The number of stamps needed to mail a letter usually depends on how much it weighs. Checking the weight with a postage scale can be ideal, but if you need to mail something and don't have a scale handy, you can estimate the weight based on how many sheets of paper are inside. If you have the paper package handy, there should be information on the package that can help you estimate how much your letter weighs.
TL;DR (Too Long; Didn't Read)
Besides using a scale to measure the paper's weight, you can do some calculations using the weight indicated on the paper ream to estimate the weight.
Letter Paper Size and Weight
Letter paper that you would use in a printer or copier is traditionally sold in reams, or packages of 500 sheets of paper. There's usually a weight indicated on the package of paper. That weight can be a bit confusing because it doesn't mean how much the package actually weighs, but it rather indicates how much the sheets of paper weighed before they were cut down to letter size.
Letter paper is 8 ½ by 11 inches, but it's usually weighed uncut at 17 by 22 inches, meaning that each uncut sheet generates four cut sheets. That means that the weight printed on a ream of letter paper is actually four times the weight of the paper in the container. One of the most common letter paper weights is 20 pounds, for instance, so a ream of this paper actually weighs 5 pounds.
Calculating Paper Weights
Once you determine how much a ream of paper weighs, you can then divide by 500 to see how much an individual sheet weighs. For instance, in that ream that weighs 5 pounds, each sheet weighs 0.01 pounds. It's often more useful to express weight in ounces. Remember that a pound is 16 ounces and multiply the number of pounds by 16 to learn that a sheet from a ream labeled 20 pounds actually weighs 0.16 ounces.
Copy paper is generally sold in 20-pound reams, so if you don't know for sure what type of paper you have but it looks to be standard copy paper, you can assume it's about 5 pounds per 500 sheets.
Estimating Postage by Weight
Once you figure out how much each sheet of paper weighs, you'll need to multiply by the number of sheets that you have to find the total weight. For instance, 10 sheets of paper that each weigh 0.16 ounces weighs a total of 1.6 ounces.
Then, you can consult the U.S. Postal Service rate chart or search for another shipping service you would want to use, such as UPS or FedEx, to determine how much you'll have to pay to ship your document. Traditionally, with USPS, one stamp will cover standard letters up to 1 ounce, and each additional stamp will cover at least another 2 ounces, so those 10 sheets of copy paper could be mailed with two stamps. If you're unsure whether you have enough postage and you can spare the stamp, you might wish to add an extra one to make sure the package arrives without postage due.
Items not sent in a traditional letter envelope can be more expensive, as can special services like Priority Mail, and carriers besides USPS set their own rates. When in doubt, bring your letter to a post office or a shipping service to have it weighed and priced.
Steven Melendez is an independent journalist with a background in technology and business. He has written for a variety of business publications including Fast Company, the Wall Street Journal, Innovation Leader and Ad Age. He was awarded the Knight Foundation scholarship to Northwestern University's Medill School of Journalism.