How Many Stamps Do I Need? The US Mail Postage Explained

How many stamps do you need?

Do you want to send mail, but you do not know how many stamps to buy? Postage can be confusing since the United States Postal Service (USPS) offers many different options.

As a direct mail automation & bulk mail service, we understand the importance of knowing your postage situation. Postalytics is here to help answer your questions and determine which service is best for you.

In this article about postage stamps, we will introduce you to the features of each of the US Postal Service’s shipping services and the prices associated with them.

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How many postage stamps do I need to send a letter? 

Two of the most frequently asked questions are, “how many stamps do I need to mail a letter?” and “Under what conditions do you need more than one stamp?” 

The answers depend on:

  • Size of the letter
  • Weight of the letter
  • Class of the service (e.g., First Class, Certified, Registered Mail, etc.).
  • The value of the stamps used 

Let us see how the stamps needed to send domestic letters, domestic envelopes, flat envelopes, domestic postcards, and international letters work. 

Domestic shipping stamps

Domestic letter mailing – price per ounce

If you are mailing a standard first-class letter within the U.S., Domestic Forever Stamps costs 58 cents and offers delivery within 1-3 business days. The U.S. Postal Service last raised the price of Domestic Forever Stamps from 55 cents to 58 cents in 2021. The cost to send each additional ounce for your mailpiece increased from 15 cents to 20 cents. 

Forever Stamps are postage stamps issued by the USPS. They are non-denominational, which means they will always be worth the current postage rate. However, you must remember that the rate covers the first ounce. If your envelope weighs more than one ounce, you will have to pay more per ounce.

Let us look at the postage you will have to pay for letters over one ounce with Domestic Forever Stamps under 3.5 ounces:

  • A letter of 1 ounce – 1 Domestic Forever Stamp costs 58 cents. 
  • A letter of 2 ounces – 1 Domestic Forever Stamp and one additional ounce stamp at 20 cents costs 78 cents.
  • A letter weighing 3 ounces – 1 Domestic Forever Stamp and two additional 20-cent stamps – costs 88 cents.
  • A letter weighing 3.5 ounces – 1 Domestic Forever Stamp, 1 Postcard Stamp, and one 10-cent stamp costs $1.08.

Note: Once your envelope exceeds the 3.5-ounce weight limit, you will need to look at rates for “flats,” which are large envelopes for mailings from 3.5-13 ounces. We will discuss postage rates for flats, other requirements, and costs for domestic envelopes below. 

Domestic envelope requirements and costs

To send a letter in an envelope, you must follow specific regulations. Therefore, you must measure your envelope before you purchase it.

These regulations are as follows:

  • The dimensions of the envelope are more important than you think. It must be no larger than 11.5 inches by 6.125 inches, no thicker than ¼ inch, and must be of uniform thickness. 
  • The envelope must be rectangular. Square envelopes must be at least five inches in size, are considered “non-machinable,” and are subject to an additional 21-cent postal charge. A “machinable” letter is one that can pass through the USPS’ sorting systems with no issues. 
  • The envelope must be bendable. If it is not, it will be classified as “non-machinable,” and you’ll have to pay an additional 21 cents. 

Flats requirements and costs for flats

As mentioned earlier, “flats” (or large envelopes) are required for shipments weighing between 3.5 and 13 ounces. They must meet the following requirements:

  • Flat envelope dimensions must be at least 6.125 inches by 11.5 inches and no larger than 12 inches by 15 inches.
  • Flat envelopes must not be thicker than 0.75 inches.
  • Bubble packet is not allowed as it makes it too thick. 

How many stamps do I need for a large envelope (flats)? As an example, for a 1 oz flat, you could use one Domestic Forever Stamp, 1×10-cents, and one postcard stamp, which would cost $1.08, or you could use 2 Domestic Forever Stamps and pay 8 cents too much. You need to weigh the convenience of Domestic Forever Stamps against the cost savings of using a combination of Forever Stamps, Postcard Stamps, and additional oz. Stamps. 

Domestic postcard shipping

If you want to mail a postcard within the United States, buying a postcard stamp for 40 cents is cheaper than a Domestic Forever Stamp, which costs 58 cents.

The postcard must meet the following requirements to be eligible for this class of stamp:

  • The postcard dimensions must be at least 3.5 inches high and 5 inches long. It must be no larger than 4.25 inches high and 6 inches long.
  • The postcard must be rectangular. 
  • The postcard must be at least 0.007 inches thick and no thicker than 0.016 inches.

International shipping letter postage

The easiest way to mail letters from the U.S. to the U.K. is with Global Forever Stamps, which will cost you $1.30. If you prefer to use Domestic Forever Stamps, make sure they total at least $1.30. You will pay more if your letter weighs more than one ounce, as Global Forever Stamps only cover up to two ounces. 

How many stamps do I need to send a package to the U.S.?

Calculating postage for parcels is much more complicated than calculating the cost of domestic letters. With parcel postage, you need to factor in the weight and size of the parcel and the distance it is being mailed. 

There are two ways to calculate how many stamps you need:

1. Calculating postage at the post office

Parcels are likely to be too large to put in a postbox. Therefore, you must take the package to the post office to mail it, and they will give you the stamp price there. 

2. Using a postage calculator

The United States Postal Service has developed an online calculator that makes it easy for you to calculate postage in advance. To use it, you need to know your mail’s size, weight, and class. Once you know this, enter the information into an online postage calculator to determine the postage cost.

Where to buy the stamps you need and the postage costs for 2024

There are several ways to buy stamps. You can buy them online from the U.S. Postal Store or other online retailers, buy them at a post office, or buy them at a gas station, grocery shop, or pharmacy.

Below is an overview of USPS mail/ship rates for 2024:


  • First-Class Mail letter (1 ounce) $0.58
  • First-Class Mail letter – each additional ounce $0.20
  • First-Class Mail flat/large envelope (1 ounce) $1.16
  • First-Class Mail flat/large envelope – each additional ounce $0.20
  • Postcard $0.40

Flat Rate Envelopes

  • Priority Mail flat rate envelope $7.75
  • Priority Mail legal flat rate envelope $8.05
  • Priority Mail padded flat rate envelope $8.45
  • Priority Mail Express Flat Rate Envelope $23.50
  • Priority Mail Express Legal Flat Rate Envelope $23.75

International Mail & Packages 

  • First Class Mail Intl letters (1 oz.) $1.30
  • First Class Mail International Flats (1 oz.) from $2.60


  • First-Class Package Intl Service – Canada (1 lb.) from $21.00
  • Priority Mail Intl (1 lb.) – Canada from $37.47
  • Priority Mail Intl Flat Rate Envelopes – Canada $28.13
  • Priority Mail Express Intl Flat Rate Envelopes – Canada $44.95

What happens if I don’t have the right amount of stamps?

If you do not have a return address on the package and don’t use enough stamps, the post office will attempt to collect postage from the recipient.

However, if they refuse to pay the postage and there is no return address, the package will be sent to “dead letters.”

A better way to manage shipping costs

If you want to send a letter or postcard, make sure you put the correct address and stamps on it. You can then drop the letter in your postbox, in a blue collection box, or at your local post office.

Using stamps can be tricky, but you do not have to be frustrated. You can use the stamp cost guide above to calculate postage, or you can use the USPS postage calculator to determine the cheapest combination of stamps for a letter or package. 

Whether you are sending a letter or a postcard, you can rest assured that Postalytics can help you organize your mail and provide the best postage rates on the market. You do not need stamps with our online mailing service because postage is included in our prices and our postcards and envelopes are prestamped. 

It is more efficient to send standard sizes than special sizes that the automated mail processing equipment cannot process.

If you use Postalytics templates, you will enjoy the postage stamp savings the USPS offers and save the headache of calculating and paying for postage. Please get in touch with us for more information on postage costs or discuss which option is best for you.

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