You’re standing in front of a mailbox with an envelope in hand and a sheet of stamps nearby. The question hits—stamps how many do I need? Whether you’re sending a birthday card, a business invoice, or a small package, knowing how many stamps you need has become more confusing as postal rates shift and mailing categories vary. Fortunately, the answer doesn’t have to be complicated.
Understanding current postage rates, weight thresholds, envelope types, and class of mail can help you quickly figure out how many stamps are needed without guessing—or overpaying.
The Basic Rule of First-Class Mail
One Forever Stamp Covers One Ounce
As of 2025, a Forever stamp covers the cost of mailing a standard first-class envelope weighing up to 1 ounce. That includes most personal letters and standard greeting cards. If your envelope exceeds 1 ounce, you’ll need to add more postage.
Here’s a quick reference for typical first-class letters:
- 1 oz or less: 1 Forever Stamp
- 2 oz: 1 Forever Stamp + 1 additional ounce stamp
- 3 oz: 1 Forever Stamp + 2 additional ounce stamps
- 3.5 oz (maximum for letter rate): 1 Forever Stamp + 3 additional ounce stamps
Keep in mind that anything over 3.5 ounces or exceeding standard dimensions becomes a “large envelope” or “flat,” and different rates apply.
What Counts as a Letter?
Standard Size and Weight Limits
Before asking stamps how many do I need, make sure your envelope qualifies as a letter. USPS defines a letter as:
- Rectangular shape
- At least 3.5″ high x 5″ long x 0.007″ thick
- No more than 6.125″ high x 11.5″ long x 0.25″ thick
- Weighs 3.5 ounces or less
If your item doesn’t meet these dimensions, it will fall into the flat or package category, which requires more postage.
Sending Large Envelopes (Flats)
More Paper Means More Stamps
Large envelopes, often used for documents, catalogs, and oversized cards, have their own postage rate. The first ounce for a flat usually costs more than a standard letter, and each additional ounce increases the rate.
For example:
- 1 oz flat: Requires 2 Forever stamps
- 2 oz flat: 2 Forever stamps + 1 additional ounce stamp
- 3 oz flat: 2 Forever stamps + 2 additional ounce stamps
Instead of stacking several Forever stamps, you can also use a combination of Forever and additional ounce stamps, or simply take it to the post office for exact postage.
Non-Machineable Surcharge
Don’t Let Shape Cost You Extra
If your envelope is square, rigid, or has decorative elements like ribbons or clasps, it may incur a non-machineable surcharge. This is an extra fee because the envelope can’t be processed through automatic sorting machines.
To cover this, you need:
- 1 Forever Stamp + 1 non-machineable surcharge stamp
You can also use two Forever stamps, but you’ll be slightly overpaying. Non-machineable surcharge stamps are typically marked with a butterfly or similar design.
International Mail
Different Countries, Different Rates
Sending a letter internationally requires more postage. A Global Forever Stamp covers a 1-ounce letter to any country outside the U.S.
- 1 Global Forever Stamp = 1 oz letter to any country
- Heavier items will require additional postage based on weight and destination
If you don’t have Global Forever stamps, you can use multiple U.S. Forever stamps totaling the required amount—just ensure it adds up correctly.
Packages and Parcels
How Many Stamps to Send a Box?
If you’re mailing a small box or padded envelope, you’ll need to calculate postage based on weight and size. USPS Retail Ground, Priority Mail, and First-Class Package Service all have different rates.
While technically you can use stamps to pay for parcels, it’s not recommended. You’ll need a precise weight and possibly dozens of stamps. It’s better to buy a shipping label or go to the post office for accurate postage.
Priority Mail Flat Rate Packages
How Many Stamps Do You Need?
USPS Priority Mail Flat Rate is a popular option for shipping boxes regardless of weight (up to 70 lbs). Each flat rate package has a set price, so you can use stamps to pay the exact amount—as long as you use enough of them to match the required postage.
As of 2025, here’s what you’ll need:
Priority Mail Flat Rate Envelope – $9.85
You’ll need approximately 15 Forever stamps at the current rate of $0.66 each to meet or slightly exceed this total. (15 x $0.66 = $9.90)
Priority Mail Small Flat Rate Box – $10.40
You’ll need 16 Forever stamps (16 x $0.66 = $10.56)
Priority Mail Medium Flat Rate Box – $17.10
You’ll need 26 Forever stamps (26 x $0.66 = $17.16)
Priority Mail Large Flat Rate Box – $22.80
You’ll need 35 Forever stamps (35 x $0.66 = $23.10)
You can also mix in stamps of other denominations to get closer to the exact amount. But because stamp values can’t be split or adjusted like digital postage, it’s often easier to round up and slightly overpay.
If you plan to ship regularly using Flat Rate boxes, consider using printed labels instead. Stamps work, but they take up space, can be cumbersome, and sometimes raise suspicion if overused.
Using a Postal Scale
The Best Tool for Accurate Postage
A small digital postal scale can save you time and money. Weighing your envelope at home lets you calculate exact postage. Combine this with the current USPS rate chart, and you’ll never have to guess again.
You can also use online USPS calculators or shipping software to get the most up-to-date postage pricing.
Real-Life Examples
Common Scenarios for Stamp Use
Sending a Birthday Card
Most greeting cards fall under 1 ounce and can be mailed with one Forever stamp—unless it’s bulky or square.
Mailing a Wedding Invitation
These often exceed 1 ounce and may be non-machineable. You’ll likely need 1 Forever stamp + 1 additional ounce + 1 non-machineable surcharge.
Sending Legal Documents in a Large Envelope
If your envelope is 9″x12″ and contains multiple sheets, it might weigh 2 or 3 ounces. Expect to use at least 2 or 3 Forever stamps.
Shipping a Medium Flat Rate Box
Use 26 Forever stamps or mix with higher denomination stamps to hit $17.10 exactly.
Avoiding Overpayment
Stop Guessing, Start Measuring
Many people throw extra stamps on an envelope just to be safe. While that ensures delivery, it wastes money over time. With Forever stamps currently valued over 60¢, guessing wrong on a dozen pieces of mail adds up.
Using additional ounce stamps or checking weight before you apply postage prevents overpayment. You can also group your stamps by value and plan combinations more effectively.
What If You Use Too Few Stamps?
USPS Policy on Underpaid Mail
If you mail something with insufficient postage, the USPS may return it to you for more postage or deliver it with postage due. In the latter case, the recipient will be required to pay the missing amount before receiving the letter.
To avoid this, always double-check weight and requirements—especially for bulky or irregular items.
Selling Extra Forever Stamps
Too Many Stamps? Turn Them Into Cash
If you find yourself constantly asking stamps how many do I need and realizing you’ve overbought, you’re not alone. Many people stock up on stamps for holiday cards, special events, or business mailings—only to find themselves with extra sheets or rolls long after they’re needed.
That’s where SellForeverStamps.net can help. We buy unused Forever stamps in mint condition and pay you cash based on current rates. Whether you have leftover wedding postage or hundreds of extra stamps from a business order, we offer a fast and reliable way to turn them into money.
When to Reevaluate Your Stamp Supply
Knowing When Enough Is Enough
If your mailing needs have changed—maybe you moved to digital billing, stopped sending paper invites, or downsized your business—you might not need all those stamps. Periodically reviewing how much mail you send vs. how many stamps you keep on hand can save money and reduce clutter.
Unused postage is still valuable, but if it’s sitting unused for years, it might be time to sell.
Recap: How to Figure Out How Many Stamps You Need
A Practical Summary for Everyday Mail
- Use 1 Forever stamp for standard 1-ounce letters
- Add additional ounce stamps as weight increases
- Use 2 Forever stamps or more for large envelopes
- Add non-machineable surcharge stamps for rigid or square mail
- Use Global Forever stamps for international mail
- For packages, weigh your item and use exact postage
- Use 15 to 35 Forever stamps for Priority Mail Flat Rate based on box size
- Avoid overpaying by weighing mail and using the USPS calculator
- Sell unused stamps at SellForeverStamps.net if you have more than you need
Asking stamps how many do I need doesn’t have to be a mystery. With a little knowledge and the right tools, you can mail smarter and save money.