What is the standard business envelope size in the US?

The #10 envelope (4.125 × 9.5 inches / 104.8 × 241.3 mm) is the standard US commercial envelope. It holds a standard 8.5 × 11 inch letter-size sheet folded in thirds.

What envelope fits an unfolded 8.5 × 11 inch sheet?

A 9 × 12 inch catalog or booklet envelope is the standard US format designed to hold standard 8.5 × 11 inch documents flat without folding. For larger packets or catalogs, a 10 × 13 inch envelope is commonly used.

What fits in a European DL envelope?

A DL envelope measures 110 × 220 mm (4.33 × 8.66 inches). It is designed to accommodate an A4 sheet (210 × 297 mm) folded into three equal horizontal panels (99 × 210 mm insert) or an unfolded A5 card folded once.

How does USPS distinguish letter-size from flat-size envelopes?

Under USPS Domestic Mail Manual (DMM 101/201) standards, a letter-size piece cannot exceed 11.5 inches long, 6.125 inches high, and 0.25 inch thick. Pieces exceeding any of these dimensions up to 15 inches long, 12 inches high, and 0.75 inch thick fall into the Flat (Large Envelope) dimensional category, provided they meet USPS flexibility, uniformity, and rectangular standards. Dimensional eligibility does not guarantee a specific retail rate.

Can a 6 × 9 inch envelope be mailed as a USPS letter?

Dimensionally, a 6 × 9 inch envelope falls within the USPS letter maximums (6.125 × 11.5 inches). However, to be processed at letter rates, it must also be oriented with the delivery address parallel to the long edge, have an aspect ratio (length divided by height) between 1.3 and 2.5, remain within 0.25 inch thickness, and meet weight and flexibility criteria.

Are listed envelope sizes internal or external?

Published envelope dimensions are nominal outside dimensions. Seam construction, paper weight, and contents thickness reduce actual interior clearance. Commercial manufacturing tolerances typically allow ±1/16 inch (approximately 1.6 mm) variation.

Envelope Size Reference

Enter an item size to find envelopes it may fit, identify the closest standard envelope from measured dimensions, or browse the complete US and ISO size chart in inches, cm, or mm.

Envelope size and fit calculator

Find the smallest standard envelope that fits an insert, or identify the closest standard format from physical measurements.

Calculation Width Height Unit

Clearance per side (mm)

Dimensions shown are nominal exterior specifications. Standard manufacturing tolerances (typically ±1/16 in / 1.6 mm) and content thickness reduce usable interior room.

Name	Width	Height	Paper Fit & USPS Dimensional Class	Common Use	Copy

Dimensional Standards and Unit Relationships

USPS Dimensional Standards (DMM 101/201)

Letter Dimension Maxima: 11.5″ length × 6.125″ height × 0.25″ thickness. Minimum: 5″ × 3.5″ × 0.007″.
Flat (Large Envelope) Maxima: 15″ length × 12″ height × 0.75″ thickness. Minimum: length > 11.5″, height > 6.125″, or thickness > 0.25″.
Rate Disclaimer: Postal classification requires meeting rectangular shape, address orientation, uniformity, and flexibility requirements; nominal dimension fit alone does not guarantee a retail rate.

Manufacturing Tolerances & Units

Unit Conversion: 1 inch = exactly 25.4 mm = 2.54 cm. ISO envelopes are canonically defined in whole millimetres; US sizes are canonically defined in inches.
EMA Standards: Normal industry manufacturing tolerance is ±1/16 inch (approx. 1.6 mm) per Envelope Manufacturers Association guidelines.
Clearance Allowance: Allow at least 1/4 inch (6 mm) clearance along each dimension for single sheets, and more for booklets or multi-page inserts.

Dimensions copied!

Summary

This reference covers standard envelope formats used in the US, Europe, and internationally. Each entry lists nominal dimensions in inches, centimeters, and millimeters along with typical paper fits and USPS dimensional classifications. Filter by group, search by keyword, or calculate fits with custom clearance.

How it works

Choose whether to find a suitable envelope for an insert or identify an existing envelope from measured dimensions.
Enter width, height, and unit (cm, mm, or inches), plus optional clearance per side.
Filter the chart by format group (All, US Commercial, US Announcement, US Catalog/Booklet, ISO C-Series, or International).
Toggle between inches (in), centimeters (cm), and millimeters (mm) in the reference table.
Search envelopes by name, insert type, or postal category.
Click the Copy button on any row to copy formatted dimensions directly to your clipboard.

Use cases

Choose the right envelope for unfolded letter sheets, folded invoices, or greeting cards.
Find US Catalog/Booklet sizes (6×9, 9×12, 10×13 in) for unfolded documents and marketing packets.
Verify whether an envelope falls within USPS letter-size or flat-size dimensional boundaries.
Select ISO C-series envelopes for A-series documents (C4 for unfolded A4, C5 for A5 or half-fold A4, DL for tri-fold A4).
Determine envelope clearance required for multi-page inserts or heavy cardstock.
Convert envelope dimensions between inches, centimeters, and millimeters for prepress and packaging setup.

Frequently Asked Questions

Last updated: 2026-10-05 · Reviewed by Nham Vu