The cost of mailing something is directly tied to the size and weight of whatever you’re mailing. That makes it tricky to use stamps correctly. The benefit of stamps is that you can prepare your mail for shipping and drop it off in a receptacle, skipping the infamously time-consuming line at the post office. Here, then, is a guide to using stamps — which can be ordered online! — to get that envelope or package on its way with a minimum of hassle. This article is essentially… “How many stamps do I need?” answered.

Categories of Stamps: What Kind of Stamp Does Your Mail Need?

Stamps are a bit confusing, so here are the basics: it costs a certain amount of money to send something through the mail, depending on the package’s weight and method of delivery (First-Class, Priority, etc.). You can either pay that fee by going to the post office and paying the exact amount owed for that particular package — or you can simply use stamps to meet the required amount in postage fees.

Stamps are just a way of paying the mailing cost; however much you paid for the stamp is the amount in postage fees it covers (with the possible exception of Forever stamps — we’ll get to that in a moment). The question of how many stamps you need is essentially asking the cost of mailing the package, in terms of the available denominations of stamps.

Because knowing the weight of your mail is often crucial to affixing the correct postage, the bottom line is that you may be best off just going to the post office if you’re unsure how much your package or envelope weighs. Guessing the correct amount of postage is tricky. However, there are basic rates for various types of mail that will help guide you in estimating an accurate amount.

How many stamps go on a letter?

Once you know how much postage to affix, it’s easy to hit on the right combination of stamps, because they are available in denominations as low as $0.01 and as high as $23.75 for a Priority Mail Express large envelope. Other convenient denominations include $0.05, $1, $5, and $10, as well as a variety of Forever stamps that cover certain categories, detailed below:

  • Basic First-Class stamps are for standard business-sized or other rectangular envelopes weighing no more than one ounce. These stamps currently cost $0.49 each.
  • Additional ounce stamps work in conjunction with First-Class stamps for mail over one ounce. These stamps currently cost $0.21 each.
  • Two-ounce stamps cover rectangular mail up to two ounces for $0.70.
  • Three-ounce stamps will mail standard-sized rectangular envelopes with contents up to three ounces for $0.91.
  • Postcard stamps for standard-sized postcards are $0.34 each; postcards larger than 6” x 4 ¼” need a $0.49 First-Class stamp.
  • Non-machinable surcharge stamps are for square envelopes of no more than one ounce, or those of any shape (also no more than one ounce) with contents that make the envelope bulky. Envelopes like these need to be hand-cancelled by the post office and currently cost $0.70 each.
  • Almost all wedding invitations fall into the category of non-machinable, and many also weigh more than one ounce due to the enclosures. Wedding invitations typically need $0.91 in stamps, a denomination which is available under the Forever category of three-ounce stamps.
  • Global Forever stamps are for international mail, and are sold at the current international first-ounce rate of $1.15.

Forever stamps are purchased at the current market value. What makes them unique is that no matter how much you paid for them, they will always have the value of a current First-Class stamp — regardless of whether the price of stamps goes up or down after you’ve purchased them.

Estimating The Cost of Postage

For a one-off or occasional piece of mail, it may be worth it to simply estimate the cost of postage and add the requisite number of stamps, keeping in mind that you’ll lose the cost of any overpaid postage (think of it like paying fare on a bus; you have to meet the minimum cost with whatever denominations you have available, but you won’t get any change back). It’s better to overpay than to underpay, though, because an underpayment will most likely result in the mail being returned to you. If you send the mail without a return address, you will need to count on the recipient to pay the remainder at the post office.

The first-ounce rate for business envelopes is $0.49; for large envelopes, it’s $0.98; and for square envelopes, it is $0.70. Additional ounces cost $0.21 for all of these types of envelope.

The weight of paper:

  • A single sheet of 8 1/2-inch x 11-inch notebook paper is roughly .16 ounces.
  • A business-sized envelope weighs slightly less than .16 ounces.
  • Approximately four pieces of notebook paper plus the envelope can be mailed with one First-Class stamp (currently $0.49).
  • Depending on the weight of the paper, up to five sheets could fit under the one-ounce limit.

If you are planning to send a lot of mail of varying sizes, an inexpensive digital scale is recommended to facilitate the process of determining postage fees. Once you know the weight, plug the package’s dimensions into the USPS postage calculator and get the exact postage amount needed. Visit the USPS online store to buy your stamps.

You can use the postage calculator to help you determine your postage costs for any of the following categories of mail:

Domestic Mail

  • First-Class Mail
  • Priority Mail
  • Priority Mail Express
  • USPS Retail Ground
  • Media Mail
  • Library Mail

International Mail

  • First-Class Mail International
  • Global Express Guaranteed
  • Priority Mail Express International
  • Priority Mail International
  • Airmail M-Bags

Determining Mail Category

In order to pick the right category of mail, and therefore determine the correct first-ounce rate, here are the overall limits for each category.

Letters

  • The first-ounce rate is $0.49; additional ounces are $0.21
  • Includes business size and other rectangular envelopes
  • After 3.5 ounces, the mail is considered a large envelope

Large Envelopes/Flats

  • The first-ounce rate is $0.98; additional ounces are $0.21
  • May not exceed 12 inches high x 15 inches long x ¾ inch thick
  • Large envelopes that weigh more than 13 ounces will be considered parcels or packages

Parcels/Packages

  • Weight is necessary to determine postage
  • Parcels may be 13.01 ounces — 15.99 ounces and use First Class mail
  • Parcels over 15.99 ounces must upgrade to Priority Mail or Priority Express Mail

All About Envelopes

Envelopes come in a huge array of sizes, and multiple sizes are suitable to mail documents, booklets, invitations, and letters. The size that you choose will directly affect the price of postage for your mail (and therefore the number of stamps you need). The following are some of the most popular envelope sizes in each category, along with the rate of postage required for each. Remember that once you know the required postage fee, you can look back at our chart of stamp denominations to figure out what type and how many stamps are necessary. Check out this First Quarter Finance article if you need help locating places that sell envelopes.

Regular Envelopes

Regular envelopes are rectangular and often called business envelopes. Though many sizes are available, the most popular is the #10 envelope, measuring 4 1/8″ by 9 1/2″. Most regular envelopes have a solid front, though some have up to two windows. Some regular envelopes have special features that can include a security tint to conceal the contents and adhesive flaps for easy closure.

Regular envelopes can be mailed at the First-Class rate of $0.49 for the first ounce + $0.21 for each additional ounce. 

Standard Sizes / Dimensions

  • #10 Regular: 4 1/8″ x 9 ½”
  • #10 Double Window: 4 1/8″ x 9 1/2″
  • #3 Mini: 2 1/8″ x 3 5/8″
  • #7 Regular: 3 3/4″ x 6 3/4″
  • #9 Regular: 3 7/8″ x 8 7/8″
  • #11 Regular: 4 1/2″ x 10 3/8″
  • #12 Regular: 4 3/4″ x 11″
  • #14 Regular: 5″ x 11 1/2″
  • See examples of each type of regular envelope on the Action Envelope website.

Large Envelopes/Flats

Large envelopes are typically used for sheaves of papers or photos that can’t be folded. Large envelopes can be mailed at a first-ounce rate of $0.98 + $0.21 for each additional ounce, but can’t exceed a maximum thickness of 3/4 inch. The USPS uses the word “flats” to describe large envelopes, newsletters, and magazines. To qualify as a large envelope or flat, it must:

  • Have one dimension that is greater than 6 1/8″ high OR 11 1/2″ long OR 1/4″ thick
  • Be no more than 12″ high x 15″ long x 3/4″ thick

Standard Sizes / Dimensions

  • 5″ x 11″
  • 9″ x 12″
  • 10″ x 13″

Action Envelope provides some examples of document-style large envelopes with the flap along the long edge, and the open-end style of large envelopes with the flap along the shorter edge.

Padded Envelopes

Remember that the size of a padded envelope refers to the outer edges. The inner dimensions will be somewhat smaller, due to the outer binding and the padding on the inside taking up some room. Always buy a padded envelope larger than the expected contents.

Provided the envelope fits into the size and thickness requirements of a large envelope, it will ship for $0.98 for the first ounce and $0.21 for each additional ounce. However, padded envelopes are usually chosen for sending contents other than papers, and may be too bulky for this rate. It’s best to weigh your padded envelopes before affixing postage.

Visit this page of Staples.com to see some examples of padded envelopes.

Standard Sizes / Dimensions

  • 5″ x 9″
  • 6″ x 9″
  • 7 1/4″ x 7 1/4″
  • 8 1/2″ x 11″
  • 9 1/2″ x 13 1/2″
  • 9″ x 12″
  • 10″ x 13″
  • 10 1/2″ x 15″

Square Envelopes

Square envelopes are usually used for fancy items like wedding invitations. Most square styles have square flaps, but some feature deep triangular contour flaps. Sizes range from the tiny 3 1/4″ square to calendar-sized 12 1/2″ square.

Square envelopes need non-machinable surcharge stamps at $0.70 for the first ounce. It is still only $0.21 for each additional ounce. For wedding invitations that weigh 2 ounces, you’ll need a $0.91 stamp. You can see some examples of square envelopes on the Action Envelope website.

Standard Sizes / Dimensions

  • 3 ¼” x 3 ¼”
  • 4″ x 4″
  • 5″ x 5″
  • 6″ x 6″
  • 7″ x 7″
  • 8″ x 8″
  • 9″ x 9″
  • 12 ½” x 12 ½”

Square Flap Envelopes

These envelopes are used mostly for invitations; they come in a variety of sizes and a wide array of colors. Square flap envelopes are a stylish choice for items such as greeting cards, announcements, and photos. Note that square flap envelopes are still rectangular in shape, and so they are eligible for the First-Class rate of $0.49 for the first ounce + $0.21 for each additional ounce. See examples of square flap envelopes on the Action Envelope website.

Standard Sizes / Dimensions

  • A1: 3 5/8″ x 5 1/8″
  • A2 – 24lb. White Wove: 4 3/8″ x 5 3/4″
  • A4 Invitation: 4 1/4″ x 6 1/4″
  • A6 Invitation: 4 3/4″ x 6 1/2″
  • A7 Invitation: 5 1/4″ x 7 1/4″
  • A8 Invitation: 5 1/2″ x 8 1/8″
  • Slimline or #9: 3 7/8″ x 8 7/8″
  • #10: 4 1/8″ x 9 1/2″
  • Photo Greeting Invitation: 4 3/8″ x 8 1/4″

In Summary

The information in this article will help you estimate the right number of stamps for your mail. The United States Postal Service offers a helpful postage calculator if you know the weight and dimensions of your envelope or package. This handy chart from USPS can help you explore prices for various methods of mailing, including domestic and international mail. There is no shortage of available mailing methods and services through the United States Postal Service. Once you know the approximate dimensions and weight of your package, stamps are an easy and hassle-free way to affix the proper postage without waiting in line at the post office.