When you are sending a letter that is especially heavy or large, or a package of any size, you will need to know the correct postage in order to determine how many stamps you will need. Not only do you need to understand the calculations but also how to correctly weigh the item to ensure it gets the postage it requires. Having a weight scale specifically designed for mailing packages is helpful, but not necessary. An ordinary kitchen scale will do just fine.
Prepare your parcel. This includes putting the documents into the envelope and sealing it or preparing the shipping box with the items to be sent, taped and sealed. This makes the measurement as accurate as possible.
Set your kitchen scale so that the needle is slightly past zero for optimum accuracy. Place the envelope or package on the scale and write down the weight. If sending a package, you will need to measure it with a measuring tape.
Enter your shipping information on the "Postage Price Calculator" at the USPS website. You will be entering the zip code, date and time of shipment, type of service and shape of box. Click "Continue" once you have entered this information, then choose from the list of shipping options (first-class, priority, express). This will give you the postage price for your shipment.
Divide your postage price by the current postage stamp rate, which is 44 cents as of 2011. For example, if your postage price is $1.50, divide that by $0.44 and get 3.40. Unless you have 10 cent stamps, you will need to round up and place four stamps on the package. You can also use an online postage stamp calculator that will tell you how many stamps you need.