C-Series Envelope Size Calculator

Select a C-series envelope size to instantly see its dimensions in millimetres and inches, which A-series paper it fits, how to fold the paper, and the envelope area.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Select C Envelope Size
Choose the specific C-series envelope size (C0-C8) you want to look up, such as 'C4'.
2
Review Your Results
Instantly view its dimensions in millimeters and inches, the corresponding A-series paper it fits, and recommended folding methods.

Example Calculation

A graphic designer needs the exact dimensions for a C4 envelope to fit an A4 document.

C Envelope Size (select)

Results

229 × 324 mm

Tips

Understand the A/B/C Series Relationship

The C-series envelopes are designed to perfectly fit a corresponding A-series paper size, or a B-series paper folded. For example, a C4 envelope fits an A4 sheet unfolded, while a C5 fits an A4 sheet folded once (to A5 size). This nested system ensures efficient mailing.

Consider Material Weight for Mailing Costs

Larger envelopes like C0 or C1 are substantial and often require specialized mailing due to their size and weight. Even for standard sizes, the combined weight of the envelope and its contents can significantly impact postage costs. Always check postal service guidelines for weight and dimension limits.

Use C-Series for International Mail

The ISO 269 standard for C-series envelopes is recognized globally, making them ideal for international correspondence and professional documents. This ensures compatibility and reduces confusion when sending mail across different countries that primarily use the ISO paper size system.

Understanding ISO C-Series Envelope Dimensions

The C-Series Envelope Size Calculator provides precise dimensions for all ISO C-series envelopes, from the expansive C0 to the compact C8. This tool is essential for graphic designers, print shops, and anyone needing exact measurements for professional correspondence or packaging. It details sizes in both millimeters and inches, specifies the corresponding A-series paper size that fits, and indicates the necessary folding requirements. For example, a C4 envelope measures 229 × 324 mm, perfectly accommodating an A4 sheet of paper without folding.

The Mathematical Basis of ISO Paper and Envelope Sizes

The ISO 216 standard, which defines the A, B, and C series of paper and envelope sizes, is ingeniously based on a single mathematical principle: an aspect ratio of √2 (approximately 1.414). This means that if a sheet of paper is cut in half along its longer side, the two resulting pieces retain the exact same proportions as the original. This principle, codified in Germany's DIN 476 standard before its international adoption, ensures that scaling up or down maintains consistent aesthetics and functionality. For instance, an A0 sheet has an area of exactly one square meter, and each subsequent A-size (A1, A2, etc.) is half the area of the previous one, while maintaining the √2 aspect ratio.

Deciphering C-Series Envelope Dimensions

The C-Series Envelope Size Calculator relies on the globally standardized ISO 269 system, which defines envelope dimensions based on the ISO 216 paper sizes. The core logic involves a lookup table for each C-size, providing its exact width and height.

Envelope Dimensions = Width_mm × Height_mm Dimensions (inches) = Width_in × Height_in Fits Paper Size = Corresponding A-series Fold Method = Description based on C-size (e.g., A4 unfolded for C4, A4 folded once for C5)

This system ensures that an A-series sheet can always fit into its corresponding C-series envelope (or a C-series envelope can fit into a B-series envelope) with minimal clearance.

💡 Understanding dimensions is key across various design tasks. If you're calculating volumes for containers, our Sphere Vessel Volume Calculator provides similar precision for spherical shapes.

Worked Example: Sizing a C4 Envelope

Let's say a professional needs to determine the exact specifications for a C4 envelope.

Select "C4" from the available envelope sizes.
Retrieve Data: The calculator looks up the predefined dimensions for C4.
- Width: 229 mm
- Height: 324 mm
Convert to Inches:
- Width: 229 mm / 25.4 mm/inch = 9.02 inches
- Height: 324 mm / 25.4 mm/inch = 12.76 inches
Determine Paper Fit: A C4 envelope is designed to perfectly fit an A4 sheet of paper without any folding.
Calculate Area and Perimeter:
- Area: (229 mm × 324 mm) / 100 = 742.76 cm²
- Perimeter: 2 × (229 mm + 324 mm) = 1106 mm

The C4 envelope measures 229 × 324 mm (approximately 9.02 × 12.76 inches) and is ideal for mailing unfolded A4 documents.

💡 For other applications involving geometric properties and precise measurements, our Vertex Form of a Parabola Calculator can help analyze curves and points.

The Mathematical Basis of ISO Paper and Envelope Sizes

Choosing the Right C-Series Envelope for Professional Use

Professionals across various fields, from graphic designers to postal services, rely on C-series envelopes for specific applications. Graphic designers often choose C4 envelopes for sending important A4 documents like résumés or contracts, ensuring they arrive pristine and unfolded. For brochures or invitations, a C5 (fitting A5 paper) offers a more compact and elegant presentation, often saving on postage for lighter items. Postal services appreciate the standardization of C-series sizes as it streamlines automated sorting processes, with specific machine compatibility requirements for different dimensions. For instance, an envelope larger than 250 x 353 mm (larger than C4) might be classified as a "large letter" or "parcel" by many postal regulations, incurring higher costs. The choice often balances document integrity, brand perception, and mailing efficiency.

Frequently Asked Questions

What is the purpose of the ISO C-series envelope sizes?

The ISO C-series envelope sizes are designed to complement the ISO A-series paper sizes, allowing A-series documents to fit snugly without extra folding. For instance, a C4 envelope is perfectly sized to hold an A4 sheet of paper unfolded, while a C5 holds an A5 sheet (which is an A4 sheet folded once). This systematic approach ensures efficiency and professionalism in document handling and mailing worldwide.

How does a C4 envelope relate to A4 paper?

A C4 envelope is specifically designed to hold an A4 sheet of paper without requiring any folds. The dimensions of a C4 envelope (229 x 324 mm) are slightly larger than an A4 sheet (210 x 297 mm), providing just enough clearance for easy insertion. This makes C4 envelopes ideal for sending official documents, reports, or certificates that should not be creased.

What does the 'aspect ratio ~1.414' mean for ISO sizes?

The aspect ratio of approximately 1.414 (which is √2) is a fundamental mathematical principle behind the ISO 216 paper standard, including the A, B, and C series. This ratio ensures that when a sheet of paper is folded or cut in half along its longer side, the resulting two smaller pieces retain the exact same aspect ratio as the original. This geometric consistency allows for proportional scaling across all sizes within the series.

When would you use a C6 envelope?

A C6 envelope (114 x 162 mm) is commonly used to hold an A6 sheet of paper, which is an A4 sheet folded twice. It's a compact size often suitable for postcards, small invitations, or folded greeting cards. Its smaller dimensions make it economical for mailing lightweight items where a larger envelope is unnecessary, frequently fitting into standard letter-sized mail slots.