C3 Envelope
C-Series Envelopes (ISO 269) · 7 specs
Direct Answer
The C3 envelope is an ISO 269 standardized large postal packet format measuring 324 × 458 mm (12.76 × 18.03 inches). Designed specifically to hold flat, unfolded A3 documents (297 × 420 mm) or thick bound A4 portfolios, it is the standard packaging for architectural presentations, financial ledgers, and artwork shipments.
Source: ISO 269:1985 (DIN 678) · Last reviewed Aug 28, 2026
Specs
| Width × Height (Metric) | 324 × 458 mm |
| Width × Height (Imperial) | 12.76 × 18.03 in |
| Geometric Mean Origin | Geometric mean of A3 (297×420 mm) and B3 (353×500 mm) |
| Target Paper Pairing | Unfolded A3 (297 × 420 mm) or bound A4 spiral/comb portfolios |
| Standard Paper Clearance | +27 mm width / +38 mm length allowance for easy insertion |
| Common Closure Types | Peel-and-seal strip, string-and-button, cardboard gusset pocket |
| Primary Applications | Architectural drawings, financial balance sheets, graphic design proofs, diploma certificates |
The Geometric Envelope Formula
The ISO 269 standard defines envelope sizes as the geometric mean between the A-series and B-series (C_n = √A_n × B_n). Because a C3 envelope (324 × 458 mm) is proportionally larger than an A3 sheet (297 × 420 mm), it provides approximately 27 mm of side clearance, allowing a thick stack of up to 50 A3 blueprints to slide inside without wrinkling.