Ever stared at your envelope, wondering: How many stamps do I need to mail this? Maybe you've slapped on an extra stamp because you're not sure how much weight a postage stamp covers. (You’re not alone!) The good news? There's actually a lot you can mail with just one USPS Forever Stamp. So, here's how to answer the question:
How Many Stamps Do I Need to Mail a Letter?
Whether you're sending a birthday card, a love letter, or some important documents, knowing the USPS rules can save you stress (and stamps). Let’s break it all down, nice and easy:
⚖️ Rule #1: Stay Under the USPS 1 Ounce Weight Limit
Forever Stamps cover up to 1 ounce. But how much is that, exactly?
Here’s a quick cheat sheet:
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💌 4 sheets of standard 8.5" x 11" copy paper = ~1 ounce
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🃏 One full suit from a deck of playing cards (like all the hearts) = just under 1 ounce
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📬 Most single greeting cards = safely under 1 ounce
If you're not sure? A cheap kitchen scale will become your best friend.
💌 Rule #2: Your Envelope Must Be Rectangular
USPS machinable mail = rectangular only.
Square envelopes? Cute, but nope. USPS doesn’t consider a square a rectangle (even if geometry says otherwise). Sending a square envelope means you’ll need extra postage.
💡 Pro tip: Stick to standard, rectangular envelopes to stay in the one-stamp zone.
📐 Rule #3: Follow the Standard Letter Size Guidelines
To qualify for one Forever Stamp, your envelope must fit within these dimensions:
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Minimum: 5” long x 3.5” high
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Maximum: 11.5” long x 6.125” high
Anything smaller or larger? USPS calls that "non-machinable." Translation: extra postage.
📦 Rule #4: Keep It Slim — No Lumps or Bumps
The max thickness allowed is ¼ inch. So skip the bulky add-ins or chunky cards with raised embellishments.
Rigid or oddly shaped envelopes often need to be hand-canceled, which bumps you up into a higher price tier.
📭 Rule #5: Your Mail Has to Be Flexible
USPS mail goes through automated sorting machines. If your envelope is stiff, thick, or can’t bend? It’s officially non-machinable and will cost extra.
Things that make mail rigid:
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Cardboard inserts
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Photo prints taped to cardstock
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Overstuffed envelopes with stickers, bookmarks, etc.
✨ Rule #6: Fancy Envelopes = Possible Extra Postage
Planning to mail something extra beautiful?
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Wax seals
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Ribbon or twine
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Decorative clasps or layers
All of these can make your envelope non-machinable. That doesn’t mean you can’t use them—but you’ll likely need to:
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✅ Take it to the post office and ask for hand-canceling, OR
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✅ Place it inside a plain envelope for mailing
So, How Many Stamps Do You Need to Mail a Letter?
Here’s a list of examples of how much weight a single postage stamp covers:
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✅ Standard letters (under 1 oz)
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✅ Greeting cards (under 1 oz, no rigid bits)
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✅ Postcards (though they actually require less than a Forever Stamp!)
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✅ Checks, invoices, or other light documents
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✅ Pen pal mail, as long as it’s bendable and slim
Bottom Line: When In Doubt, Check the Weight
One forever stamps is pretty forgiving—but not limitless. Stick to standard size, flexible shape, and under 1 oz, and you’re golden.
Everything else? It’s not a big deal—you’ll just need a few extra cents of postage.
