Mailing documents requires the perfect pairing between paper and envelope. Our Envelope Size Calculator helps you choose the correct ISO C-Series or North American envelope for your needs. Whether you're sending an unfolded A4 certificate in a C4 envelope or a tri-folded Letter in a #10 business envelope, our tool ensures your stationary is professional and correctly sized for the post.
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Envelope Size Converter
Find the right envelope dimensions and see which paper sizes fit inside (ISO 269 & US standards).
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Standard Envelope Chart
Best ForA4 Paper (flat)
Envelope Dimensions
229 × 324 mm
C-Series Standard
The **C-series** (ISO 269) is designed specifically for A-series paper. A size **Cn** is the geometric mean of An and Bn paper, ensuring a perfect fit.
Folding Tip
If you don't want to fold your paper, match the number: **A4 fits in C4**. For one fold (half size), go up one number: **A4 fits in C5** when folded once.
About Standard Envelope Size Chart
1
Enter Paper Size
Select the paper you want to mail (e.g., A4).
2
See Envelope
The tool recommends the appropriate envelope size.
3
Review Results
Verify the output looks correct before using or downloading it.
4
Save Your Output
Click the download or copy button to export your results to your device or clipboard.
Key Features
Paper-to-Envelope Mapping
Matches your document size (unfolded, half-folded, or tri-folded) to the correct envelope.
C-Series ISO Support
Complete dimensions for the international C4, C5, C6 and DL standards.
Business Envelope Guide
Includes North American standards like the #10, #9, and invitation (A-series) sizes.
Frequently Asked Questions
A C4 envelope fits an unfolded A4 sheet. A C5 fits an A4 folded in half.
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