9.75″ -The "classic" business envelope
0.229 ounces - #9 envelope 3 7/8" x 8 7/8" - Usually the one you see inserted inside a #10 as a business mail reply envelope.
0.222 ounces #8 envelope 3 5/8" x 8 5/8" - Fits a check even a larger business sized check
0.247 ounces 6 x 9 envelope - The typical size for holiday cards, letters, and post cards
Remember the weight of the envelope will increase as items are placed inside, and it's important to accurately measure the weight each time you mail a letter to determine the cost of shipping through the USPS.
Notice how the 6 x 9 is heavier than the others? This was done with a fancy paper stock that is used to send out holiday mailers. So its a bit heavier than typical commercial paperstock.
But most people just want to know how many pieces of paper they can put in an envelope with one stamp.
How to Calculate your Mail Price
One piece of paper is on average 0.1696 ounces. So take 0.238 + 0.1696 = .40 ounces for a one page letter. This is below the threshold for a regular piece of first class mail, meaning you will only need one stamp for this.
The general rule is that a letter with 5 pieces of paper inside is the max allowed for one forever stamp (currently at $0.63 for up to 1 oz).
Anything above 5 pages in your envelope you should consider putting two stamps on the letter.
first class has a max of 3.5 ounces, which should be more than enough for any regular envelope.
USPS Size and Weight Rules
The weight of everything the Postal Service does is typically measured in ounces, here is their current pricing.
If you are mailing a larger envelope or a thicker envelope made from heavier materials, such as a padded envelope, the weight can quickly add up. There is a max allowed for first class mailing in terms of envelope size. Here is the USPS guide for maximum sizes allowed for mailing a first class letter
Take all of this into consideration when mailing to make sure you meet the USPS guidelines. But for most instances a regular envelope with a few pieces of paper will only require one stamp.