C3 vs C4: C3 is exactly twice the area of C4. Both share the ISO 1:√2 aspect ratio, so scaling between them is seamless.
| Property | C3 | C4 | Ratio |
|---|---|---|---|
| Width (mm) | 324 | 229 | 1.415× |
| Height (mm) | 458 | 324 | 1.414× |
| Width (in) | 12.76 | 9.02 | — |
| Height (in) | 18.03 | 12.76 | — |
| Area (mm²) | 148,392 | 74,196 | 2× |
| Pixels @ 300 DPI | 3,827 × 5,410 | 2,705 × 3,827 | — |
| Aspect ratio | 1:√2 | 1:√2 | Same |
| Fits | A3 unfolded | A4 unfolded | — |
C3 to C4 Envelope Conversion — Everything You Need to Know
The ISO 269 C-series is the international standard for envelopes. The C3 and C4 sizes are two of the most widely used envelope formats in offices, print shops, and postal systems worldwide. This guide explains how C3 and C4 compare, how to convert between them, what documents they hold, and how to calculate pixel dimensions for print or design.
📐 What Is a C3 Envelope?
C3 measures 324 × 458 mm (12.76 × 18.03 inches). It is designed to hold an unfolded A3 sheet (297 × 420 mm) with a comfortable margin on all sides. C3 is used for:
- Large-format mailings — portfolios, architectural drawings, certificates
- Unfolded A3 documents — large brochures, presentations, proofs
- Postal tubes and rigid mailers — thick documents that must stay flat
- Promotional mailers — posters, art prints, and large flyers
📐 What Is a C4 Envelope?
C4 measures 229 × 324 mm (9.02 × 12.76 inches). It is designed to hold an unfolded A4 sheet (210 × 297 mm) flat, without folding. C4 is the most common business envelope in Europe and is used for:
- Business letters — A4 documents, invoices, contracts
- Certificates — unfolded A4 awards and diplomas
- Reports and proposals — professional mailings
- Brochures — unfolded A4 marketing pieces
🔄 The Relationship Between C3 and C4
Both C3 and C4 belong to the ISO 269 C-series, which shares the 1:√2 aspect ratio with the A and B series. The key relationship is:
📐 Area
C3 = 2 × C4C3 has exactly twice the area of C4. Folding or cutting a C3 in half yields two C4 envelopes.
📏 Dimensions
1.414× largerEach dimension of C3 is 1.414× (√2) the corresponding dimension of C4.
Mathematically:
- C3 width: 324 mm ≈ 229 × √2 (229 × 1.4142 = 323.86 mm)
- C3 height: 458 mm ≈ 324 × √2 (324 × 1.4142 = 458.2 mm)
📊 C3 to C4 Conversion Table
| DPI | C3 Pixels | C4 Pixels | Best For |
|---|---|---|---|
| 72 DPI | 919 × 1,298 | 649 × 919 | Web previews |
| 150 DPI | 1,913 × 2,705 | 1,353 × 1,913 | Draft print |
| 200 DPI | 2,551 × 3,606 | 1,803 × 2,551 | Standard print |
| 250 DPI | 3,189 × 4,508 | 2,254 × 3,189 | Good print |
| 300 DPI | 3,827 × 5,410 | 2,705 × 3,827 | Professional print |
| 400 DPI | 5,102 × 7,213 | 3,606 × 5,102 | Premium print |
| 600 DPI | 7,654 × 10,819 | 5,410 × 7,654 | Archival / photo |
| 1200 DPI | 15,307 × 21,638 | 10,819 × 15,307 | High-res scanning |
📏 C3 and C4 in All Units
✉️ Which Envelope Should You Use — C3 or C4?
Choose C4 if…
You are sending an unfolded A4 document — invoices, contracts, business letters, certificates. C4 is the standard business envelope.
Choose C3 if…
You are sending an unfolded A3 document — drawings, posters, large certificates, or a portfolio that must remain flat.
Cost consideration
C4 is more economical and standard. Use C3 only when the A3 document cannot be folded or rolled.
Postal rates
C4 falls under standard letter rates in most postal systems. C3 may require large-letter or parcel postage depending on weight.
🔄 How to Use This C3 to C4 Converter
- Select the size — C3 or C4 — from the dropdown.
- Enter the DPI — 72 for web, 300 for print, 600+ for archival.
- Click Convert — the tool instantly shows mm, inches, pixels, and what the envelope holds.
- Use the comparison table to see both sizes side by side.
- Bookmark this page for quick envelope-size checks.
❓ Frequently Asked Questions
How much bigger is C3 than C4?
C3 is exactly twice the area of C4. Each dimension is √2 (≈1.414) times larger. C3 is 324 × 458 mm; C4 is 229 × 324 mm.
Can I fold a C3 to fit in a C4?
Yes — folding a C3 in half along its long edge produces a sheet that fits neatly inside a C4 envelope, because both sizes share the ISO 1:√2 aspect ratio.
What size document fits in a C4 envelope?
A C4 envelope holds an unfolded A4 sheet (210 × 297 mm) with a margin. It also holds A5 folded, A6 folded, or similar-sized documents.
What size document fits in a C3 envelope?
A C3 envelope holds an unfolded A3 sheet (297 × 420 mm) with margin. It also holds A4 unfolded twice, or A2 folded.
What are C3 and C4 in pixels at 300 DPI?
At 300 DPI: C3 = 3,827 × 5,410 px and C4 = 2,705 × 3,827 px. These are the recommended print-ready dimensions.
Is C3 the same as A3?
No. C3 is an envelope size (324 × 458 mm), while A3 is a paper size (297 × 420 mm). C3 is designed to hold A3 paper.
What is the difference between C-series and B-series envelopes?
The C-series is specifically for envelopes and is sized to fit A-series paper. The B-series is a paper size and is sometimes used for envelopes but is not envelope-specific.
Can I print directly on a C3 or C4 envelope?
Yes, but check your printer's supported sizes. Many office printers handle C4 and smaller; C3 typically requires a large-format printer. Always verify margins and feed orientation.
🎨 Design Tips for C3 and C4 Envelope Printing
- Bleed: Add 3–5 mm bleed on all sides for full-bleed designs.
- Safe zone: Keep text 5–8 mm from the edges to avoid trimming issues.
- Color mode: Use CMYK for print; RGB only for digital proofs.
- Resolution: 300 DPI for professional print; 150 DPI is acceptable for in-house mailers.
- File format: PDF/X-1a or PDF/X-4 is recommended for print shops.
- Address placement: Keep the address block within the postal service's window zone if using window envelopes.
🌍 C3 and C4 Around the World
- 🌍 Europe, Asia, Africa, Australia: ISO 269 C-series is the standard for envelopes.
- 🇺🇸 United States: Uses its own envelope sizes (e.g., #10, 9×12), but C4 is increasingly common for international mail.
- 🇨🇦 Canada: Uses a mix of ISO and North American envelope sizes.
- 🇬🇧 United Kingdom: C3, C4, C5, and C6 are the most common envelope sizes.
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This C3 to C4 conversion page is built for fast indexing and ranking on Bing and other search engines using a full on-page, technical, and off-page SEO strategy:
- On-page SEO: Keyword-targeted title, meta description, H1–H3, structured tables, internal links to all C-series pages, and long-form FAQ content.
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msvalidate.01verification,bingbot/msnbotdirectives, IndexNow-ready meta, and content formatted for Bing's preference for clear tables and FAQs. - Off-page SEO: Internal links from the Paper Sizes hub and every C-series page, plus quick-jump links from the C-series family.
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Last updated: January 2026. This C3 to C4 envelope conversion guide is regularly reviewed for accuracy and completeness.